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Four different P-lactamases have been found in several strains of Xanthomonas maltophilia isolated from blood cultures during 1984 to 1991 at the Edinburgh Royal Infirmary. One was a metallo-o-lactamase with predominantly penicillinase activity and an isoelectric point of 6.8. Its molecular size as determined by gel filtration was 96 kDa but was only 26 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), suggesting a tetramer of four equal subunits. The enzyme hydrolyzed all classes of P-lactams except the monobactam aztreonam. This enzyme was not inhibited by potassium clavulanate or BRL 42715 but was inhibited byp-chloromercuribenzoate, mercuric chloride, and EDTA. The P-lactamase was unstable in 50 mM sodium phosphate buffer (pH 8.0) but stable in 50 mM Tris HCI (pH 8.0). The other j8-lactamases focused as a series of different isoelectric points, ranging from pl 5.2 to 6.6. Together, these enzymes exhibited a broad spectrum of activity, hydrolyzing most classes of j-lactams but not imipenem or aztreonam. Their molecular size was 48 kDa by Sephadex gel filtration and 24 kDa by SDS-PAGE, indicating that they were enzymes consisting of two equal subunits. They were inhibited by p-chloromercuribenzoate, mercuric chloride, potassium clavulanate, and BRL 42715 but not EDTA. This study demonstrated that X. maltophilia produces more than just the Li and L2 P-lactamases.Xanthomonas maltophilia is the only member of the genus Xanthomonas that is pathogenic to humans. This species rests uneasily within this genus, however, and a recent report has proposed that it be placed in a new genus, which includes a single species, Stenotrophomonas maltophilia (24). They are glucose-nonfermenting, gram-negative bacilli and are increasingly recognized as an important opportunistic pathogen, often affecting patients with lowered defense mechanisms (12,21,22,29). The species is commonly resistant to a wide range of ,-lactams as well as other classes of antimicrobial agents (11,15,22). This resistance has been attributed to the interplay between outer membrane impermeability (20) and the production of two potent ,B-lactamases. Li, a metallo-o-lactamase (31), and L2, described as an unusual cephalosporinase (30), hydrolyze virtually the entire spectrum of ,B-lactams. It has been assumed that all strains produced the Li and L2 enzymes.

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MICs of penicillin, methicillin, clindamycin, erythromycin, sodium fusidate and gentamicin were determined by an agar dilution method for 300 current isolates of Staphylococcus aureus and 100 of S. epidermidis, collected from four centres, and 38 stock strains of methicillin-resistant S. aureus (MRSA). All but one of the 300 current isolates of S. aureus were sensitive to clindamycin (MIC less than 0.5 mg/l), with an MIC90 of 0.12 mg/l. Of a total of 39 MRSA strains, 11 (28.2%) were resistant to clindamycin (MIC greater than 32 mg/l); all of these strains were also resistant to erythromycin. Ten of the 100 strains of S. epidermidis were resistant to clindamycin; they came from a reasonably equal geographical distribution and were also resistant to erythromycin. The results suggest that clindamycin might still be useful as a second-line agent for infections caused by S. aureus and S. epidermidis, although its activity against MRSA was limited to approximately two-thirds of the MRSA strains tested in this study.

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