Bactericidal effect and regrowth of Streptococcus faecalis exposed to amoxicillin following beta-lactamase

Streptococcus faecalis usually requires high concentrations of penicillin or ampicillin to achieve killing (i.e. a high MBC/MIC ratio). However, most strains show the Eagle or paradoxical effect. We subjected 12 strains of S. faecalis to 0.1, 1, 10 and 100 micrograms/ml of amoxicillin. Turbidometry studies have shown that 3 h after the inactivation of amoxicillin by penicillinase, there was a longer effect for 1 micrograms/ml following beta-lactamase (12 h 31 min +/- 2 h 09 min) than for 10 micrograms/ml (7 h 0 min +/- 1 h 12 min) or 100 micrograms/ml (5 h 22 min +/- 0 h 52 min). After 3 h, the reduction of CFU/ml (inoculum 10(6) CFU/ml) was -1.8 +/- 0.6 for 1 micrograms/ml, -0.56 +/- 0.56 for 10 micrograms/ml and -0.21 +/- 0.20 for 100 micrograms/ml. The more rapid killing at 3 h was not the only reason for the longer effect following beta-lactamase observed with 1 micrograms/ml. Indeed, the growth curve obtained with an inoculum of 10(3) CFU/ml was 2 h delayed from the control curve (10(6) CFU/ml). In conclusion, a paradoxical effect (killing curves and effect following beta-lactamase) was observed for all S. faecalis strains included in this series.