POSTANTIBIOTIC EFFECTS AND POSTANTIBIOTIC SUB-MIC EFFECTS OF BENZYLPENICILLIN ON VIRIDANS STREPTOCOCCI ISOLATED FROM PATIENTS WITH INFECTIVE ENDOCARDITIS

We investigated the postantibiotic effects (PAEs) and the postantibiot ic sub-MIC effects of benzylpenicillin on three strains of viridans st reptococci isolated from infective endocarditis patients. The PAEs of benzylpenicillin on penicillin tolerant Streptococcus sanguis TW-70 (0 .4-3.9h), penicillin tolerant S. sanguis TW-80 (0.3-6.3 h) and nontole rant Streptococcus oralis TW-186 (0.5-3.1 h) were dependent on exposur e time. The PAEs were not concentration dependent for S. sanguis TW-70 and S. sanguis TW-80 above the MIC, and for S. oralis TW-186 above 16 x MIC. The antimicrobial effects of benzylpenicillin at sub-MIC conce ntrations were examined in bacteria pretreated with benzylpenicillin ( 8 x MIC) for 2 h and compared with untreated bacteria. At the sub-MICs tested, the regrowth of pretreated S. oralis TW-186 cells was more pr olonged than that of untreated cells and bactericidal action was seen only in pretreated cells. These effects (so-called 'postantibiotic sub -MIC effects') were not observed in penicillin tolerant S. sanguis TW- 70. The presence of the postantibiotic sub-MIC effect may be an import ant factor in determining the dosing regimen for infective endocarditi s.