Which option lists two phenotypic tests commonly used to confirm ESBL or AmpC production in Gram-negative rods?

Which option lists two phenotypic tests commonly used to confirm ESBL or AmpC production in Gram-negative rods?

• A. Gram stain.
• B. ESBL: clavulanic acid synergy; AmpC: cefoxitin-based screening with boronic acid inhibitor or AmpC confirmatory test.
• C. E-test only.
• D. PCR.

Answer: (B)

The statement highlights two practical phenotypic approaches labs use to confirm ESBL or AmpC production in Gram-negative rods. For ESBLs, the clavulanic acid synergy principle is key: ESBL enzymes are inhibited by clavulanic acid, so a beta-lactam disk (like cefotaxime or ceftazidime) placed near a disk containing the same drug plus clavulanate shows an enhanced inhibition zone or a clear synergy. This pattern indicates ESBL activity. For AmpC, a cefoxitin-based screen is informative because AmpC producers can resist cefoxitin, but when a boronic acid inhibitor (which inhibits AmpC enzymes) is included, it can restore activity or reveal a synergistic effect, serving as a confirmatory clue that AmpC is present. An explicit AmpC confirmatory test can also be used to demonstrate AmpC activity. These are distinctly phenotypic tests focused on enzyme behavior, rather than just identifying the organism or its genetic material. Gram staining identifies bacteria as Gram-positive or Gram-negative and gives morphology, but it doesn’t reveal resistance enzymes. An E-test alone can indicate reduced susceptibility but doesn’t differentiate ESBL from AmpC. PCR detects genes, not enzyme activity, so it’s not a phenotypic confirmation.

The statement highlights two practical phenotypic approaches labs use to confirm ESBL or AmpC production in Gram-negative rods. For ESBLs, the clavulanic acid synergy principle is key: ESBL enzymes are inhibited by clavulanic acid, so a beta-lactam disk (like cefotaxime or ceftazidime) placed near a disk containing the same drug plus clavulanate shows an enhanced inhibition zone or a clear synergy. This pattern indicates ESBL activity. For AmpC, a cefoxitin-based screen is informative because AmpC producers can resist cefoxitin, but when a boronic acid inhibitor (which inhibits AmpC enzymes) is included, it can restore activity or reveal a synergistic effect, serving as a confirmatory clue that AmpC is present. An explicit AmpC confirmatory test can also be used to demonstrate AmpC activity.

These are distinctly phenotypic tests focused on enzyme behavior, rather than just identifying the organism or its genetic material. Gram staining identifies bacteria as Gram-positive or Gram-negative and gives morphology, but it doesn’t reveal resistance enzymes. An E-test alone can indicate reduced susceptibility but doesn’t differentiate ESBL from AmpC. PCR detects genes, not enzyme activity, so it’s not a phenotypic confirmation.