Members of the family Enterobacteriaceae were cultured from 52.5% of 141 milk substitute infant formulas which were obtained in 35 countries. The concentration did not exceed a level of 1 CFU/g in any product. The species which were isolated most frequently were Enterobacter agglomerans, Enterobacter cloacae, Enterobacter sakazakii, and Klebsiella pneumoniae. If infections due to these organisms occur, it can be useful to include a check of the hygienic precautions which are taken during the preparation and storage of the formula. Milk powders without members of the Enterobacteriaceae might offer extra protection to the newborn if some multiplication does occur in the formula.

The aim of this study was to investigate the in vitro beta-lactam-induced response of clinical Gram-negative bacteria with the potential for inducible beta-lactamase production, in order to be able to discriminate diagnostically between inducible and constitutive beta-lactamase production. A total of 242 clinical Gram-negative isolates of species with inducible chromosomal beta-lactamase were subjected to a disc diffusion test involving agar dilution of the inducers. Cefoxitin (FOX) and imipenem (IPM) were used as inducers and the antibiotics ceftazidime, cefuroxim, cefazolin, amoxycillin and piperacillin as indicators. beta-lactamase induction was observed at concentrations as low as 0.06 mg/l FOX or 0.008 mg/l IPM. In our test, 2 types of antibiotic phenomenon often interfered with inductive effects. Firstly a minor antibiotic effect was seen as an increase in inhibition zones at increasing inducer concentration and, secondly, absence of growth was caused by too high antibiotic activity of the inducer. The induced decrease in zone diameters varied strongly (up to 22 mm). Expressions of resistance were combined in inducibility profiles. Compilation of these profiles allowed an explanation to be proposed for the multi(-beta-lactam)-resistance, i.e. that most isolates combine inducible beta-lactamase synthesis with one or more resistance-potentiating factors. Only a few isolates demonstrated non-inducible resistance, which was probably due to mutation-mediated de-repression of beta-lactamase synthesis. The test presented here may be well-suited to studying inducibility of beta-lactamase production in the diagnostic laboratory.