12.3 Antibiotic Classes, Intrinsic Resistance, and Genetic Markers
Key Takeaways
- Map class to target: beta-lactams bind PBPs; glycopeptides bind D-Ala-D-Ala; aminoglycosides and tetracyclines hit 30S; macrolides/lincosamides hit 50S; fluoroquinolones hit DNA gyrase/topoisomerase IV; TMP-SMX blocks folate; metronidazole damages DNA in anaerobes.
- mecA encodes PBP2a and equals oxacillin/cefoxitin resistance; vanA rebuilds D-Ala-D-Lac and equals high-level vancomycin resistance; blaKPC is a serine carbapenemase (CRE).
- Intrinsic patterns to reject as ID/method errors: Klebsiella ampicillin; Proteus nitrofurantoin and tetracycline; P. aeruginosa many non-antipseudomonal agents; Stenotrophomonas carbapenems (L1 metallo); Enterococcus cephalosporins and aminoglycoside monotherapy; anaerobes aminoglycosides.
- Genotype must match the report: mecA-positive staphylococci are oxacillin/cefoxitin-resistant even if a misleading oxacillin disk looks large.
Once you can generate an MIC or a zone, the exam asks whether the result is biologically plausible. Official outline II.K pairs major antibiotic classes and their targets with intrinsic resistance patterns and with the genetic markers mecA, vanA, and blaKPC. A marker is not trivia; it rewrites the report. Cascade lists of which drugs to report by body site remain SM-only and are not M-tested.
Mechanisms of action: one target per class
| Class | Primary target | High-yield consequence |
|---|---|---|
| Beta-lactams (penicillins, cephalosporins, carbapenems, monobactams) | Penicillin-binding proteins (PBPs); block peptidoglycan transpeptidation | Cell-wall lysis in growing cells; fails if PBP is altered (PBP2a, pneumococcal PBPs) or an enzyme opens the ring |
| Glycopeptides (vancomycin) | Bind D-Ala-D-Ala termini and sterically block cross-linking | Too large to pass the Gram-negative outer membrane; defeated when ligase makes D-Ala-D-Lac (vanA) |
| Aminoglycosides | 30S ribosomal subunit; mRNA misreading | Need oxygen-dependent uptake — intrinsically inactive vs anaerobes; enterococcal synergy only if high-level resistance is absent |
| Macrolides and lincosamides | 50S ribosomal subunit | erm methylation or efflux; the D-test is this biochemistry on a plate |
| Fluoroquinolones | DNA gyrase (GyrA) and topoisomerase IV (ParC) | Stepwise QRDR mutations; not a cell-wall class |
| Tetracyclines | 30S; block aminoacyl-tRNA docking | Intrinsic resistance in Proteus; efflux common |
| Trimethoprim–sulfamethoxazole | Sequential folate synthesis (DHPS then DHFR) | Antagonized by thymidine in agar (method trap); enterococci can use exogenous folate in vivo |
| Metronidazole | Reduced to DNA-damaging radicals only in a low-redox (anaerobic) cytoplasm | Active vs many Gram-negative anaerobes; not 30S “aminoglycoside-like” action and not useful for facultative aerobes |
Beta-lactams are PBP drugs. Carbapenems are still beta-lactams; KPC is a beta-lactamase, not a ribosomal enzyme. Vancomycin is not a beta-lactam and does not bind PBPs — it ties up the stem peptide. That distinction is how you avoid answering “mecA makes vancomycin R.”
Genetic markers: mecA, vanA, blaKPC
mecA lives on a staphylococcal cassette chromosome (SCCmec) and encodes PBP2a. Beta-lactams that rely on native PBPs no longer bind efficiently, so the isolate is oxacillin-resistant and, by the CLSI surrogate, cefoxitin-resistant. Phenotype = MRSA (or methicillin-resistant coagulase-negative staphylococci if the species is not S. aureus). Correlate: mecA detected → report oxacillin/cefoxitin resistant; do not leave oxacillin susceptible because a mis-set oxacillin disk looked large. mecA does not confer vancomycin resistance.
vanA encodes a ligase that rebuilds peptidoglycan precursors with D-Ala-D-Lac, which vancomycin binds about a thousand-fold less well. Phenotype = high-level vancomycin resistance, typically with teicoplanin resistance, in E. faecium/E. faecalis (VRE). vanA is often plasmid-borne and transferable, unlike intrinsic vanC. Correlate: vanA → vancomycin resistant; a vanA PCR on an E. faecium blood isolate is incompatible with a vancomycin-susceptible report.
blaKPC encodes Klebsiella pneumoniae carbapenemase, a class A serine enzyme that hydrolyzes carbapenems and other beta-lactams. Phenotype = meropenem/imipenem/ertapenem resistance or an elevated MIC in Enterobacterales (CRE when definitions are met). mCIM-positive and eCIM-negative fits a serine enzyme such as KPC; a validated molecular panel names blaKPC. EDTA does not rescue KPC. Correlate: blaKPC → do not report carbapenems susceptible.
Intrinsic resistance patterns (organism biology, not body-site cascade)
These chromosomal, expected resistances should make you reject a “fully susceptible” panel as a method or identification error.
Klebsiella pneumoniae / K. oxytoca: intrinsic ampicillin (and amoxicillin) resistance from chromosomal penicillinase (SHV-type in K. pneumoniae). An ampicillin-susceptible Klebsiella is a red flag for misidentification or a failed well. That does not automatically make every cephalosporin intrinsically resistant.
Proteus spp. (especially P. mirabilis): intrinsic resistance to nitrofurantoin and tetracycline (and polymyxins/colistin). Do not release nitrofurantoin as a urinary option for Proteus because a well was empty. Swarming is not a susceptibility result.
Pseudomonas aeruginosa: a long intrinsic list driven by impermeability, efflux, and chromosomal AmpC — ampicillin, cefazolin, ceftriaxone, ertapenem, tetracycline, tigecycline, TMP-SMX, and other agents that never were antipseudomonal. The drugs you actually test are antipseudomonal (piperacillin, ceftazidime, cefepime, meropenem, aminoglycosides, ciprofloxacin). A Pseudomonas reported ampicillin-S is wrong.
Stenotrophomonas maltophilia: intrinsic carbapenem resistance from L1 metallo-beta-lactamase (zinc enzyme), plus an L2 cephalosporinase. Meropenem “S” on a Stenotrophomonas panel is a method/ID error. TMP-SMX is the historical drug of choice; that is a therapeutic association, not an SM-only site cascade.
Enterococcus spp.: intrinsic resistance to cephalosporins (low-affinity PBPs) and to clinically useful aminoglycoside monotherapy because of poor uptake. High-level aminoglycoside resistance testing (gentamicin and streptomycin) is done only to predict synergy with a cell-wall agent, not because gentamicin is a stand-alone enterococcal drug. TMP-SMX may look active in vitro but fails in vivo because enterococci use exogenous folate. E. faecium is often ampicillin-resistant; E. faecalis is usually ampicillin-susceptible — use that split as an identity check.
Anaerobes: aminoglycosides require oxygen-dependent electron-transport uptake, so Bacteroides, Clostridium, and related anaerobes are intrinsically aminoglycoside-resistant. Metronidazole, not gentamicin, is the anaerobic DNA-damage agent. Some Gram-positive anaerobes (Actinomyces, Cutibacterium) are metronidazole-resistant; that is species biology, not a reason to add gentamicin to the anaerobic panel.
Genotype–phenotype correlation as a reporting check
If molecular and phenotypic AST disagree, troubleshoot before release:
- mecA or PBP2a positive, cefoxitin screen negative → repeat both; do not report oxacillin-S.
- vanA positive, vancomycin MIC in the susceptible range → repeat identification and MIC; mixed culture or PCR contamination is more likely than new biology.
- blaKPC positive, meropenem clearly wild-type susceptible → confirm mix and the carbapenemase assay; a true KPC producer should not look like wild-type E. coli.
Phenotype can also exist without that named gene: BORSA without mecA, vanB instead of vanA, NDM instead of KPC. The outline names mecA, vanA, and blaKPC because they are the highest-yield correlates, not because they are the only alleles on earth.
Detection of mecA in Staphylococcus aureus correlates with which phenotypic report?
Which organism has intrinsic carbapenem resistance due to an L1 metallo-beta-lactamase, so a meropenem-susceptible result should be treated as an identification or method error?
Metronidazole is clinically useful against many anaerobes because of which mechanism?