<i>Streptococcus faecalis</i>: in vitro Susceptibility to Antimicrobial Drugs, Single and Combined, with and without Defibrinated Human Blood

Abstract

Ampicillin, fusidic acid, gentamicin, imipenem, mezlocillin, ofloxacin, penicillin G, piperacillin, and vancomycin were examined for inhibitory and bactericidal activity in various broth media against 7 clinical isolates of Streptococcus faecalis. On a weight-for-weight basis, ampicillin, imipenem, mezlocillin, and ofloxacin proved to be more efficacious. All enterococcal isolates were resistant against gentamicin; fusidic acid and vancomycin lacked bactericidal activity. The combinations of either ampicillin, imipenem, mezlocillin, ofloxacin, piperacillin, or vancomycin with a subinhibitory concentration (4 μg/ml) of gentamicin, with or without added 65% (v/v) fresh defibrinated human blood, respectively, yielded additive effects against all enterococcal isolates. The addition of fresh human blood failed to enhance the antienterococcal activity of 4 μg/ml of gentamicin; in contrast, addition of 65% (v/v) fresh or heat-inactivated (56°C, 30 min) normal rabbit, bovine, and human sera augmented the activity of gentamicin, an effect that was ablated through the addition of either 0.005 M DTT or 0.01 M MgCl₂ + 0.01 M EGTA + 0.01 M CaCl₂, supplements known to antagonize human serum β-lysin, but not lysozyme activity.

Bactericidal Activity of Gentamicin against Enterococcus faecalis In Vitro and In Vivo

“…A discrepancy was observed between the in vivo results and those obtained in vitro in BHI medium. As shown previously, rabbit and human serum enhanced the in vitro activity of gentamicin against E. faecalis (24,26). Susceptibility testing performed in the presence of serum was predictive of in vivo activity in rabbits.…”

“…However, we noted in our study an increase in the activity of gentamicin not only with rabbit serum but also with human serum. Second, it was shown that the alkalinization of nutrient broth and modifications in the concentrations of cations (such as calcium or magnesium), the various components of complement, and specific immunoglobulins of the serum may interact with antibiotics in killing bacteria (7,10,25,26).…”

“…Finally, the increase in bactericidal activity may be due to the interaction between gentamicin and serum noncomplement cationic proteins, such as ␤-lysin (9,15,26). Indeed, Traub et al showed that the antagonization of ␤-lysin abolished the augmentation of gentamicin antienterococcal activity induced by the addition of rabbit, bovine, or human serum (26).…”

“…Indeed, Traub et al showed that the antagonization of ␤-lysin abolished the augmentation of gentamicin antienterococcal activity induced by the addition of rabbit, bovine, or human serum (26).…”

Gentamicin- and Methicillin-Resistant <i>Staphylococcus aureus</i>: Phenotypic and Genotypic Characterization of Three Putative Nosocomial Outbreak Strains

“…Agar disk diffusion tests using Mueller-Hinton agar were carried out in accordance with the method of Bauer et al [36]; the results were interpreted according to specifications of the National Committee for Clinical Laboratory Stan dards [37] where applicable. A microtitcr broth dilu tion method, carried out according to the recommen dations of the National Committee for Clinical Labo ratory Standards [38] served to determine the minimal inhibitory (MIC) and minimal bactericidal (MBC) concentrations of all 31 drags in MHB [6,39]. The concentration ranges tested were 0.125-128 pg/ml (cotrimoxazole = 0.015/0.3-16/304 pg/ml; rifampin = 0.0017-2 pg/ml).…”