12.2 Phenotypic Detection of Resistance

Phenotypic resistance tests answer a different question from a routine MIC: is a mechanism present that would make a susceptible-looking result unsafe? Official outline II.K expects nitrocefin beta-lactamase, ESBL screening/confirmation in historical versus current interpretive context, the D-test for inducible clindamycin resistance, carbapenemase methods (mCIM/eCIM and Carba NP as concepts), the MRSA cefoxitin screen, and VRE detection. Do not convert these items into SM-only cascade reporting of which drugs to list by body site.

Nitrocefin (Cefinase) beta-lactamase

Nitrocefin is a chromogenic cephalosporin. Intact reagent is yellow; hydrolysis of the beta-lactam ring turns it red. Rub a colony onto a moistened disk or use a drop of reagent and read color within the insert window (often up to 5–15 minutes). A positive test means the isolate produces an enzyme that opens a beta-lactam ring — most classically TEM-type penicillinases.

High-yield uses:

• Haemophilus influenzae / H. parainfluenzae: nitrocefin-positive isolates are ampicillin-resistant penicillinase producers. Nitrocefin-negative ampicillin-resistant Haemophilus implies a non-enzyme mechanism (PBP changes, BLNAR) that HTM disk or MIC must catch.
• Moraxella catarrhalis: nearly all isolates are nitrocefin-positive, which is why ampicillin fails.
• Neisseria gonorrhoeae: penicillinase-producing N. gonorrhoeae (PPNG).
• Selected anaerobes and staphylococci: penicillinase that predicts penicillin failure.

Nitrocefin is not a universal resistance oracle. Extended-spectrum beta-lactamases and carbapenemases may hydrolyze nitrocefin poorly or slowly. A yellow disk does not prove the isolate lacks ESBL or KPC. Do not use nitrocefin in place of a cefoxitin MRSA screen or in place of a carbapenemase method.

ESBL: historical clavulanate confirmation versus current carbapenem-sparing interpretation

Extended-spectrum beta-lactamases (TEM, SHV, CTX-M families) hydrolyze penicillins and expanded-spectrum cephalosporins and are inhibited by clavulanate. They do not, by themselves, efficiently hydrolyze carbapenems.

Historical CLSI approach:

1. Screen Enterobacterales with cefotaxime, ceftriaxone, ceftazidime, or aztreonam using former (higher) breakpoints that could still call some ESBL producers “susceptible.”
2. Confirm with clavulanate combination tests: cefotaxime ± clavulanate and ceftazidime ± clavulanate as disks (a large zone increase, traditionally taught as ≥5 mm) or MIC wells (a ≥3 two-fold dilution / 8-fold MIC decrease with clavulanate). Those increments are confirmatory method criteria, not a year-stamped M100 therapy table.
3. If ESBL was confirmed, laboratories edited the report: penicillins, cephalosporins, and aztreonam were reported resistant even when an individual agent tested susceptible in vitro, because of documented clinical failures. That edit pushed therapy toward carbapenems.

Current CLSI interpretive philosophy (after cephalosporin and aztreonam breakpoints were lowered): report each of those drugs as tested against the current M100 MIC or zone. Routine ESBL confirmation is no longer required to rewrite every cephalosporin result for therapy. Lower breakpoints are designed so that isolates with clinically important ESBL-level hydrolysis test intermediate or resistant to the drugs that would fail. That is the carbapenem-sparing interpretation: a cefepime result that is truly susceptible at the current breakpoint may be reported as such rather than automatically forcing every beta-lactam except carbapenems to R. ESBL phenotypic or molecular detection remains valuable for infection control and epidemiology; it is no longer the mandatory editor of every cephalosporin. Know both eras — stems still ask what clavulanate confirmation used to do to the report — but apply reporting-as-tested logic unless the question is clearly historical.

AmpC (Enterobacter, Klebsiella aerogenes, Citrobacter freundii, Serratia) is not an ESBL. AmpC is poorly inhibited by clavulanate and can look like a failed ESBL confirmatory test. Cefoxitin resistance plus no clavulanate restoration of cefotaxime points to AmpC, not CTX-M.

D-test: inducible clindamycin resistance

Macrolide-lincosamide-streptogramin B (MLS_B) resistance is often encoded by erm, which methylates 23S rRNA. Expression may be constitutive (erythromycin R and clindamycin R) or inducible (erythromycin R, clindamycin S in vitro until a macrolide inducer is present). Clindamycin is a weak inducer in the well but can fail in vivo if erm is inducible.

For staphylococci (and beta-hemolytic streptococci when tables require it) that test erythromycin-resistant and clindamycin-susceptible or intermediate, place an erythromycin disk and a clindamycin disk at CLSI spacing so the zones can interact. After overnight incubation, inspect the clindamycin zone on the side facing erythromycin. Flattening into a D shape is a positive D-test (inducible clindamycin resistance, iMLS_B). Report clindamycin resistant. Do not report clindamycin susceptible if the D-test is positive.

A negative D-test with erythromycin R / clindamycin S is typical of efflux (msrA in staphylococci): clindamycin remains active and may be reported susceptible. Disks placed too far apart miss the D; disks stacked on top of each other are unreadable technique, not a new phenotype.

Carbapenemases: mCIM, eCIM, and Carba NP

Carbapenemase-producing Enterobacterales, and some Pseudomonas/Acinetobacter, need a mechanism test because meropenem MICs can sit near breakpoints.

Modified carbapenem inactivation method (mCIM): incubate a heavy suspension of the test organism with a meropenem disk, then place that disk onto a lawn of E. coli ATCC 25922. If the test organism destroyed meropenem, the indicator E. coli grows up to the disk (small or no zone) = mCIM positive. If meropenem survives, a large zone remains = mCIM negative.

EDTA-modified CIM (eCIM): run in parallel with EDTA, which chelates zinc. Metallo-beta-lactamases (NDM, VIM, IMP) need zinc; EDTA suppresses them, so the eCIM disk recovers a larger zone than mCIM. Serine carbapenemases (KPC, some OXA) are not zinc enzymes; eCIM stays inactivated like mCIM. Teach the pattern: mCIM-positive / eCIM-positive → MBL; mCIM-positive / eCIM-negative → serine enzyme such as KPC. Do not treat a single millimeter cutoff as a frozen M100 therapy table — the concept is EDTA rescue versus no rescue.

Carba NP is a rapid colorimetric hydrolysis assay: imipenem in a pH-indicator mix turns yellow when a carbapenemase dumps acid. It is a phenotypic screen, not a gene name. Some OXA enzymes give false negatives; a negative Carba NP does not bless a high meropenem MIC.

MRSA cefoxitin screen and VRE

MRSA is mecA (rarely mecC) encoding PBP2a, a transpeptidase with low beta-lactam affinity. Cefoxitin is the phenotypic surrogate because it induces mecA better than oxacillin. A cefoxitin disk or cefoxitin MIC that interprets as resistant is reported as oxacillin-resistant S. aureus. PBP2a latex or mecA PCR can confirm. Borderline oxacillin-resistant S. aureus (BORSA) without mecA is not MRSA.

VRE usually means acquired vanA or vanB in E. faecium or E. faecalis. Vancomycin agar screen (commonly 6 µg/mL BHI) or vancomycin MIC/disk per CLSI, incubated a full 24 hours, detects resistant enterococci. vanA typically confers high-level vancomycin and teicoplanin resistance (D-Ala-D-Lac). vanB is vancomycin-resistant, teicoplanin-susceptible. Motile pigmented species (E. casseliflavus, E. gallinarum) carry intrinsic vanC and low-level vancomycin resistance — they are not the outbreak vanA E. faecium clone. Identify to species before labeling an isolate “VRE” for infection control.

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