Isoelectric Focusing of Beta-Lactamases in Mycobacterium fortuitum

The uninduced culture supernatants and cell extracts from 58 strains of the 3 biovariants (biovar) of Mycobacterium fortuitum were all positive for β-lactamase with the chromogenic cephalosporin substrate. By analytical isoelectric focusing (IEF), 29 of 30 strains of biovar fortuitum exhibited an identical β-lactamase pattern with 1 major band. In contrast, the β-lactamases of biovar peregrinum and the unnamed third biovar were heterogeneous, with multiple bands and a variety of patterns. The pH range of isoelectric points for the β-lactamases was relatively narrow, however, with most bands appearing between pH 4.3 and 5.2. Although additional genetic studies are required, these enzymes appear to be chromosomal, as they are present in all strains including some without detectable plasmids. Repeat isolates from the same patient obtained up to six months apart always had the same β-lactamase pattern by IEF. Of the third biovar complex, 30% are cefoxitin resistant with minimal inhibitory concentrations > 32 ...