Evaluation of a disk approximation test of inducible beta-lactamases in Enterobacteriacae and Pseudomonas aeruginosa.

A modified disk approximation test of inducible beta-lactamases in Enterobacteriacae and Pseudomonas aeruginosa was evaluated. The amount of inducer was adapted to produce the smallest possible zone of inhibition and the distance between the centre of the disks was standardized. Among several beta-lactam antibiotic disks tested, imipenem (0.06 microgram per disk) or cefoxitin (10 micrograms per disk) placed at a distance of 14-16 mm from a cefotaxime disk (30 micrograms) most efficiently revealed inducible beta-lactamases. A positive induction test (primarily expected with Enterobacter spp, Citrobacter freundii, Serratia spp, Pseudomonas aeruginosa and indole positive Proteus spp) may serve as a warning of the risk of selecting mutants with beta-lactamase mediated cross-resistance during systemic treatment with ureidopenicillins, later generations of cephalosporins or monobactams. Such resistance was exemplified by characterization of some laboratory mutants with hyperproduction of beta-lactamases. However, evaluation of the specificity and sensitivity of the disk approximation test (and other indirect induction tests) still remains to be done.