A technical microbiology textbook chapter on Extended-Spectrum Beta-Lactamases (ESBL), covering their biochemistry, classification, resistance mechanisms, and detection methods. Written for medical students, laboratory professionals, and researchers in antimicrobial resistance.
This is a technical microbiology textbook chapter focused on Extended-Spectrum Beta-Lactamases (ESBL), enzymes produced by Gram-negative Enterobacteriaceae that inactivate penicillins, third- and fourth-generation cephalosporins, and aztreonam. The chapter is designed for medical students, laboratory professionals, and researchers studying antimicrobial resistance. It begins with a quick summary defining ESBLs and their key characteristics, including inhibition by beta-lactamase inhibitors and…
The background section explains that ESBL-producing Enterobacteriaceae are among the most challenging pathogens identified by the World Health Organization. It describes how misuse of antibiotics drives resistance evolution through mutations or horizontal gene transfer. The text notes that ESBL-E are commensal bacteria in humans and animals, posing threats to food safety and food security through environmental transmission routes.
The chapter details ESBL producers, identifying E. coli as the most common host followed by Klebsiella pneumoniae, with the ST131 clone being dominant. It covers the main ESBL families: TEM (first discovered in Greece in the 1960s), SHV (common in Klebsiella), and CTX-M (now predominant globally). Specific bacterial species and serovars are listed, including Salmonella serovars linked to foodborne gastroenteritis.
Classification systems are explained in depth: the Ambler system (Classes A, B, C, D) and the Bush-Jacoby-Medeiros system (Groups 1-3). Modern grouping divides ESBLs into ESBLA (Class A, including TEM, SHV, CTX-M), ESBLM (miscellaneous, including AmpC and OXA types), and ESBLCARBA (carbapenem-degrading). The evolution of TEM-1 and SHV-1 from narrow-spectrum to extended-spectrum enzymes through amino acid mutations is described, along with CTX-M's origin from Kluyvera bacteria.
The resistance mechanism section explains how Gram-negative bacteria release beta-lactamase enzymes into the periplasm to destroy antibiotics before they reach their target. It covers gene locations on chromosomes or mobile genetic elements (plasmids, transposons, integrons) and horizontal gene transfer via transformation, transduction, and conjugation. Additional resistance tools include reduced membrane permeability and efflux pumps.
Diagnostic methods are presented with detailed protocols: the Combination Disk Test (CDT) requiring a zone diameter increase of 5 mm or more with clavulanic acid, the Double-Disk Synergy Test (DDST) showing augmented zones or keyhole patterns, and the gradient test method requiring an 8-fold MIC reduction. The chapter concludes with a key terms glossary defining resistance genes, horizontal transmission, superbugs, E. coli ST131, transposons, insertion sequences, nosocomial pathogens, and…