Antibiotic Mechanisms
of Action and
Resistance
MLAB 2434 – Microbiology
Keri Brophy-Martinez
Overview

Antimicrobial Therapy



Broad 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


Bacteriocidal
 Kills the bacteria
Bacteriostatic
 Inhibit 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 action
Effects on Cell Wall Synthesis
 Interruption of Cell Membrane
Structure and Function
 Inhibition of Protein Synthesis
 Inhibition of Folate Synthesis
 Interference 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 penicillinbinding proteins (PBPs)


PBPs mediate peptidoglycan crosslinking
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 βlactamase inhibitor act in synergy


Bind 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
Antibiotics
 Aminoglycosides, tetracyclines,
macrolides, clindamycin
chloramphennicol, and oxazolidinone
Antibiotics of Protein
Synthesis Inhibition

Aminoglycosides
Bactericidal
 Used primarily against GN bacteria

Antibiotics of Protein
Synthesis Inhibition

Tetracyclines
Bacteriostatic
 Broad spectrum
 Effective against GP and GN
organisms
 Tetracycline 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
Agent

Genes 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 permeability
 Decreased uptake
 Increased 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