Penicillin-induced effects on streptomycin uptake and early bactericidal activity differ in viridans group and enterococcal streptococci

Abstract

In vitro studies with penicillin and [3lHlstreptomycin in four strains of streptococci (S. faecalis, S. sanguis, and S. mitis) were performed by simultaneously measuring the rates of bacterial killing and uptake of streptomycin. In S. faecalis, penicillin stimulated streptomycin uptake, as has been shown by Moellering and Weinberg (R. C. Moellering, Jr., and A. N. Weinberg, J. Clin. Invest. 50:2580-2584). Moreover, the antibiotic combination was associated with an enhanced bactericidal rate which temporally correlated with ,-lactam-induced aminoglycoside uptake. In contrast, in viridans group streptococci (S. sanguis and S. mitis) penicillin had no effect on streptomycin uptake and a minimal effect on bactericidal rate when compared with either drug alone. These data suggested that the stimulation of streptomycin uptake in streptococci by penicillin is strain or species specific and that important differences exist between enterococci and viridans group streptococci regarding the mechanisms of ,-lactam-aminoglycoside potentiation.Antibiotic synergy is perhaps best exemplified by the use of a P-lactam combined with an aminoglycoside in the therapy of enterococcal endocarditis. The enhanced antibacterial activity of the combination has been well established both in clinical trials (26) and in an experimental model of endocarditis (14). In studies on the mechanism of synergy in enterococci, Moellering and colleagues (35, 36) demonstrated that penicillin enhanced intracellular entry of otherwise sublethal concentrations of streptomycin, suggesting that the mechanism of sytergy in Streptococcusfaecalis was similar to that shown earlier in Escherichia coli by Plotz and Davis (36). However, from a mechanistic standpoint it is not clear how penicillin would enhance uptake of aminoglycosides in most gram-positive bacteria since no barrier to aminoglycoside penetration comparable to that of gramnegative bacilli has been described. Time kill studies (35,36) also demonstrated that enhancement of streptomycin entry was associated with an increase in the rate of killing during 24 h as determined by the inability of cells to form colonies on agar after overnight incubation. Twenty-four-hour time kill studies have since become a generally accepted in vitro method for examining bactericidal synergism in both grampositive and gram-negative bacteria.Numerous 24-h in vitro time kill studies have shown apparent synergy (i.e., a >2 logl0 enhanced bactericidal effect) between P-lactams and aminoglycosides in grampositive organisms (5,12,20,21,41,44). In vitro (22,24, 33) and in vivo (23, 25, 33) studies with Staphylococcus aureus and Staphylococcus epidermidis in our laboratory, however, indicated that a second mechanism by which cell wall-active antibiotics (,-lactams, vancomycin) enhance the bactericidal effect of aminoglycosides (or rifampin) is by preventing regrowth of relatively resistant subpopulations. Twentyfour-hour time kill studies often do not distinguish between the aforementioned mechanisms of drug potentiation, and ...

Early effects of beta-lactams on aminoglycoside uptake, bactericidal rates, and turbidimetrically measured growth inhibition in Pseudomonas aeruginosa

“…Both ,I-lactams increased the uptake of subinhibitory concentrations of tobramypin. This result was quantitatively associated with a 2-to 4-h time-kill potentiation and confirmed earlier studies on the mechanism of J3-lactam-aminoglycoside synergy in Escherichia coil (P. H. Plotz and B. D. Davis, Science 135:1067-1068, 1962 (17,23). This finding suggests that this mechanism of drug potentiation is strain or species specific.…”

“…It is not possible at this time to determine the relative importance of antibiotic synergism or drug potentiation because of the prevention of mutational resist- ance. Finally, these data support our belief, based on studies with gram-positive cocci (17), that standard 24-h time-kill studies may not reliably predict 3-lactam-aminoglycoside synergism. Prolonged incubation is associated with mutant regrowth (15,18), antibiotic degradation (6), and medium acidification (8).…”

“…The commonly held belief that this mechanism occurs in other E. coli strains or in different species of gram-negative bacilli has never been examined. Recent studies have demonstrated that 1-lactam-enhanced aminoglycoside uptake and bacterial killing are absent in viridans group streptococci (17) but present in both enterococci and Staphylococcus aureus (17,23). This finding suggests that this mechanism of drug potentiation is strain or species specific.…”

“…Moreover, an examination of the kinetics of increased aminoglycoside uptake and the bactericidal effect on replicating cells of P-lactams or chemicals which increase the AR. H+ demonstrates that aminoglycoside-associated killing is clearly apparent within minutes to hours (4,12,13,15,17,18,20,23; present data). These kinetic studies support the suggestion of others (6,7,11,14) that abbreviated time-kill studies may be preferable to conventional 24-h data in demonstrating synergy secondary to P-lactam-enhanced increased uptake of aminoglycosides.…”

Effect of gentamicin dosing interval on therapy of viridans streptococcal experimental endocarditis with gentamicin plus penicillin

“…Such an effect was not observed in vivo in the treatment of experimental enterococcal endocarditis (22). The absence of an in vivo PAE of the combination of a ␤-lactam plus an aminoglycoside against enterococci does not preclude the presence of a PAE against viridans streptococci because of differences in the mechanisms of action of this combination on the two microorganisms (28,29,44). The mechanism of synergism between penicillin and aminoglycosides against viridans streptococci is still controversial.…”