The action and interaction of benzylpenicillin and gentamicin on Streptococcus faecalis was studied using mainly turbidimetric methods. The minimum antibacterial concentration (MAC) of each antibiotic lay considerably below the conventionally determined minimum inhibitory concentration, and levels of the two agents exceeding the MAC were necessary in order to obtain a synergic interaction. Evidence was obtained that gentamicin interfered with bacterial lysis induced by penicillin, and this suggests that the aminoglycoside is responsible for the bactericidal activity of the combination, the role of the penicillin being solely to facilitate access of the aminoglycoside to its target site. Our findings do not, however, fully support the generally held view that the increased permeability of enterococci to aminoglycosides is due to penicillin-induced cell wall damage. 'Persisters'--cells surviving prolonged exposure to the optimum lethal concentration of penicillin--were not killed by subsequent exposure to gentamicin if the penicillin was removed but were killed if the penicillin remained present.
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