Epidemiology of Rob β-Lactamase Among Ampicillin-Resistant Haemophilus influenzae Isolates in the United States

: We surveyed 161 clinical isolates of ampicillin-resistant, beta-lactamase-producing isolates of Haemophilus influenzae obtained between 1975 and 1985 to determine whether they produced TEM-1 or Rob beta-lactamase. Plasmid DNA was obtained from a Rob-producing isolate, F990, and a plasmid (pBR322) known to encode TEM-1. Both plasmids were labeled with 32P and hybridized to whole cell DNA obtained from the clinical isolates. All 161 isolates hybridized with one of the plasmid probes and could be classified as TEM-1- or Rob-producing isolates. Analysis of the distinctive pH profiles of the two beta-lactamases was used to confirm the findings of the DNA hybridization assay. Overall, 13 (8%) isolates obtained from patients in California, North Carolina, Tennessee, Missouri, Louisiana, and Mississippi produced the Rob beta-lactamase. The remaining isolates elaborated the TEM-1 enzyme. We conclude that ampicillin resistance in H. influenzae may be mediated by the production of Rob beta-lactamase and that the occurrence of this enzyme is not limited to the two isolates described to date.