Antibiotic and its Action against Bacteria

What are antimicrobials

An agent that kills microorganisms (bacteria, viruses, protozoas, fungi) or stops their growth

What are antibiotics

Agents with biological activities to kill or inhibit the growth of bacteria

What is antimicrobial resistance?

The ability of bacteria to resist the effects of an antibiotic

• when the bacteria change in a way that reduces the effectiveness of antibiotics 

What are antimicrobial susceptibility testing (AST) methods?

Methods used to determine how sensitive or resistant a bacteria is to a specific antibiotic

What are the 2 main types of AST (antimicrobial susceptibility test)?

• Disc diffusion

• Minimum inhibitory concentration

AST: What is minimum inhibitory concentration and what can it be divided into?

What: Determine of the lowest concentration of an antimicrobial that will inhibit the visible growth of a microorganism after overnight incubation

• Agar dilution 

• Broth dilution 

• E-test 

AST: Disc Diffusion 

• Name of method

• Principle 

• Interpretation

Name of method: Kirby-Bauer Method

Principal:

• Paper discs containing known concentrations of antibiotics are placed on the surface of an agar plate (usually Mueller–Hinton agar) that has been inoculated evenly with the test bacteria.

• The antibiotic diffuses outward from the disc into the agar.

• After incubation, bacteria grow everywhere except where the antibiotic concentration is high enough to inhibit them.

• This clear area around each disc is called the zone of inhibition.

Interpretation:

• Measure the diameter of the inhibition zone (in mm).

• Compare with standard reference charts (CLSI or EUCAST) to classify the bacterium as:

  • Sensitive (S)

  • Intermediate (I)

  • Resistant (R)

AST: MIC: Broth Dilution Method

• Serial dilutions of an antibiotic are prepared in liquid (broth) medium.

• Each tube (or well) is inoculated with a standard bacterial amount and incubated.

• The lowest concentration that shows no visible turbidity (no growth) is the MIC.

• Can be done as:

  • Macrobroth (in test tubes)

  • Microbroth (in microtiter plates — automated systems often use this)

AST: MC: Agar Dilution Method

• Serial concentrations of antibiotic are mixed into solid agar plates.

• Each plate is spot-inoculated with the test bacterium.

• After incubation, the lowest antibiotic concentration plate with no visible growth indicates the MIC

AST: MC: E test

• A plastic strip impregnated with a gradient of antibiotic concentration is placed on an agar plate inoculated with the test bacterium.

• After incubation, an elliptical zone of inhibition forms around the strip.

• The point where the ellipse edge meets the scale on the strip gives the MIC value directly.

What are the properties of antimicrobial agents?

• Selective toxicity 

• Bacteriacidal (bacteria killing) instead of bacteriostatic (inhibit growth) 

• Stable, low cost 

• Microbe should not become readily resistant 

• Have activity broad (act against both G+ and G-) or narrow spectrum (act against G+ OR G-) 

What is the mechanism of antibiotic?

• Cell wall and cell membrane synthesis inhibitors

• Nucleic acid inhibitors

• Protein synthesis inhibitors

• Folic acid metabolism inhibitors

Cell wall inhibitors

• Active against

• How it works 

• Inhibits

• Spectrum of activity

• Examples

Active against: Growing bacteria 

How it works: 

• Reacts with enzymes required in peptidoglycan synthesis 

• So cells develop weak points 

• And become osmotically fragile (cell lyse) 

Inhibits: Peptidoglycan synthesis

Spectrum of activity:

• Bacteriacidal

• Aerobic and anaerobic G

Examples: Bacitracin, Cephalosporin, Cycloserine, Penicillins, Vancomycin

Cell Membrane Inhibitors 

• Active against 

• Examples

Active against: Gram negative bacteria and fungi 

Examples:

• Polymixins - effective against G -ve bacteria 

• Polyenes - effective against fungi 

Protein synthesis inhibitors

• Inhibits

• Examples

Inhibits: Protein synthesis

Examples: Aminoglycosides, tetracyling 

Nucleic Acid inhibitors 

• How it works 

• Examples

How it works: Targets molecules involved in nucleic acid synthesis and replication

Examples: Quinolones 

Folic acid metabolism inhibitors

Inhibits folic acids which are enzymes that are necessary for bacterial protein synthesis and synthesis of amino acids

What are the ways that microorganisms exhibit resistance to drugs

• Produce enzymes that destroy active drug (Staphylococci resistant to penicillin produce B-lactamase that destroys penicillin)

• Change their permeability to the drug

• Develop an altered structure target for the drug

• Develop an altered enzyme that can still perform its metabolic function but less affected by drug 

How bacteria resist the antibiotics

• Antibiotic inactiviation 

• Alteration of the target site 

• Descreased acess to site of action 

Enzymatic Inactivation of Antimicrobial Agents

• What 

• Example

What: When bacteria produces specific enzymes that inactivate or modify the drug 

Example: B-lactamases

• Enzyme cleave the beta lactam ring of susceptible antibiotics

• In gram positive

Modificiation of Cell Wall permeabiltiy

• What 

• Cause

What: Decrease access to site of action 

Cause: Change in lipopolysacharide