Microbio Lab 24

What is the Kirby-Bauer test used for?

Determining bacterial susceptibility or resistance to antimicrobials

What creates the bacterial lawn on a Kirby-Bauer plate?

Even inoculation of the entire plate surface with a pure culture

Why must antimicrobial disks be spaced apart?

To prevent overlapping zones and ensure accurate measurement

What diffuses from the disk into the agar?

The antimicrobial drug, forming a concentration gradient

What does bacterial growth right up to the disk indicate?

Resistance

What is a zone of inhibition?

An area around the disk with no bacterial growth → indicates susceptibility

What is the MIC?

Minimal Inhibitory Concentration — lowest drug concentration that inhibits growth

How are Kirby-Bauer results interpreted?

Measure the zone diameter and compare it to CLSI standards

Who establishes zone diameter interpretive charts?

CLSI (Clinical & Laboratory Standards Institute)

What is the required turbidity standard for inoculum preparation?

0.5 McFarland

What happens if the plate is too thick?

Drug diffuses less → smaller zone → false resistance

What happens if the plate is too thin?

Drug diffuses more → larger zone → false susceptibility

What happens if too many bacteria are used?

Smaller zones → inaccurate interpretation (false resistance)

What happens if too few bacteria are used?

Larger zones → inaccurate interpretation (false susceptibility)

What is a major advantage of Kirby-Bauer testing?

Allows simultaneous testing of multiple antimicrobial drugs

What do penicillins and cephalosporins have in common?

A beta-lactam ring

What do beta-lactam antibiotics target?

Peptidoglycan in the bacterial cell wall

What enzyme forms peptide cross-links in peptidoglycan?

Transpeptidase

How do beta-lactam antibiotics work?

They block transpeptidase → no cross-linking → weak cell wall → cell lysis.

What is the key enzyme bacteria use to inactivate beta-lactam antibiotics?

Beta-lactamase

What does beta-lactamase do to the antibiotic?

Destroys its beta-lactam ring

Nitrocefin is what type of compound?

A chromogenic cephalosporin

What color indicates a beta-lactamase NEGATIVE test?

Yellow

What color indicates a beta-lactamase POSITIVE test?

Pink/red

Why is the beta-lactamase test clinically important?

Quickly identifies beta-lactam–resistant bacteria (e.g., MRSA)

What genetic element often carries beta-lactamase genes?

Plasmids

Which bacterial structure is NOT produced correctly when beta-lactams are used?

Peptide bridges in peptidoglycan

What are the two main goals of susceptibility testing?

Choose the right drug and the right therapeutic dose

Why is it important not to kill normal flora?

Loss of normal flora can allow pathogens like C. difficile to cause disease

What does the Kirby-Bauer test NOT provide?

Exact MIC (μg/mL)

What does the E-test provide?

An approximate MIC

Which method gives a precise MIC?

Tube Dilution Test

What is MIC?

Minimum inhibitory concentration — the lowest drug concentration that inhibits growth

What medium is used for dilution tubes?

Mueller-Hinton broth

What is turbidity a sign of?

Bacterial growth

What is the MIC if the first clear tube is 16 μg/mL?

MIC = 16 μg/mL

What is the advantage of automation in MIC testing?

Faster, standardized results; can read up to 100 panels in ~18 hours

What is the disadvantage of manual tube dilution?

Time-consuming and tests only one drug at a time

Why are serial 1:2 dilutions used?

To produce known decreasing concentrations for MIC determination