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MT MLO: Bacteriology: Isolation, Identification, and Antimicrobial Susceptibility Testing

MT MLO · Work through your exam, one topic at a time.

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Bacteriology: Isolation, Identification, and Antimicrobial Susceptibility Testing

This domain tests the full bench workflow: choosing primary media, identifying an isolate by phenotype and MALDI-TOF, and translating an MIC or zone size into a correct, clinically useful report.

Isolation and identification

  • Standard primary set: blood agar, chocolate agar, MacConkey, plus CNA/PEA for gram-positive selection and anaerobic plates when indicated.
  • MALDI-TOF works reliably on 18–24 hour colonies; direct-from-positive-blood-culture pellets shorten turnaround.
  • Key bench reactions: coagulase and PYR (Staphylococcus/Enterococcus), bile esculin and 6.5% NaCl, optochin and bile solubility (S. pneumoniae), bacitracin A disk (S. pyogenes).

Susceptibility testing

  • Use current CLSI (or EUCAST) breakpoints, a 0.5 McFarland inoculum, and standardized incubation; read disks and MIC panels at the validated time.
  • Report only agents on the validated panel and suppress intrinsic resistance (ampicillin for Klebsiella, cephalosporins for Enterococcus and Serratia).
  • Perform a D-test for S. aureus and beta-hemolytic streptococci when erythromycin is resistant and clindamycin is susceptible, to detect inducible clindamycin resistance.

Worked example

A urine K. pneumoniae has a meropenem MIC of 4 µg/mL (resistant). The mCIM is positive and the eCIM (with cloxacillin) is negative. Reasoning: growth inhibition by cloxacillin means the enzyme is a serine carbapenemase (KPC-type), not a metallo-beta-lactamase. Report carbapenems as resistant, do not downgrade to susceptible, and add an infection-prevention comment.

Traps

Reporting ampicillin for Klebsiella; missing the D-test; using cefoxitin as the mecA surrogate only for S. aureus; calling a mixed culture a pathogen without quantifying; and reading a carbapenem disk without confirming with mCIM/eCIM when the organism is an Enterobacterale.