In vitro antimicrobial susceptibility of viridans streptococci isolated from blood cultures

The susceptibility of 211 viridans streptococci isolated from blood cultures to eight antimicrobial agents was determined. All the isolates were susceptible to cefotaxime, ceftriaxone, imipenem and vancomycin. Thirty eight percent of the isolates were resistant to penicillin (MICs greater than or equal to 0.25 micrograms/ml). Tetracycline resistance was found in 41% of the isolates and in 7% of these strains tetracycline resistance was combined with erythromycin resistance. Five Streptococcus mitis isolates exhibited increased (MIC 64 micrograms/ml and 128 micrograms/ml) or high-level (MIC greater than or equal to 500 micrograms/ml) resistance to gentamicin, kanamycin and tobramycin. Four of these isolates were also resistant to penicillin (MICs 16-32 micrograms/m). In vitro synergy was not demonstrated for combinations of penicillin and gentamicin against three Streptococcus mitis isolates with gentamicin MICs of 1000, 128 and 64 micrograms/ml. Results of this study indicate the importance of monitoring antibiotic resistance trends in viridans streptococci particularly with respect to penicillin and aminoglycoside resistance.