Monday, November 2, 2009

John Martinko, Department of Microbiology

Room 126 LS II

536-2349

martinko@micro.siu.edu

Office Hours: M, W 2-5, or by appointment

Web page:

http://www.science.siu.edu/microbiology/micr453/

Primary resource:

Brock Biology of Microorganisms, 12^th edition

Madigan, Martinko, Dunlap, and Clark, 2009

Pearson-Benjamin Cummings, San Francisco, CA

ISBN 0-132-32460-1

Brock Biology of Microorganisms

12 th Edition

Chapter 27:

Microbial Growth Control

Sections 27.1-3

I. Physical antimicrobial control

Heat – autoclave ( F 27.3), Pasteurization, flame (loop)

Radiation - gamma rays, microwave, UV

Filtration –

II. Chemical Antimicrobial Control

27.4 Chemical Growth Control

Chemical agents are routinely used to control microbial growth.

antimicrobial agent: a natural or synthetic chemical that kills

or inhibits the growth of microorganisms.

-cidal agents kill microorganisms

prefix indicates the kind of organism killed

bacteriocidal, fungicidal, and viricidal agents

-static agents inhibit microbial growth

bacteriostatic, fungistatic, and viristatic agents

Some -cidal agents may also be

-lytic agents - lyse micoorganisms and viruses

Effect of Chemical Antimicrobial Agents on Growth

Chemicals that have toxicity limit microbial growth in vitro.

Experiment :

Chemical antimicrobial agents are added to exponentially

growing bacterial cultures: Results: Figure 27.9

bacteriostatic, inhibitors of protein synthesis, act by binding to

ribosomes - tetracycline

bacteriocidal, agents bind tightly to cellular targets and are not

removed by dilution - penicillin

bacteriolytic agents induce killing by cell lysis, - a decrease in cell

number and turbidity - detergents dissolve membranes

penicillin

Measuring Antimicrobial Activity (Figure 27.10)
minimum inhibitory concentration (MIC) - the lowest amount of agent needed to completely inhibit the growth of a test organism (tube dilution technique)

Figure 27.11

Agar medium is inoculated with the test organism.

Known amounts of antimicrobial agent are added

to filter paper disks and placed on the surface of the agar.

The agent diffuses from the filter paper into the agar; the further from the filter paper, the lower the concentration of the agent.

The diameter of the zone of inhibition is proportional to the amount of antimicrobial agent

added to the disk, the solubility, the diffusion coefficient, and the overall antimicrobial effectiveness.

•Distinguish between -static, -cidal, and -lytic agents.

•Describe how the minimum inhibitory concentration of an antibacterial agent is determined

27.5 Chemical Antimicrobial Agents for External Use

1. We must control microorganisms not pathogenic in humans

Table 27.3 - mostly industrial applications

2.We must control growth of human pathogens in vitro

(on inanimate objects).

sterilants, disinfectants, sanitizers, and antiseptics.

Table 27.4 – healthcare, food, water, personal hygiene

Table 27.4 Sterilants are used on inanimate objects

(not live tissue)

sterilants, / sterilizers / sporicides destroy all forms of

(microbial) life, -even endospores AND viruses

Chemical sterilants are used when it is impractical to use heat or

radiation

Hospital, laboratory applications:

thermometers, lensed instruments, tubing, catheters,

reusable medical equipment (respirometers)

cold sterilization - enclosed devices that resemble autoclaves

employ a gaseous chemical agent:

ethylene oxide, formaldehyde, peroxyacetic acid, or

hydrogen peroxide

Liquid sterilization -sodium (hypo)chlorite solution / amylphenol

instruments that cannot withstand high temperatures or gas

Table 27.4

Disinfectants, Sanitizers

kill microorganisms, but not all kill endospores,

used on inanimate objects

Disinfectants- kill (almost) all microorganisms - proper mix and

exposure

Hospital infection control

-ethanol and cationic detergents

decontaminate floors, tables, bench tops, walls

Household disinfectants for general disinfection

swimming pools, and water purifiers

Sanitizers - reduce, but may not eliminate, microbial numbers to

a "safe" level- water soluble

Food contact sanitizers - mixing and cooking equipment,

dishes, and utensils

non-food contact sanitizers - counters, floors, walls, carpets,

air, and laundry

Antiseptics and germicides- chemical agents that kill or inhibit

growth of microorganisms and are sufficiently nontoxic to be

applied to living tissues.

handwashing (hand “sanitizers”/ treating surface wounds

certain antiseptics may also be effective disinfectants (FDA)

Antimicrobial Efficacy

Disinfectants and others can be neutralized by organic materials

e.g., pathogens are often grow in large numbers as

biofilms, limiting penetration of a chemical agent

ONLY sterilants are effective against bacterial endospores

Certain vegetative cells - Mycobacterium tuberculosis-

are resistant to the action of common disinfectants

because of the nature of their cell wall

•Distinguish between sterilizer, a disinfectant, a sanitizer, and an antiseptic.

•What disinfectants are routinely used for treatment of water? Why are these disinfectants not harmful to humans?

Physical / chemical growth control

Sterilizers
Kill all (even spores) – 37% formaldehyde
Disinfectants
Kill all but spores – surfaces
3-8% formaldehyde, Lysol, high bleach Sanitizer
Kill most – reduce microbial load - not spores - surfaces - low bleach
Antiseptics (germicides)
Kill most
Safe for contact with skin – alcohols
reduce microbial load

Selective toxicity
Growth factors / Analogs

III. Antimicrobial Agents Used in vivo

Treatment and Prevention of Infectious Disease

Compounds for internal use

Synthetic drugs

Antibiotics

27.6 Synthetic Antimicrobial Drugs

-manufactured chemical compounds that can be used internally

selective toxicity - the ability to inhibit or kill pathogenic

microorganisms without adversely affecting the host

First example - proof of concept Fig 27.15

Bacterial structures are different from eukaryotic structures

-targets for antimicrobial therapy Fig 27.12

Some microorganisms are more susceptible to certain agents

than others - different structures, etc.

Fig 27.13

27.6 Synthetic Antimicrobial Drugs

-manufactured chemical compounds that can be used internally

selective toxicity - the ability to inhibit or kill pathogenic

microorganisms without adversely affecting the host

First example - proof of concept Fig 27.15

Bacterial structures are different from eukaryotic structures

-targets for antimicrobial therapy Fig 27.12

Some microorganisms are more susceptible to certain agents

than others - different structures, etc.

Fig 27.13

27.6 Synthetic Antimicrobial Drugs

-manufactured chemical compounds that can be used internally

selective toxicity - the ability to inhibit or kill pathogenic

microorganisms without adversely affecting the host

First example - proof of concept Fig 27.15

Bacterial structures are different from eukaryotic structures

-targets for antimicrobial therapy Fig 27.12

Some microorganisms are more susceptible to certain agents

than others - different structures, etc.

Fig 27.13

Broad spectrum


Narrow spectrum

27.6 Synthetic Antimicrobial Drugs

Growth factor analogs

Nucleic acid base analogs

Quinolones

Growth factor analogs

-synthetic compounds, structurally similar to a growth factor

-subtle structural differences between the analogs and the

authentic growth factors prevent the analogs from functioning

in the cell

GF analogs for vitamins, amino acids, purines,

pyrimidines, and other compounds

Sulfa Drugs

-first widely used growth factor analogs that specifically

inhibit the growth of bacteria

-predate antibiotics

-large-scale screening of chemicals to find a cure

for streptococcal diseases in experimental animals

Figure 27.16 Sulfanilamide

p-aminobenzoic acid (PABA) analog - part of the folic acid,

vitamin - nucleic acids and amino acids

Blocks folic acid synthesis, inhibiting nucleic acid synthesis

Active only in Bacteria

Bacteria must synthesize folic acid

most animals obtain folic acid from their diet

Widespread drug resistance developed with time - 10-20 years

-sulfamethoxazole (a clinically useful sulfa drug) plus trimethoprim,

a related folic acid synthesis competitor, is now used

-drug combination produces sequential blocking of the

folic acid synthesis pathway

-resistance to the combination drug requires two mutations

in the same pathway in the same organism

Nucleic Acid base analogs/ amino acid analogs

Example: Fig 34.42

AZT, a nucleoside analog of thymidine

Quinolones

-another class of synthetic antimicrobial drugs

-interact with bacterial DNA gyrase, prevent the gyrase from

supercoiling bacterial DNA and packaging DNA in the

bacterial cell

DNA gyrase is found in all Bacteria

-fluoroquinolones (ciprofloxacin) are effective for treating both

gram-positive and gram-negative bacterial infections

-broad spectrum

- (over)use in beef and poultry

prevention and treatment of respiratory diseases

Figure 27.18

Ciprofloxacin- urinary tract infections in humans

-treat infections from penicillin-resistant strains of

Bacillus anthracis (bioterrorism agent)

•What is selective toxicity?

•Distinguish synthetic chemotherapeutic agents from antiseptics and disinfectants.

•Describe the action of a growth factor analog.

27.7 Naturally Occurring Antimicrobial Drugs: Antibiotics

Antibiotics: natural antimicrobial compounds produced

by fungi and bacteria (other microorganisms) that inhibit or

kill other microorganisms

-thousands known- <1% are clinically useful

-semisynthetic antibiotics

natural antibiotics structurally modified in the laboratory

?enhanced efficacy?

Antibiotics and Selective Antimicrobial Toxicity

Figure 27.12

The susceptibility of microorganisms to individual antibiotics and

other antimicrobial agents varies significantly

-gram-positive Bacteria and gram-negative Bacteria may differ in

their susceptibility to an individual antibiotic

-broad-spectrum antibiotics are effective on both groups.

broad-spectrum antibiotics may find wider medical

use than a narrow-spectrum antibiotic

(but vancomycin (MRSA) and isoniazid are very useful)

Antibiotics Affecting Protein Synthesis Figure 27.13
Antibiotics inhibit protein synthesis by interacting with the ribosome - medical and research uses
-streptomycin inhibits protein chain initiation
-tetracycline inhibits protein elongation

Selectivity
-tetracycline is specific for ribosomes of Bacteria

Problem:
antibiotics that inhibit protein synthesis in Bacteria also inhibit protein synthesis in mitochondria

-tetracycline is still useful because the eukaryotic mitochondria are not affected at the concentrations used for antimicrobial therapy

-Distinguish antibiotics from growth factor analogs.

- What is a broad-spectrum antibiotic?

- Identify the potential target sites for antibiotics that

inhibit protein synthesis and transcription.

27.8 Beta-lactam Antibiotics

penicillins, cephalosporins, and cephamycins

Figure 27.14 Penicillins and cephalosporins account for 54% of

antibiotics produced and used worldwide.

Figure 27.19 Penicillins, cephalosporins, and cephamycins

-medically important antibiotics, share a structural

component, the beta-lactam ring

Penicillins

-1929 Alexander Fleming characterized an antibacterial product of

the fungus Penicillium chrysogenum as the antibiotic penicillin G

-the first clinically effective antibiotic discovered

-1939 Howard Florey et al. - process for large-scale production

of penicillin

Penicillin G is active primarily against gram-positive Bacteria

-Staphylococcus, Streptococcus pneumoniae, S. pyogenes

- gram-negative Bacteria are impermeable to the antibiotic

Modifications of the penicillin G structure changes properties of

the resulting analogs F 27.19

-semisynthetic penicillins -ampicillin and carbenicillin- are

effective against some gram-negative Bacteria

-modified N-acyl groups of these semisynthetic penicillins allow

them to be transported across gram-negative outer membranes

-resistance: penicillin G is sensitive to beta-lactamase

produced by some penicillin-resistant Bacteria

-semisynthetic oxacillin and methicillin are resistant to

beta-lactamase

Mechanism of Action

Beta-lactam antibiotics inhibit cell wall synthesis, therefore are highly selective

-transpeptidation reaction cross-links two glycan-linked peptide chains (Figure 6.7a).

-transpeptidase enzymes [penicillin binding proteins (PBPs)] bind beta-lactam antibiotics instead of bacterial glycopeptides

Result:

no cross-linking, cell wall continues to be formed, resulting in a weakened, self-degrading cell wall

-osmotic pressure (high inside, low outside) and endogenous autolysins inside the cell, lyse the cell

-bacteriolytic, bacteriocidal

•Draw the structure of the beta-lactam ring and indicate the site of beta-lactamase activity.

•How do the beta-lactam antibiotics function?

27.9 Antibiotics from Prokaryotes

Macrolide Antibiotics

Erythromycin (produced by Streptomyces erythreus) contains

large lactone rings connected to sugar molecules (Figure 27.22)

-variations in macrolide ring and sugars = variety of macrolides

-protein synthesis inhibitor binds the 50S subunit of the

ribosome

-use- in place of penicillin in patients allergic to penicillin

or other beta-lactam antibiotics

Tetracyclines Figure 27.23

-produced by Streptomyces

-early broad-spectrum antibiotics, inhibiting almost all

gram-positive and gram-negative Bacteria

-naphthacene ring substituted at several positions to form new

tetracycline analogs

e.g. chlortetracycline has a chlorine atom,

oxytetracycline has a hydroxyl (OH) group, no chlorine

-natural products

-semisynthetic variations are constantly developed

-protein synthesis inhibitor

interferes with 30S ribosomal subunit function

-tetracyclines (and beta-lactam antibiotics) are the most

important groups of antibiotics in clinical use

-also used in veterinary medicine

-in some countries tetracyclines are used as nutritional

supplements for poultry and swine

-this prophylactic veterinary use is now discouraged

-fosters drug resistance

Fig 27.25 Platensimycin

Streptomyces platensis

Inhibits bacterial lipid biosynthesis

Broad spectrum

MRSA, VRE

No toxicity

No resistance

•What are the biological sources of tetracyclines? Macrolides? Platensimycin?

•Compare to the sources of penicillin beta-lactam antibiotics.

•How does each antibiotic lead to death of the affected cell?

IV  Control of Viruses

27.10 Antiviral Drugs

Viruses use eukaryotic hosts to reproduce and perform

metabolic functions

-many drugs that control viruses target host structures and

are also toxic for the host

Strategy

- several agents are more toxic for viruses than for the host

-a few chemical agents specifically target viruses

HIV

Antiviral Chemotherapeutic Agents (Table 27.5).

Nucleoside analogs, some of which are

nucleoside reverse transcriptase inhibitors (NRTI),

all work by the same mechanism:

inhibit elongation of the viral nucleic acid polymer (DNA or RNA)

Model compound

-azidothymidine (AZT)

Fig 34.42 AZT inhibits retroviruses -HIV

inhibits RNA-dependent DNA polymerase

-analog of thymidine, substitutes 3’N₃ for the 3’OH in deoxyribose, stops chain elongation

AZT inhibits multiplication of retroviruses by blocking reverse transcription and production of the virally encoded DNA intermediate

Side effects

-normal cell nucleic acid replication is targeted,

causing host toxicity (all of the virus inhibitor drugs)

-NRTIs lose potency with emergence of drug resistance

Table 27.5

-Nevirapine, a non-nucleoside reverse transcriptase inhibitor

(NNRTI), binds to reverse transcriptase

-inhibits reverse transcription

Phosphonoformic acid - inorganic pyrophosphate analog inhibits

internucleotide linkages, preventing synthesis of viral nucleic acids

protease inhibitors (PI) (Figure 27.31) prevent viral replication by

binding the active site of HIV protease, inhibiting processing of

viral polypeptides and virus maturation-less toxicity

fusion inhibitor -enfuvirtide -a 36-amino acid synthetic peptide

-binds gp41 membrane protein of HIV

-stops the conformational changes necessary for the fusion of

viral and target T lymphocyte cell membranes

Influenza Antiviral Agents

-adamantane derivatives - amantadine and rimantadine

synthetic amines that interfere with an influenza A ion transport

protein, inhibiting virus uncoating and replication

-neuraminidase inhibitors - oseltamivir and zanamivir

block the active site of neuraminidase in influenza A and B

viruses, inhibiting virus release from infected cells

Both categories of drugs are used for treatment and prophylaxis

of influenza infection (Section 34.9).

•Why are there relatively few effective antiviral chemotherapeutic agents? Why aren't such agents used to treat common viral illnesses such as colds?

•What steps in the viral maturation process are inhibited by nucleoside analogs? By protease inhibitors?

27.12 Antimicrobial drug resistance

-the acquired ability of a microorganism to resist the effects

of a chemotherapeutic agent to which it is normally

susceptible

Origins - antibiotic producers are microorganisms,

-antibiotic-producing microorganisms develop (or have)

resistance mechanisms to neutralize or destroy their

own antibiotics (Streptomyces and Penicillium)

-resistance genes may be transferred between and among

(related) microorganisms by genetic exchange

vertical (chromosomal)

horizontal (plasmids)

Resistance Mechanisms (Table 27.7)

(1) The organism may lack the targeted structure. Mycoplsma

-mycoplasmas, lack bacterial cell wall = resistant to penicillins

(2) The organism may be impermeable to the antibiotic

-gram-negative Bacteria are impermeable to penicillin G

(3) The organism may alter the antibiotic to an inactive form

-many staphylococci contain beta-lactamase that cleaves the

beta-lactam ring of many penicillins (Figure 27.27)

Common Modifications - Figure 27.27

(4) The organism may modify the target of the antibiotic

(e.g., mutation in ribosome)

(5) The organism may develop a resistant biochemical pathway

-Bacteria resistant to sulfonamides modify their metabolism to take

up preformed folic acid from the environment, avoiding the need

for the pathway blocked by sulfonamides or the enzyme that

binds PABA acquires a higher affinity for PABA than sulfonamide

(6) The organism may be able to pump out an antibiotic entering

the cell (efflux)

Mechanism of Resistance Mediated by R Plasmids

Drug-resistant bacteria isolated from patients have R (resistance)

plasmids

-R plasmids encode enzymes that inactivate antimicrobial agents

(beta-lactamase inactivates penicillin -Figure 27.27)

-R plasmids encode enzymes that prevent drug uptake

or

-R plasmids encode proteins that actively pump out the drug

-R plasmids may confer multiple antibiotic resistance

-a single R plasmid may contain several different genes,

each encoding a different antibiotic-inactivating enzyme

Origin of Resistance Plasmids

R plasmids existed before the antibiotic era.

-Escherichia coli freeze-dried in 1946 contained a plasmid with

genes conferring resistance to both tetracycline and

streptomycin

-neither antibiotic was used clinically until several years later

-strains carrying R plasmid genes for resistance to

Semisynthetic penicillins existed before the semisynthetic

penicillins had been synthesized.

-R plasmids with antibiotic resistance genes are found in

nonpathogenic soil Bacteria

? selective advantage for major antibiotic-producing organisms

that are normal soil organisms (Streptomyces) ?

Spread of Antimicrobial Drug Resistance

-medical, veterinary, and agricultural antibiotic use provides

selective conditions for the spread of R plasmids and

confers a selective advantage to bacteria containing R plasmids

-Antibiotic resistance is predictable

-an outcome of “natural” selection

-(Over)use of antimicrobial drugs fosters resistance

-Increased clinical use of an antibiotic is paralleled by emerging

resistance to the same antibiotic

Example: Neisseria gonorrhoeae (gonorrhea) resistance

to ciprofloxacin (fluoroquinolone) / ceftriaxone

Figure 27.28c

Overuse

Good news:

Antibiotics are now prescribed at lower levels than in the past

(generally useful in 20% of cases)

-physicians now prescribe about one-third fewer antibiotics for

treatment of childhood infections than they did 10 years ago

Bad news:

-ineffective dose, patient non-compliance, lead to sublethal

doses of antibiotics for short periods of time, selecting for

resistant strains

Overuse

More bad news:

-Antibiotics used in animal feeds as growth-promoters and

prophylactic additives may lead to resistance

-fluoroquinolones have been extensively used for less than 20 yrs

as growth-promoting and prophylactic agents in agriculture AND

-fluoroquinolone-resistant Campylobacter jejuni have emerged as

foodborne pathogens. Why?

? Routine treatment of poultry flocks with fluoroquinolones to

prevent respiratory diseases?

Antibiotic-Resistant Pathogens

Almost all pathogens are resistant to some antimicrobial agents

since widespread use of antimicrobial chemotherapy began in the

1950s (Figure 27.29)

Penicillin and sulfa drugs are not widely used because many

pathogens have acquired some resistance

Some pathogens are resistant to known antimicrobial agents

-vancomycin-resistant Enterococcus faecium (VRE)

-Mycobacterium tuberculosis (XDR TB)

-methicillin-resistant Staphylococcus aureus (MRSA)

-Pseudomonas aeruginosa

-Candida albicans

Steps to prevent resistance

Limited, appropriate use of antibiotics

Combination therapy

Potential reversal of resistance patterns with restricted use

•Identify the six basic mechanisms of antibiotic resistance among bacteria.

•Identify the primary sources of antibiotic resistance genes.

•What practices encourage the development of antibiotic-resistant pathogens?

Brock Biology of Microorganisms

12th Edition

Chapter 28:

Microbial Interactions with Humans

II
HARMFUL MICROBIAL INTERACTIONS WITH HUMANS

F 28.12

Pathogens are disease-

producing microorganisms

Various pathogen

strategies cause disease

Virulence

the relative ability of a

pathogen to cause disease

Growth in the body

= infection

Model pathogens:

Neisseria gonorrhoeae

Streptococcus

Clostridium

Escherichia coli

Salmonella

Influenza

28.6 Entry of the Pathogen into the Host

Adherence Table 28.3

Pathogens initiate infections at breaks / wounds in skin /

mucous membranes of the respiratory, digestive, or genitourinary

tract - specific adherence to epithelial cells (Figure 28.13)

Examples:

Tissue specificity

Neisseria gonorrhoeae (gonorrhea)

adheres to urogenital epithelia through a surface protein

Opa (opacity associated protein)

Opa binds CD66 on the surface of human epithelial cells.

Species specificity-

H5N1 strain adheres to bird epithelia

not well to humans

Polysaccharides, proteins, or protein-carbohydrate mixtures

are synthesized and secreted by the bacteria.

slime layer - not attached to the bacteria (Fig 28.4b)

capsule (Figures 28.13 and 28.14) a dense, well-defined

polymer coat surrounding the cell

Functions:

1. adherence between other bacteria

2. protect bacteria from host defense / phagocytosis

Vibrio cholerae

No capsule

Escherichia coli

Extensive capsule

Both adhere to villi Fig 28.13

Fig 28.14 Bacillus anthracis Extensive capsule

Fimbriae and pili

-bacterial cell surface protein structures that function in mobility

and attachment

-Pili of Neisseria gonorrhoeae attach to urogenital epithelium

-Fimbriae- strains of Escherichia coli with fimbriae

(Figure 28.15)

cause urinary tract infections more frequently than

strains lacking fimbriae

Differences:

Type I fimbriae (Escherichia, Klebsiella, Salmonella, Shigella)

-uniformly distributed on the surface of cells

enterotoxic strains of E. coli express fimbrial proteins

called CFA (colonization factor antigens)

adhere specifically to cells in the small intestine

Fimbriae in E. coli Fig 28.15

Pili are generally longer than fimbriae, with fewer pili found on

the cell surface.

Flagella also increase adherence to host cells and

boost virulence.

Invasion - getting a foothold in the host

-most pathogens must penetrate the epithelium to initiate

pathogenicity

-attachment is established and invasion occurs at small breaks or

lesions in the skin or on mucosal surfaces

-attachment and invasion can be enhanced on mucosal surfaces

if normal flora is altered

How? Example: antimicrobial chemotherapy

illness

Type organism: Streptococcus pneumoniae

•How do Opa proteins on Neisseria gonorrhoeae influence adherence to mucosal tissues?

•What is invasion?

•How does adherence initiate invasion?

21.7 Colonization and Growth (Infection)

colonization - pathogen multiplies in wounds, breaks, mucosal

tissues, even blood

Pathogen must find nutrients and suitable conditions

in host (temperature, pH, and presence or absence of oxygen)

-soluble nutrients -sugars, amino acids, organic acids- are limited

-pathogens able to use complex nutrients (glycogen) are favored

Iron: concentration influences growth

-transferrin and lactoferrin in animals bind iron tightly

-some pathogens require iron

-iron salt solution given to an infected animal

increases the virulence of some pathogens

siderophores called aerobactin from Escherichia coli

(encoded on Col V plasmid) removes iron bound to host

transferrin

Localized Growth/ Infection and Spread in the Body

MRSA

Local infection- boil that may arise from Staphylococcus skin

infections may lead to generalized (systemic) infection

– spread of the pathogen through the blood and lymph systems

can lead to extensive bacterial growth in tissues

Some of the organisms are shed into the bloodstream in large

numbers - bacteremia

Widespread systemic infection may occur – septicemia

•Why are colonization and growth necessary for the success of most pathogens?

•Identify host factors that limit or accelerate colonization and growth of a microorganism at a local site.

21.8 Virulence

Virulence is the relative ability of a parasite to cause disease

Measuring Virulence

virulence estimates:

LD₅₀ (lethal dose₅₀) = dose of an agent that kills 50% of the

animals in a test group

Fig 28.16 Virulence quantified

For S. pneumoniae

-hard to count the few organisms needed for LD₅₀

highly virulent pathogen - way less than 100 organisms

20?

LD₅₀ for Salmonella typhimurium, a less virulent pathogen,

is much higher than for S. pneumoniae

2000?

The number of cells of Salmonella typhimurium required to kill

100% is about 10,000 times greater than the number of cells

needed to achieve 100% death with S. pneumoniae.

Attenuation- loss of virulence in a pathogen occurs because

nonvirulent mutants may grow faster in vitro

Successive transfers in vitro select such mutants

Attenuated strains are used for production of many viral vaccines

measles, mumps, rubella, some polio vaccines.

BCG (bacille Calmette-Guerin) and Mycobacterium tuberculosis

Toxicity and Invasiveness

Virulence is due to the ability of a pathogen to cause host damage

through toxicity and invasiveness.

Toxicity is the ability of an organism to cause disease by means

of a preformed toxin that inhibits host cell function or kills host cells.

Major virulence factor for Clostridium tetani is tetanus toxin

-moves to distant parts of the body and initiates

irreversible muscle contraction and often death of the host.

Virulence in C. tetani is due almost exclusively to toxicity.

Invasiveness

Ability of an organism to grow in host tissue in such large numbers

that the pathogen inhibits host function (causes disease).

Major virulence factor for Streptococcus pneumoniae

= polysaccharide capsule

- prevents the phagocytosis of pathogenic strains

Encapsulated S. pneumoniae cause extensive host damage

because they are highly invasive

-grow in lung tissues in enormous numbers and initiate

-host inflammatory responses that lead to pneumonia

(nonencapsulated strains are destroyed by phagocytes)

Most pathogens fall between the extreme toxicity of C. tetani

and the extreme invasiveness of S. pneumoniae

Most successful pathogens use a combination of toxins

and invasiveness

Salmonella, Streptococcus pyogenes, Staphylococcus

•Explain

attenuation

toxicity

invasiveness

•Give examples

III. Virulence Factors and Toxins

28.9 Virulence Factors (Table 28.4)

virulence factor

-pathogen-produced extracellular protein that aids in

the establishment and maintenance of disease

Example; streptococci, staphylococci, and certain clostridia

produce hyaluronidase (enzyme)

-promotes spread of organisms in tissues by breaking down host

hyaluronic acid (intercellular cement)

-digestion of the intercellular matrix enables these

organisms to spread from an initial site

Fibrin, Clots, and Virulence

Coagulase (E) produced by pathogenic

Staphylococcus aureus

Coagulase causes fibrin (clot) to be deposited on S. aureus cells

-protects the coated bacteria from attack by host cells

-accounts for the extremely localized nature of many

staphylococcal infections, as in boils and pimples.

Coagulase-positive Staphylococcus aureus strains are more

virulent than coagulase-negative strains

Streptokinase (E) fibrinolytic substance produced by

Streptococcus pyogenes

Streptokinase dissolves clots, allowing S. pyogenes to move out

of wound and into surrounding tissues

•What advantage does the pathogen gain by producing enzymes that digest structural components of host tissues?

•How can the activity of coagulase work to enhance the growth of Staphylococcus aureus?

28.10 Exotoxins Table 28.4

Exotoxins -toxic proteins released extracellularly

Travel from a focus of infection and cause damage at distant sites.

4 categories;

1.Cytolytic toxins (CT) - enzymatically attack cells, cause lysis.

2.Enzymes (E)- break down host tissues or produce protective

structures for the pathogen

3.AB toxins: AB toxins consist of two covalently bonded subunits

-B subunit binds to a cell surface receptor, allowing the transfer of

the toxic A subunit across the cell membrane, where it damages

the cell

4.Superantigen toxins (SA)

stimulates large numbers of immune cells, resulting in extensive

inflammation

Fig 28.18 Cytolytic Toxins--hemolysins

(a)Streptococcus pyogenes - streptolysin O

(b) Clostridium perfringens- lecithinase

Fig 28.19 Staphylococcal alpha toxin - a cytotoxin

AB toxins

Diphtheria Toxin (Figure 28.20)

Corynebacterium diphtheriae AB toxin is a single polypeptide

-fragment B binds a host cell receptor

binding induces proteolytic cleavage, separating fragments

A and B

-A enters host cytoplasm - disrupts protein synthesis by

blocking amino acid transfer from tRNA to the growing

polypeptide chain

-A specifically inactivates elongation factor 2 in eukaryotic cells

(involved in growth of the polypeptide chain) by catalyzing

the attachment of adenosine diphosphate (ADP) ribose

from NAD⁺ to EF-2, modifying its ability to bind and transfer

amino acids from tRNA, stopping protein synthesis

Fig 28.20 Action of diphtheria AB toxin

Neurotoxic exotoxins

Botulinum toxin: AB toxin, botulism, Figure 28.21

Most potent biological toxin known

1 mg of botulinum toxin can kill > 1 million guinea pigs

1 g = 1 BILLION

Botulism

-bioactive toxin complex binds to presynaptic membranes on

the termini of the stimulatory motor neurons at the

neuromuscular junction, blocking release of acetylcholine

Fig 28.21 Botulinum-poisoned (blocked) muscle cannot

receive excitatory A signal - contraction is prevented

Result: flaccid paralysis (death by suffocation)

Botulinum toxin-poisoned (blocked) muscle cannot receive an

excitatory signal

Contraction is prevented

transmission of the nerve impulse to the muscle through

acetylcholine - muscle receptor binding/excitation

is blocked

Result: flaccid paralysis and death by suffocation

Tetanus toxin AB toxin Figure 28.22

a. Normally, interneurons release glycine, an inhibitory

neurotransmitter.

G binds to receptors on the motor neurons,

stopping release of A by motor neurons and inhibiting muscle

contraction, allowing relaxation.

b. Tetanus AB toxin moves through motor neurons to the

spinal cord and binds specifically to ganglioside lipids at the

termini of the inhibitory interneurons.

TT blocks G release: motor neurons are not inhibited, leading

to continual release of A and uncontrolled contraction of the

poisoned muscles.

Uncontrolled contraction results in spastic paralysis

trismus/lockjaw

Fig 28.22 Tetanus -spastic, twitching paralysis

-affected muscles are constantly contracted

Fig 35.21 Tetanus.

Muscles of the mouth- trismus or lockjaw

Respiratory muscles-death due to asphyxiation.

•Identify features are shared by all exotoxins.

•Identify the unique features of exotoxins.

•Are bacterial growth AND infection in the host necessary for the production of toxins? Explain your answer.

28.11 Enterotoxins

Enterotoxins are exotoxins whose activity affects the

small intestine

-causes massive secretion of fluid into the intestinal lumen

-leads to vomiting and diarrhea

Figure 28.23 Cholera toxin action of cholera toxin

AB enterotoxin produced by Vibrio cholerae (cholera)

one A subunit : five B subunits

-B subunit binds specifically with the ganglioside GM1 (a complex

glycolipid) in the cytoplasmic membrane of epithelial cells

-the A subunit crosses the cell membrane, activates adenyl

cyclase, converts ATP -> cAMP

-higher cAMP levels cause secretion of chloride and bicarbonate

ions from mucosal cells into the intestinal lumen (out)

- secretion of large amounts of water into the lumen (out)

- rate of water loss into the small intestine can be greater

than the reabsorption of water by the large intestine

- massive net fluid loss occurs - vomiting, diarrhea

Victims generally die from dehydration/ electrolyte imbalance

Treatment - oral fluid replacement with solutions containing

electrolytes and other solutes

Other enterotoxins

Some enterotoxigenic Escherichia and Salmonella toxins are

functionally and structurally, related to cholera toxin

-produced in the gut by colonizing bacteria

-Staphylococcus aureus enterotoxin is a superantigen

-stimulate large numbers of immune lymphocytes

-causes systemic and intestinal inflammatory responses

Shiga toxins are protein synthesis inhibiting AB toxins

Shigella dysenteriae

E. coli O157:H7 (Shiga-like toxin)

Foodborne diseases

Food poisoning - acquired by the ingestion of preformed toxin

-growth in host is unnecessary

Staph enterotoxin, botulism (latency - hours)

Food infections - require active growth to form toxins

V. cholerae, E.coli, Salmonella

Food infections require bacterial growth in the host,

followed by toxin production (latency - days)

•What key features are shared by all enterotoxins?

•Describe the action of Vibrio cholerae toxin on the small intestine. Why does this lead to massive fluid loss?

ORT – Oral rehyderation therapy

ORS- oral rehydration solution

UNICEF /WHO contents of reduced osmolarity ORS packets 2006.

A 1-liter preparation of ORT solution contains:

sodium chloride (NaCl) - 2.6g

trisodium citrate dehydrate - 2.9g

potassium chloride (KCl) - 1.5g

anhydrous glucose - 13.5g

recommendation for zinc supplementation

Sports drinks are formulated to rehydrate healthy individuals

-too much sugar and too little electrolytes for ORT

28.12 Endotoxins

Endotoxin: the lipopolysaccharide (LPS) produced by

gram-negative Bacteria as part of the outer layer of

cell envelope is toxic

LPS endotoxins are cell-bound and released in large

amounts only when cells lyse

-in contrast, exotoxins are secreted products of living cells

(Table 28.5).

Endotoxins have been studied primarily in Escherichia, Shigella,

and especially Salmonella (gram- Bacteria)

Endotoxin Structure and Function

LPS consists of three covalently linked subunits (Figure 4.23)

lipid A, a core polysaccharide, and the O-polysaccharide.

Fever - universal symptom

endotoxin stimulates host cells to release endogenous

pyrogens (cytokines) that affect temperature control in the brain

Other symptoms

-diarrhea

-rapid decrease in lymphocyte, leukocyte, and platelet numbers

-release of cytokines leading to general inflammation

Large doses:

death from hemorrhagic shock and tissue necrosis

Toxicity of endotoxins is MUCH lower than that of exotoxins (mice)

LD₅₀ for endotoxin is 200-400 mg per animal

LD₅₀ for botulinum toxin is about 25 pg

about 10,000 fold less!

•Why do gram-positive Bacteria not produce endotoxins?

•Why are drug preparations tested for endotoxin?

(Parenteral administration)

Start here 11/5

IV. Host Factors in Infection

Uncontrollable risk factors: age, genetic makeup,

tissue specificity

Controllable risk factors: diet, stress

Host Risk Factors for Infection

Age, Stress, and Diet

Age -not controllable-

Infectious diseases are more common in the very

young (<1 yr) and in the very old ( > 50, 65)

Infant diarrhea

pathogens have a greater opportunity to become

established and produce disease

(limited by establishment of local flora? Immunity?)

diarrhea from pathogenic strains of Escherichia coli is more

frequent in infants <1 year.

Infant botulism

intestinal infection with Clostridium botulinum - infants <1 yr

C. botulinum colonizes and grows, secretes botulinum toxin,

leading to flaccid paralysis

contracted by ingestion of C. botulinum from soil, air,

(raw honey)

prevented by establishment of the (competitive) intestinal

normal flora in older children and adults

> 50, 65 years

infectious diseases are much more common

much more susceptible to respiratory infections,

particularly influenza

?declining immune response to pathogens?

Stress (controllable)

fatigue, exertion, poor diet, dehydration, or drastic climate

changes are stressors

-increase the incidence and severity of infections

Experimental Salmonella infections:

Rats subjected to Salmonella challenge after intense physical

activity (long periods) show a higher morbidity and mortality

than rested animals

Hormone studies

- stress hormone cortisone produced at higher level under stress

Activates "fight or flee" response

anti-inflammatory agent

- suppresses inflammation, inhibits activation of

immune response, promotes infection

Diet (controllable)

Vibrio cholerae numbers necessary to produce cholera in an

exposed individual are drastically reduced if the

individual is malnourished

Dietary restriction of sucrose and good oral hygiene

eliminate tooth decay.

Without dietary sucrose, Streptococcus mutans and

S. sobrinus are unable to synthesize the polysaccharide

slime layer necessary for adherence to tooth surfaces.

The Compromised Host

Hosts in which one or more resistance mechanisms are inactive

-probability of infection is therefore increased.

Healthcare-associated infections

morbidity -2 million cases / year

mortality -100,000 deaths / year

Hospital procedures - catheterization, injection, surgery etc.,

may introduce microorganisms

Organ transplant patients are treated with immunosuppressive

drugs that reduce patient resistance to infection

General stress of illness

Compromised hosts outside the hospital result from:

smoking, excess consumption of alcohol, i.v. drug use,

lack of sleep, poor nutrition, acute or chronic infection with

another agent

Chronic infection example: HIV infection

HIV-AIDS deaths are generally due to opportunistic pathogens,

microorganisms that do not ordinarily cause disease in an

uncompromised host

Genetic diseases that attack the immune system predispose

individuals to infections - die from infection. Which organisms?

•Identify factors that control susceptibility to infection and cannot be controlled by the host.

•Identify factors that control susceptibility to infection and can be controlled by the host.

28.14 Innate Resistance to Infection

Natural Host Resistance

Examples of species differences

Rabies

Raccoons and skunks are very susceptible to rabies infection.

Opossums rarely acquire rabies.

Anthrax (Bacillus anthracis)

-birds are totally resistant to anthrax

-fatal blood poisoning in cattle

- cutaneous anthrax - mild pustules in humans

-pulmonary, or airborne, anthrax (bioterrorism) is

>90% fatal in humans

HIV infection occurs only in higher-order primates

(great apes and humans)

Why?

CXCR4 protein on T cells and

CCR5 protein on macrophages

are uniquely expressed in primates

They act as cell surface receptors and specifically interact with

HIV gp120 protein

Other animals lack these receptors, cannot bind HIV,

and are thus protected from HIV infection

Figure 28.25 Physical and Chemical Defenses of the Body

Skin - effective barrier to the penetration of microorganisms.

Sebaceous glands in the skin secrete fatty acids and lactic

acid, lowering to pH 5

-inhibits colonization of many pathogenic bacteria

(blood and internal organs are about pH 7.4)

Ciliated epithelial cells on nasopharynx and trachea move cells into

oral secretions

-expectorated or swallowed and killed in the stomach

-continual movement also prevents adherence

Stomach acidity (pH 2) kills/ reduces numbers of ingested bacteria

Resident microflora and pH prevent colonization

stomach (pH 2)

small intestine (pH 5) and

large intestine (pH 6–7) (bacterial numbers of 10¹⁰ or more

per gram of contents)

Lysozyme found in lumen of the kidney and the surface of the eye

Tissue Specificity (Table 28.6) Pathogens must be able to

adhere and colonize at the site of exposure

Adherence to an exposure site does not guarantee colonization:

The site must meet nutritional and metabolic requirements of the

Pathogen

Pathogens cause disease at tissue-specific sites:

Clostridium tetani cells or endospores introduced into a deep

wound promotes growth and tetanus toxin production

in the anoxic zones created by local tissue death

C.tetani pathogen is killed by the acidity of the stomach

Enteric bacteria - Salmonella and Shigella

-colonize the intestinal tract, cause food infections

-do not cause wound infections

- Identify physical and chemical barriers to pathogens.

How might these barriers be compromised?

- How might preexisting infection compromise an

otherwise healthy host?