Penicillin-aminoglycoside synergy and post-antibiotic effect for enterococci

The post-antibiotic effect (PAE) on seven strains of enterococci was studied using a novel ATP method and conventional viable counting. There was a good correlation between the results of the two techniques. In strains exhibiting low-level resistance to penicillin and streptomycin, PAEs of 1 to 2.4 h (mean 1.8 h) resulted from exposure to penicillin; shorter PAEs were induced by streptomycin (0.2-0.3 h; mean 0.2 h). Addition of streptomycin to pencillin increased the duration of penicillin-induced PAEs by two-fold to three-fold. Enterococci exhibiting high-level streptomycin resistance but low-level penicillin resistance did not exhibit a PAE with streptomycin but exhibited a short PAE with gentamicin (0.3-0.6 h). For these strains, the addition of gentamicin, but not streptomycin, to penicillin increased the penicillin-induced PAEs. Penicillin alone or in combination with streptomycin or gentamicin did not induce PAEs for a single strain of Enterococcus faecium which exhibited high-level resistance to both penicillin and streptomycin. During penicillin-induced PAEs, extracellular ATP was detectable and only during this time period were enterococci susceptible to the action of gentamicin. The addition of aminoglycosides to penicillin not only extended the PAE for enterococci but also the periods when organisms leaked ATP and were susceptible to growth inhibition by gentamicin.