Detection of β-lactamase-mediated resistance

ß-Lactams are the most widely used antibiotics, and ß-lactamases are the greatest source of resistance to them. Colorimetric tests are good, rapid, indicators of ß-lactamase production and its contingent resistance in Haemophilus, Moraxella and Neisseria spp. They can also be applied to Gram-negative bacilli but are less useful, since the usual question is not whether a ß-lactamase is produced but which ß-lactamase? Accurate identification of the ß-lactamases of Enterobacteriaceae demands gene or protein sequencing, but the broad type of enzyme can often be inferred from antibiotic susceptibility profile. Resistance to indicator third generation cephalosporins implies extended-spectrum ß-lactamase (ESBL) production in Escherichia coli and Klebsielia spp., especially if susceptibility to cefoxitin is retained. With the emergence of CTX-M ESBLs, it is necessary to use cefotaxime as well as ceftazidime as an indicator, or to use cefpodoxime. ESBL production can be confirmed with double disc synergy tests or with various commercial kits. Derepression of AmpC ß-lactamases in Enterobacter spp. and Citrobacter freundii is also important and can be inferred from cross-resistance encompassing ß-lactamase inhibitor combinations as well as all cephalosporins except fourth-generation agents. Antagonism between cefoxitin and cefotaxime implies the presence of inducible AmpC enzymes in e.g. Enterobacter spp., indicating the risk of segregation of derepressed mutants. In general, though, this risk is better predicted from accurate species identification. Metallo-carbapenemases are slowly emerging and present a new detection challenge, firstly because carbapenem resistance does not always accompany enzyme production and secondly because EDTA, which is used as an inhibitor in some diagnostic tests, may also exert other effects on the cell.