## Extended-Spectrum β-Lactamase (ESBL) Classification **Key Point:** CTX-M enzymes are the predominant ESBL type globally and in India, accounting for >80% of ESBL-producing Enterobacteriaceae isolates. ### ESBL Types and Epidemiology | ESBL Type | Mechanism | Global Prevalence | Clinical Significance | |-----------|-----------|-------------------|----------------------| | CTX-M | Serine β-lactamase; inhibited by clavulanic acid | >80% | Most common; rapidly spreading | | TEM/SHV variants | Point mutations of TEM-1/SHV-1 | ~10–15% | Earlier; declining prevalence | | OXA variants | Oxacillinase-type; variable clavulanic acid inhibition | <5% | Less common in Enterobacteriaceae | **High-Yield:** CTX-M enzymes evolved from chromosomal β-lactamases of *Kluyvera* species and are now plasmid-encoded in *E. coli*, *Klebsiella*, and other Gram-negatives. They are particularly active against cephalosporins (especially cefotaxime, hence the name). ### Why TEM-1 and SHV-1 Are NOT the Answer **Clinical Pearl:** TEM-1 and SHV-1 are **broad-spectrum** (not extended-spectrum) β-lactamases. Their point-mutated derivatives (TEM-2, SHV-2, etc.) can confer ESBL phenotype, but the parent enzymes themselves do not. CTX-M has become the dominant ESBL globally due to its superior fitness on plasmids and higher hydrolytic activity against 3rd-generation cephalosporins. ### Confirmatory Testing - **Phenotypic:** Double-disk synergy test (DDST) or ESBL E-test strip - **Genotypic:** PCR for *bla*~CTX-M~, *bla*~TEM~, *bla*~SHV~ genes - **Susceptibility pattern:** Resistant to ceftriaxone/cefotaxime; susceptible to carbapenems (unless co-resistance present) [cite:Koneman's Textbook of Diagnostic Microbiology Ch 6] 
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