SN5 SHORT NOTE 5M ★ High-yield

Describe ESBL and carbapenemase-producing organisms with their detection and treatment.

Microbiology · 2nd Professional — Para-Clinical

The Problem

ESBLs (extended-spectrum beta-lactamases) and carbapenemases are enzymes produced by gram-negative bacilli that hydrolyse successively broader ranges of beta-lactam antibiotics, and are the principal cause of untreatable gram-negative infection.

Ambler class	Enzymes	Substrates and inhibition
Class A	TEM, SHV, CTX-M (the commonest ESBL worldwide and in India); KPC (a carbapenemase)	ESBLs hydrolyse penicillins, and third- and fourth-generation cephalosporins and aztreonam — but not carbapenems or cephamycins. Inhibited by clavulanate and tazobactam, which is the basis of their detection
Class B — metallo-beta-lactamases	NDM-1 (New Delhi metallo-beta-lactamase), VIM, IMP	Hydrolyse carbapenems and almost all other beta-lactams; require zinc, so they are inhibited by EDTA but not by clavulanate or avibactam. Aztreonam is stable to them, which is therapeutically useful
Class C — AmpC	Chromosomal (Enterobacter, Citrobacter, Serratia, Pseudomonas) or plasmid-mediated	Hydrolyse cephalosporins including cephamycins; not inhibited by clavulanate. Often inducible, so the organism appears sensitive in vitro and resistance emerges during treatment
Class D	OXA-48 and relatives	Carbapenemases, common in Acinetobacter; variable inhibition

CLINICAL PEARL

An ESBL-producing organism may test sensitive to a cephalosporin in the laboratory and still fail the patient. The enzyme hydrolyses the drug slowly enough that a disc test can look acceptable, but at the higher organism density of a real infection the drug is destroyed. This is why laboratories report ESBL producers as resistant to all cephalosporins regardless of the measured zone — the in-vitro result is a poor predictor of what happens in a patient.

Detection

ESBL screening — reduced susceptibility to ceftazidime, cefotaxime or ceftriaxone on routine testing
ESBL confirmation — the double-disc synergy test or combined disc test, in which the zone around a cephalosporin is enlarged by 5 mm or more when clavulanate is added, demonstrating inhibition of the enzyme
AmpC detection — cefoxitin resistance with no clavulanate synergy
Carbapenemase detection — the modified carbapenem inactivation method (mCIM); the EDTA combined disc test identifies metallo-enzymes; and molecular detection of blaNDM, blaKPC, blaOXA-48
Report ESBL producers as resistant to all penicillins, cephalosporins and aztreonam, whatever the individual zone sizes suggest

Treatment Options

ESBL producers — carbapenems are the drugs of choice in serious infection; piperacillin-tazobactam may suffice in uncomplicated urinary infection; and for cystitis, nitrofurantoin or fosfomycin are useful oral options that spare carbapenems
Carbapenemase producers — options are few: colistin (polymyxin E), which is nephrotoxic and neurotoxic and had been abandoned before its revival; tigecycline (but not for bacteraemia or urinary infection, since serum and urinary levels are low); fosfomycin; aminoglycosides where sensitive; and newer agents such as ceftazidime-avibactam (effective against KPC and OXA-48 but not against metallo-enzymes such as NDM), ceftazidime-avibactam with aztreonam for NDM producers, meropenem-vaborbactam and cefiderocol
Combination therapy is often used, and treatment should be guided by the specific enzyme where it can be identified

Control and Applied Aspects

India has among the highest rates of carbapenem resistance in the world, driven by over-the-counter antibiotic sale, unregulated use, incomplete courses, and environmental and agricultural contamination; NDM-1 was first characterised from an isolate acquired in India
Antimicrobial stewardship — restricting carbapenems, de-escalating once sensitivities are known, shortening courses, and using narrow agents wherever possible
Infection control — hand hygiene, contact precautions, screening and cohorting of colonised patients, and environmental cleaning; these organisms spread clonally within units
Distinguish colonisation from infection; a resistant organism in a catheter urine or a wound swab from a well patient usually needs no treatment, and treating it exhausts the few remaining drugs
Use local antibiograms for empirical therapy rather than textbook spectra, since resistance patterns differ sharply between hospitals and even between wards
Schedule H1 and the National Action Plan on AMR are the regulatory response in India, restricting over-the-counter sale of key antibiotics; their effectiveness depends on enforcement and on prescriber behaviour
Colistin was abandoned for toxicity and revived out of necessity, which is a clear signal of how far options have narrowed; its use in animal feed has since been banned in India for precisely this reason

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