Antibiotic Mechanisms of Action and Resistance

MLAB 2434 – Microbiology Keri Brophy-Martinez

Overview

Antimicrobial TherapyBroad term for use of chemical

compounds to treat diseases caused by microorganisms

Antimicrobial agents used to treat infections are directed by antimicrobial susceptibility testing (AST)

Targets specific body sites or specific characteristics of microbes

Agents

Antimycobacterials Treat mycobacterial diseases

Antifungals Treat fungal disease

Antiprotozoals Tread protozoal disease

Antivirals Treat viral disease

Antibiotics Treat bacterial disease

Antibiotics

Antibiotics are naturally occurring substances produced by a fungus or bacteria

Used to treat bacterial infections Alternate Forms

Synthetic• Totally manufactured or artificial

• Semi-synthetic compounds• Naturally occurring substances that have

been chemically altered

Definitions

BacteriocidalKills the bacteria

BacteriostaticInhibit microbial growth

Definitions (Cont’d)

Spectrum of activity Range of susceptible organisms

•Narrow-spectrum• Kill either Gram positive or Gram

negative organisms• Organism specifc

•Broad-spectrum• Kill both Gram positive and Gram

negative organisms• Extensive

Empirical therapy Initiation of therapy prior to organism ID

Definitions (Cont’d)

Additive Effects Combining two antimicrobials causes twice

the effect of the two drugs by themselves Indifference

No effect of combining antimicrobial therapies

Synergy Combined effect is greater than the two

individual effects added together Antagonism

One drug counteracts the other

Antimicrobial Agents:Factors to Consider

What is the targeted bacteria? Where is it located? Can the

antimicrobial reach that site in sufficient concentration?

Can the antimicrobial be retained in the body long enough to be effective?

What are the side effects? How is it excreted?

What is the cost?

Antimicrobial Categories

Mechanisms of actionEffects on Cell Wall SynthesisInterruption of Cell Membrane

Structure and FunctionInhibition of Protein SynthesisInhibition of Folate SynthesisInterference with Nucleic Acid

Metabolism

Effects on Cell Wall Synthesis Cell wall protects the bacteria

cytoplasmic membrance Cell wall primarily composed of a

peptidoglycan layer Inactivating or interfering with

enzymes that synthesize the cell wall can destroy the bacteria

β-Lactam Antibacterial Agents

Effect cell wall synthesis

Sizable portion of antibacterial agents used today

Includes penicillins, monobactams, and carbapenems, and cephalosporins

β-Lactam Antibacterial Agents: Overview

Bind specific enzymes known as penicillin-binding proteins (PBPs)PBPs mediate peptidoglycan

cross-linking If PBPs are bound by the beta-lactam,

the cross-linking of the cell wall is incomplete, results in cell death

β-Lactam Antibacterial Agents Penicillins

Simple penicillins are effective against many streps, Neisseria, Pasteurella, and a number of anaerobes

Monobactams Limited to aerobic Gram negative bacilli

Carbapenems Broadest antimicrobial spectrum Effective against gram positive and negative

organisms, and anaerobes Resistant to beta-lactamase

Cephalosporins Classified by their spectrum of activity and are

spoken of in terms of “generations”

Generations of Cephalosporins

First-generation Have good GP and GN activity

Second-generation Have better GN, and anerobes activity

Third-generation Better with Enterobacteriaceae and Pseudomonas

spp. Fourth –generation

Effective against GNR that are resistant to 3rd generation cephalosporins

Fifth-generation Spectrum of activity includes the 3rd and 4th

generation

β-Lactam/β-Lactamase Inhibitors

Combination of a β-lactam and a β-lactamase inhibitor act in synergyBind to beta-lactamase produced

by certain microbes β-Lactamase Inhibitors

Offer no antibacterial activity by themselves• Examples include: clavulanic acid,

sulbactam, tazobactam

Effects on Cell Wall Synthesis Glycopeptides

Bind certain amino acids and inhibit enymes in the developing peptidoglycan layer

Vancomycin• Most clinically important• Effective against MRSA, other GP

organisms, and organisms resistant to penicillin

Interruption of Cell Membrane Structure and Function Damages the cytoplasmic membrane of

the organism Bacitracin

Prevents the addition of peptidogylcan to the cell wall

Disrupts the cell membrane Primarily effective against GP organism Because of toxicity, these are limited

to topical medications (ex. Neosporin, etc.)

Interruption of Cell Membrane Structure and Function Polymyxins

Bind to outer surface of cell membrane, affecting phospholoid

Leads to leakage of intracellular contents and cell death

Effective against gram negative bacteria

Inhibition of Protein Synthesis These antimicrobials bind to ribosomal

subunits This binding is either irreversible,

resulting in cell death(bactericidal), or reversible, resulting in bacteriostatic effects

AntibioticsAminoglycosides, tetracyclines,

macrolides, clindamycin chloramphennicol, and oxazolidinone

Antibiotics of Protein Synthesis Inhibition Aminoglycosides

BactericidalUsed primarily against GN

bacteria

Antibiotics of Protein Synthesis Inhibition Tetracyclines

BacteriostaticBroad spectrumEffective against GP and GN

organismsTetracycline is NOT used in young

children or in pregnancy, as it affects tooth and bone development

Antibiotics of Protein Synthesis Inhibition Macrolides

Bacteriostatic Broad spectrum Effective against GP and some GN

organisms, spirochetes, Mycoplasma, Legionella, and Chlamydia

Agents include: erythromycin, azithromycin, clarithromycin

Antibiotics of Protein Synthesis Inhibition

Clindamycin Bacteriostatic Excellent activity against aerobic

GP organisms Extremely potent against

anaerobes “D” test

• Detects resistance to clindamycin based on past treatment with erythromycin

Antibiotics of Protein Synthesis Inhibition Chloramphenicol

Bacteriostatic Has broad activity but is extremely

toxic

Oxazolidinone Linezolid Effective against MRSA, VRE, and

mycobacteria

Inhibition of Folate Synthesis

Folic acid pathway provides essential precursor molecules for DNA synthesis

Antibiotics can block steps in this pathway resulting in cell death

Agents: sulfonamides, trimethoprim Used in combination Active against broad spectrum,

including GP and GN organisms, except for P. aeruginosa

Interference with Nucleic Acid Metabolism Interfere with either DNA or RNA

metabolism Inhibit enzymes required in the

replication process Agents:

quinolones/fluoroquinolones, rifamycins

Antibiotics of Nucleic Acid Metabolism Interference

RNA Synthesis Interference Rifampin

Mainly used for M. tuberculosis and M. avium complex

Has a broad spectrum of activity

Antibiotics of Nucleic Acid Metabolism Interference

DNA Synthesis Interference Quinolones/Fluoroquinolones

• Bactericidal• Used to treat GN organisms• Agents- ciprofloxacin, levofloxacin

Metronidzole • Activates under anaerobic conditions• Effective against anaerobes and protozoa,

bacterial vaginosis Nitrofurantoin

• Used against GN and GN organisms• Concentrates well in urine

Mechanisms of Antimicrobial Resistance Modify target

If target is altered, reduction or prevention of antimicrobial binding can occur

End result- antimicrobial is ineffective How does the microbe modify the

target?• Chromosomal mutations• Transposons• Plasmids

Mechanisms of Antimicrobial Resistance Inactivation of Antimicrobial

AgentGenes of the microbe encode

enzymes that convert active antimicrobial agents to an inactive form• Encoding of enzymes via chromosomal

or plasmid-mediated genes• Example: beta-lactamase producing

organisms

Mechanisms of Antimicrobial Resistance Blockage of antimicrobial entry

into the cell Mechanisms

Decreased permeabilityDecreased uptakeIncreased ability to pump

antimicrobial out of cell

References

Kiser, K. M., Payne, W. C., & Taff, T. A. (2011). Clinical Laboratory Microbiology: A Practical Approach . Upper Saddle River, NJ: Pearson Education.

Mahon, C. R., Lehman, D. C., & Manuselis, G. (2011). Textbook of Diagnostic Microbiology (4th ed.). Maryland Heights, MO: Saunders.

http://www.parn.org.pk/index_files/D.test.html