Synergism and mechanism of subinhibitory concentration of streptomycin on Streptococcus faecalis

The influence of various incubation conditions on synergism with cell wall-active agents combined with streptomycin was studied in a Streptococcus faecalis strain. It could be shown that both a synergistic increased killing effect and a synergistic prolongation of post-antibiotic effect (PAE) were not dependent on an active process, as they occurred both in an anaerobic atmosphere and when an electron transport inhibitor (sodium azide) was added to the media. Moreover, it was demonstrated that if a drug, alone or in combination with streptomycin, had a PAE, there was an increased susceptibility to a subinhibitory concentration of streptomycin (1/4xMIC) when added to a culture recovering from its PAE. These findings confirm the commonly held belief in a mechanistic mechanism of synergism: That cell wall-active agents modify in some way the cell-envelope, thereby enhancing aminoglycoside uptake and killing in Streptococcus faecalis.