## Extended-Spectrum β-Lactamase (ESBL) Overview **Key Point:** CTX-M enzymes are the predominant ESBL type globally, accounting for >80% of ESBL-producing Enterobacteriaceae, particularly in India and other developing nations. ### Classification of β-Lactamases and ESBL Production | Enzyme Type | Substrate Spectrum | Inhibitor Profile | Clinical Significance | |---|---|---|---| | TEM-1, SHV-1 | Penicillins, 1st-gen cephalosporins | Inhibited by clavulanic acid | Narrow-spectrum; NOT ESBL | | CTX-M | 3rd-gen cephalosporins, aztreonam | Inhibited by clavulanic acid | **Most common ESBL globally** | | AmpC | 3rd-gen cephalosporins, cephamycins | NOT inhibited by clavulanic acid | Plasmid or chromosomal; not true ESBL | | MBLs (IMP, VIM, NDM) | Carbapenems, β-lactams | NOT inhibited by clavulanic acid | Carbapenem-resistant; ultra-resistant | **High-Yield:** CTX-M enzymes are serine β-lactamases that preferentially hydrolyze cefotaxime and other 3rd-generation cephalosporins. They are inhibited by clavulanic acid, making them true ESBLs. ### Why Other Options Are Incorrect - **TEM-1 and SHV-1:** These are narrow-spectrum enzymes that do NOT confer resistance to 3rd-generation cephalosporins and are NOT classified as ESBLs. - **AmpC β-lactamases:** While they do hydrolyze 3rd-gen cephalosporins, they are NOT inhibited by clavulanic acid and are technically NOT ESBLs (they are cephalosporinases). - **Metallo-β-lactamases:** These confer carbapenem resistance, not ESBL phenotype. **Clinical Pearl:** ESBL detection relies on the **clavulanic acid inhibition test** — resistance to 3rd-gen cephalosporins that is reversed by clavulanic acid is diagnostic of ESBL. [cite:Koneman's Textbook of Diagnostic Microbiology Ch 8] 
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