chapter 15microbial mechanisms of

pathogenicity

pathogenesis

portals of entry & exit

inoculation vs. disease: preferred portal of entryentry DOES NOT EQUAL disease

entry into preferred portal of entry DOES NOT EQUAL disease

ID50: infectious dose for 50% of population

– inhalation anthrax: <104 spores

– V. cholerae: 108 cellsLD50 : lethal dose for 50%

– botulinum toxin: 0.03 ng/kg

– E. coli shiga toxin: 250 ng/kg

pathogenesis: enzymes

coagulase & kinase

hyaluronidase&

collagenase

leukocidins

toxicity: bacterial toxins

exotoxin endotoxin

source Gram positive/enterics Gram negative

expressed gene outer membrane component

chemical make-up protein lipid

neutralized by antitoxin? yes no

fever? no yes

LD50 (relative) small large

allow spread and cause damage to the host• toxigenicity: ability to produce a toxin• toxemia: toxin in blood• toxoid: immunization• antitoxin: Ab to toxin

cytotoxins: hemolysins

neurotoxins: Clostridium

enterotoxins: V. cholerae

endotoxins: fever

Salmonella virulence

mechanisms of pathogenicity

Inactivating host defenses

chapter 15 learning objectives1. Describe pathogenesis from exposure to disease. What factors contribute to disease?

2. Relate preferred portal of entry and ID50 to the likelihood of infection.

3. Know how to interpret ID50 and LD50 results.

4. Describe what is meant by invasiveness and the mechanisms and factors that affect invasiveness

(adherence, penetration, avoidance of phagocytosis, ability to cause damage).

5. Be able to list enzymes produced by microbes than enhance pathogenicity and virulence as well as

describe the effects of these enzymes on the host (i.e., hyaluronidase, collangenase, coagulase,

kinase).

6. Differentiate between an endotoxin and an exotoxin as far as source, chemistry and type of

molecule (protein, or polysaccharide/lipid). List and understand how examples from class work (e.g.,

cytotoxin, hemolysin, neurotoxin, enterotoxin, endotoxin). It is not necessary to know the particular

details of how each of the three types of exotoxins work.

STUDY ANIMATION URLs

endotoxin production

virulence factors animation

exotoxin production

penetrating host tissues

inactivating/avoiding the host defenses (just for your information)

avoiding host defenses (just for your information)

chapter 20antimicrobial compounds

chemotherapeutic agentsPaul Ehrlich- 1910’s • salvarsan (synthetic arsenic)

to treat syphilis

Alexander Fleming- 1928• Penicillium notatum

Howard Florey- 1940• P. notatum effectivity

antimicrobialsinhibition of protein synthesis: chloramphenicol, erythryomycin, tetracyclines, streptomycin

Transcription Translation

ReplicationEnzyme

ProteinDNA mRNA

inhibition of NA replication & Xscription: quinolones, rifampin

inhibition of cell wall synthesis: penicillins, cephalosporins, bacitracin, vancomycin

injury to plasma membrane: polymyxin B

inhibition of metabolite synthesis: sulfanimide, trimethoprim

protein synthesis inhibition

Translation

Streptomycin Tetracyclines

Chloramphenicol

Messenger RNA

Direction of ribosome movement

70S prokaryotic ribosome

tRNA

Protein synthesis site

30S portion

50S portion

Changes shape of 30S portion, causing code on mRNA to be read incorrectly

Interfere with attachment of tRNA to mRNA–ribosome complex

Binds to 50S portion and inhibits formation of peptide bond

GFA: metabolite inhibition & synergism

Phosphate

Nucleoside

Guaninenucleotide

Cellularthymidine kinase

DNA polymeraseIncorporated into DNA

Phosphate

DNA polymerase blocked by false nucleotide. Assembly of DNA stops.

False nucleotide(acyclovir triphosphate)

Acyclovir (resembles nucleoside)

ViralThymidine kinase

GFAs: nucleic acid inhibition

penicillin & cell wall synthesis inhibitionCELL WALL FORMATION

autolysins cut wall

new “bricks” inserted

transpeptidase bonds bricks

PENICILLIN ACTION

transpeptidase binds pen.

forms PBP-antibiotic structure

no new bond

formation

cell ruptures

Abx resistance

1. outdated, weakened, inappropriateAbx use

2. use of Abx in animal feed

3. long-term, low-dose Abx use

4. aerosolized Abx in hospitals

5. failure to follow prescribed treatment

the episilometer (E) test- the MIC

Abx resistance1. loss of porins

- Abx/drug movement into cell

2. Abx modifying enzymes

-cleave β-lactam ring

-Anx non-functional

3. efflux pumps

- movement out of cell

4. target site mutations

-enzymes

-polymerases

-ribosomes

-LPS layer

Resistance mechanisms

the effect of -lactamase on -lactam Abx

VERY STABLE RESISTANCE• NDM-1 (metallo- -lactamase)

• K. pneumoniae & E. coli, plasmids & chromosomal

• KPC (K. pneumoniae carbapenemase, class of -lactamase)

RESISTANCE RESISTED• clavulinic acid/sulbactam bind -

lactamase• can be hydrolyzed by high

copy # plasmid -lactamase

-lactamsNarrow-spectrum• β-lactamase sensitive

benzathine penicillinbenzylpenicillin (penicillin G)phenoxymethylpenicillin

(penicillin V)procaine penicillin

• Penicillinase-resistant penicillins methicillin, oxacillinnafcillin, cloxacillindicloxacillin, flucloxacillin

• β-lactamase-resistant penicillins temocillin

Moderate-spectrumamoxicillin, ampicillin

Broad-spectrumco-amoxiclav

(amoxicillin+clavulanic acid)Extended-spectrum

azlocillin, carbenicillinticarcillin, mezlocillin, piperacillin

Cephalosporins • 1st generation: moderate

cephalexin, cephalothincefazolin

• 2nd generation: moderate, anti-Haemophiluscefaclor, cefuroxime, cefamandole

• 2nd generation cephamycins: moderate, anti-anaerobecefotetan, cefoxitin

• 3rd generation: broad spectrumceftriaxone, cefotaximecefpodoxime, cefiximeceftazidime (anti-Pseudomonas activity)

• 4th generation: broad, anti-G+ & β-lactamase stabilitycefepime, cefpirome

• Carbapenems and Penems: broadest spectrumimipenem (with cilastatin), meropenemertapenem, faropenem, doripenem

• Monobactamsaztreonam (Azactam), tigemonamnocardicin A, tabtoxinine-β-lactam

bacterial resistance

2009 CASE STUDY, U. of Pittsburgh Medical Center

• 6/2008- post-surgical hospitalization, septicemia (E. coli & E. cloacae)

• 7/2008- UTI, E. coli & P. mirabilis

• 8/2008- UTI, E. coli (imipenem S) & K. pneumoniae (imipenem R & ertapenem R)

• 9/2008- abdominal tissue infection, E. coli & K. pneumoniae (both R to Abx)

• 11/2008- sputum P. aeruginosa & S. marcescens, K. pneumoniae

• 12/2008- MDR-pneumonia, A. baumanii & M. morganii

• 1/2009- sputum, S. marcescens (ertapenem & imipenem R)

chapter 20learning objectives1. What is the major difference between an antibiotic and a drug? What were the first drug and antibiotic?

2. Antimicrobial agents target which areas of the bacterial cell? How specifically do antibiotics inhibit protein

synthesis?

3. Describe the mechanism of action of penicillin on the bacterial cell.

4. List and explain the effects of antibiotic/drug action on the bacterial cell and the action of penicillin

specifically.

5. Discuss the mode of action of growth factor analogs in general and sulfa drugs and acyclovir specifically.

6. How are antibiotic use and antibiotic resistance related? How are antibiotics abused?

7. Define bacteriolytic, bacteriostatic, bactericidal, MIC, MBC. Describe how MIC is calculated and what it will

tell you about a given bacterium.

8. Understand the four major ways that antibiotic resistance is achieved. Include -lactamases and

clavulanate/clavulinic acid specifically.

STUDY ANIMATION URLs

mechanisms of Abx resistance

the origins of Abx resistance

the emergence of Abx resistance

cell wall formation, ß-lactam ABx and resistance