Antimicrobial susceptibility of viridans group streptococcal blood isolates to eight antimicrobial agents

A total of 155 viridans group streptococci blood culture isolates identified by the Rapid ID32 Strep system were tested for their minimum inhibitory concentrations (MICs) to penicillin, amoxicillin, ceftriaxone, erythromycin, clindamycin, rifampicin, vancomycin and teicoplanin using the E-test. The following species were identified: S. oralis (n = 67), S. mitis (n = 66), S. sanguis (n = 7), S. salivarius (n = 5), S. parasanguis (n = 4), S. gordonii (n = 3) and S. mutans (n = 3). S. oralis and S. mitis demonstrated the highest levels of resistance to the agents tested. There were 27% of S. oralis isolates resistant to penicillin, 51% resistant to erythromycin and 6% resistant to clindamycin. For S. mitis 11% were resistant to penicillin, 40% resistant to erythromycin and 3% resistant to clindamycin. Penicillin resistant isolates (MIC greater than or equal to 2 mg/l) also demonstrated decreased susceptibility to other antimicrobial agents tested in this study. High level resistance (MIC greater than or equal to 2 mg/l) to ceftriaxone was found in 12 isolates. The isolates identified as ceftriaxone resistant comprised S. oralis (n = 7), S. mitis (n = 4) and S. parasanguis (n = 1). This study has highlighted the difference in susceptibility between different species of viridans group streptococci. These findings are of concern in the light of spread of antibiotic resistance genes from S. oralis and S. mitis to the more invasive pneumococcus.