Rate of bactericidal activity for Streptococcus faecalis of a new quinolone, CI-934, compared with that of amoxicillin

Abstract

The rate of bactericidal activity of a new quinolone, CI-934, was compared with that of amoxicillin for 20 strains of Streptococcus faecalis. At 10 and 100 ,ug/ml, the bactericidal activity of CI-934 was more rapid at 6 h than that of amoxicillin. A paradoxical effect (a killing rate higher at 1 Fg/ml than at 100 ,Ig/ml at 6 h) was observed for 19 of the 20 strains with amoxicihlin and for 1 of the 20 strains with containing 30 ml of Iso-Sensitest agar medium was inoculated (before and after dilution, 1/10 and 1/100, respectively) with 0.05 ml of the culture which was distributed on the agar surface by the spiral inoculator system (Spiral System Instruments, Inc., Bethesda, Md.) (6, 10). In this system, a variable cam-activated syringe dispenses the culture from the center to the edge of the plate in a logarithmically decreasing quantity in the form of an Archimedes spiral. The Spiral 500 laser colony counter and the Casba 800 microprocessor (Spiral System Instruments, Inc.) were used for the CFU count. Antibiotic carry-over could be detected by the absence of colonies in the center of the petri dishes. RESULTSThe killing curves (mean CFU ± the standard deviation) for 20 strains of S. faecalis exposed to 1, 10, and 100 ,ug of CI-934 or amoxicillin per ml are shown in Fig. 1.With CI-934, the bactericidal activity was more rapid as the concentration was increased, except for 1 of the 20 strains. For this strain, the killing was more rapid with 1 ,ug/ml than with 10 and 100 ,ug/ml.With amoxicillin, a paradoxical effect (manifested by 1 log10 CFU/ml or more) was observed for 19 of the 20 strains. At 6 h, there was a significant difference (Mann-Whitney test) between the CFU per milliliter of S. faecalis tested with 1 and 10 ,ug of amoxicillin per ml (P < 0.00003) or with 1 and 100 jig/ml (P < 0.00003).Overall comparison of these two antimicrobial agents shows a more rapid killing rate at 6 h by CI-934 than by amoxicillin at 10 ,ug/ml (P < 0.00003) and 100 ,ug/ml (P < 0.00003). DISCUSSION

In vitro activities of quinolones against enterococci resistant to penicillin-aminoglycoside synergy

“…1) were consistent with those reported from previous studies (9,31,35). Although their activities against synergy-susceptible E. faecalis strains generally were comparable to those of penicillin alone, they were substantially less than those of the penicillin-aminoglycoside combinations.…”

An In Vitro Study on the Effects of Nisin on the Antibacterial Activities of 18 Antibiotics against Enterococcus faecalis

“…The phenomenon that pathogens are relatively resistant to higher concentrations of some antibiotics while remaining susceptible to lower concentrations of antibiotics was first discovered by Eagle and Musselman in 1948 [30]. Nowadays, the phenomenon is often referred to as the “Eagle effect” and has been supported by additional studies [31], [32], [33], [34]. However, in our study the “Eagle effect” on E. faecalis did not take place with the addition of nisin, and E. faecalis was killed by the combination of the antimicrobial peptide nisin and many test antibiotics.…”