Further studies on post-antibiotic effect and synergism in two strains of Streptococcus faecalis

The differences in the synergistic potential for either killing effect or increase in duration of post-antibiotic effect (PAE) seen with various beta-lactam-aminoglycoside combinations in two strains of Streptococcus faecalis were further studied to examine this relationship in more detail. It was seen that a strain which produced a synergistic increase in recovery period (PAE ≥ 0.5 h), comparing the beta-lactam antibiotics alone and when combined with streptomycin, showed increased sensitivity (≥ 1 log10 reduction in viable counts) to a subinhibitory concentration of streptomycin added to a culture recovering from the PAE; this was not demonstrated with a strain showing no increase in PAE when briefly exposed to a combination therapy, although its corresponding killing curve was clearly synergistic. Moreover, it was seen that the differences in the synergistic prolongation of recovery period when combining various aminoglycosides at 1/4 × MIC with ampicillin were consistent with the different duration of PAE with the particular aminoglycoside alone at the MIC.