Detection of β-lactamase-mediated resistance

Abstract

β-Lactams are the most widely used antibiotics, and β-lactamases are the greatest source of resistance to them. An understanding of β-lactamase detection and identification is therefore valuable. Colorimetric, acidimetric and iodometric tests of β-lactamase production are good, rapid indicators of penicillin and ampicillin resistance in Haemophilus, Moraxella and Neisseria spp. These methods can also be applied to Gram-negative aerobic bacilli but are less useful, since the usual question is not whether a β-lactamase is produced by these organisms, but which β-lactamase? Accurate identification of the β-lactamases of Enterobacteriaceae demands gene or protein sequencing, but the broad type of enzyme produced by an isolate can often be inferred from antibiotic susceptibility data. Resistance to ceftazidime or cefpodoxime implies extended-spectrum β-lactamase (ESBL) production in Escherichia coli and Klebsiella spp., especially if susceptibility to cefoxitin is retained. ESBL production can be confirmed with double disc tests or with various commercial kits. Derepression of AmpC β-lactamases in Enterobacter spp. and Citrobacter freundii is another important mechanism and can be inferred from cross-resistance to β-lactamase inhibitor combinations and to all cephalosporins except fourth-generation agents. Antagonism between cefoxitin and cefotaxime can be used to infer the presence of inducible AmpC enzymes in these species, indicating the risk of segregation of derepressed mutants, but in general this risk is better predicted from accurate speciation.