Enterococcus faecalis BM4405 was resistant to low levels of vancomycin (MIC, 16 μg/ml) and was susceptible to teicoplanin (MIC, 0.5 μg/ml). No PCR product was obtained when the total DNA of this clinical isolate was used as a template with primers specific for glycopeptide resistance genes vanA,vanB, vanC, and vanD. However, a 604-bp PCR fragment was obtained when V1 and V2 degenerate primers were used and total DNA was digested with HindIII as a template. The product was cloned and sequenced. The deduced amino acid sequence had greater identity (55%) with VanC than with VanA (45%), VanB (43%), or VanD (44%). This was consistent with the fact that BM4405 synthesized peptidoglycan precursors that terminated ind-serine residues. After induction with vancomycin, weakd,d-dipeptidase and penicillin-insensitived,d-carboxypeptidase activities were detected in cytoplasmic extracts of BM4405, whereas a serine racemase activity was found in the membrane preparation. This new type of acquired glycopeptide resistance was named VanE.

Treatment with a combination of erythromycin and rifampin has considerably improved survival rates of foals and immunocompromised patients suffering from severe pneumonia caused by Rhodococcus equi. Frequently, because of monotherapy, emergence of rifampin-resistant strains has been responsible for treatment failure. Using consensus oligonucleotides, we have amplified and sequenced the rifampin resistance (Rif r )-determining regions of 12 rifampin-resistant R. equi strains isolated from three foals and of mutants selected in vitro from R. equi ATCC 3701, a rifampin-susceptible strain. The deduced amino acid sequences compared to those of four rifampin-susceptible R. equi strains showed several types of mutations. In 3 of the 10 strains isolated from one foal, His526Asn (Escherichia coli numbering) and Asp516Val mutations were associated with low-level resistance (rifampin MIC, 2 to 8 g/ml), whereas His526Asp conferred high-level resistance (rifampin MIC, 128 g/ml) in the 7 remaining strains. In strains from the two other foals, His526Asp and Ser531Leu mutations were found to be associated with high-level and low-level resistance, respectively. The in vitro mutants, highly resistant to rifampin, harbored His526Tyr and His526Arg substitutions. As described in other bacterial genera, His526, Ser531, and Asp516 are critical residues for rifampin resistance in R. equi, and the resistance levels are dependent on both the location and the nature of the substitution.

Linezolid belongs to a new class of antimicrobials, the oxazolidinones, that act by inhibiting protein synthesis. To detect cross-resistance with other inhibitors of protein synthesis (chloramphenicol, macrolides, lincosamides, streptogramins, aminoglycosides and tetracyclines), the in vitro activity of linezolid was determined against isolates harbouring known genes conferring resistance to these antimicrobials. Neither the presence of modifying enzymes (LinA, LinA', LinB, Vgb, Vat, SatA, ANT(4') (4")-I, AAC(6')-APH(2"), APHA-3 and Cat), nor the presence of an efflux mechanism (MsrA, MefE, MefA, MreA, Vga, TetK and TeL), nor the modification or protection of antimicrobial target (because of ribosomal methylases or TetM and TetO) affected linezolid activity as demonstrated by similar in vitro activity against resistant isolates and sensitive control isolates.

Twenty-three strains of Staphylococcus aureus with borderline resistance to oxacillin were studied. These strains were not detected by the cefoxitin test, tests for penicillin-binding protein 2a (PBP2a), mecA , and mecA LGA251 were negative, and the strains were genetically unrelated. To detect all strains resistant to oxacillin, laboratories should routinely test for both cefoxitin and oxacillin.