Antimicrobial Susceptibility to 27 Drugs and the Molecular Mechanisms of Macrolide, Tetracycline, and Quinolone Resistance in sp.

is a catalase-negative, facultative anaerobic, Gram-positive coccus that is commensal in humans but can become opportunistic and cause severe infectious diseases, such as infective endocarditis. Few studies have tested the antimicrobial susceptibility of . We tested its antimicrobial susceptibility to 27 drugs and defined the resistant genes using PCR in 58 strains, including 52 clinical isolates and six type strains. The type strains and clinical isolates included 22 , 18 (GH) group (genetically indistinguishable from and ), 13 , three , and two . No strain was resistant to beta-lactams and vancomycin. In total, 6/22 (27.3%) strains were erythromycin- and clindamycin-resistant -positive, whereas 5/18 (27.8%) in the GH group, 6/13 (46.2%) , and 1/3 (33.3%) of the strains were erythromycin-non-susceptible - or -positive and clindamycin-susceptible. The MIC of minocycline and the ratios of -positive strains varied across the different species-: 2 µg/mL and 27.3% (6/22); GH group: 8 µg/mL and 22.2% (4/18); : 8 µg/mL and 53.8% (7/13), respectively. Levofloxacin resistance was significantly higher in (8/13 61.5%) than in (2/22 9.1%). Levofloxacin resistance was associated with a substitution at serine 83 for leucine, phenylalanine, or tyrosine in GyrA. The mechanisms of resistance to erythromycin and clindamycin differed across species. In addition, the rate of susceptibility to levofloxacin differed across sp., and the quinolone resistance mechanism was caused by mutations in GyrA alone.