Comparative bactericidal activities of beta-lactam antibiotics determined in agar and broth media.

Abstract

Comparative bactericidal activities were determined utilizing a relatively large number of test strains, in both agar and broth media, with special reference to the time of exposure of the bacteria to certain /3-lactatn antibiotics. it was apparent that the activities increase with time. The concentrations producing a99.9% kill with cephalothin for Escherichia coli, Klebsiella sp., and carbenicillin for Pseudontonas aerugino.sa were higher in broth than in agar. In contrast, those of benzylpenicillin for a-streptococcus (non-enterococcal) were higher in agar than in broth. If the bactericidal concentrations with 3-hour or 6-hour exposure to antibiotics were used as the criterion, these concentrations of carbenicillin for P. aeruginosa, and benzylpenicillin for a-streptococcus were, in particular, unusually high compared with the conventionally determined bacteriostatic concentrations (MICs).Determination of bactericidal activity is widely accepted to be more important than that of bacteriostatic activity in the treatment of patients with infective endocarditis and presumably in patients with seriously impaired host defence mechanisms. The present paper describes minimal lethal concentrations (MLCs), defined as the concentrations producing a 99.9% kill'1-2, together with bacteriostatic concentrations of /3-lactam antibiotics for a-streptococcus which is the major blood isolate from infective endocarditis',", and also Escherichia coli, Klebsiella sp. and Pseudomonas aeruginosa which are the most frequently encountered infective pathogens in debilitated patients5--8). In many of the studies already reported, bactericidal activity is expressed in terms of the results obtained after overnight exposure of bacteria to antimicrobial agents2'9,10'. Such a long period of continuous exposure of microorganisms to the antimicrobic does not generally occur in clinical practice. Most drugs are administered orally or parenterally, achieving in serum or at the focus of infection antimicrobic concentrations which are higher than the in vitro bactericidal concentrations for a few hours at the most. Based on this reason, we now report on in vitro bactericidal activities after varying periods of exposure of bacteria to /-lactams in agar and broth media. Materials and MethodsBacterial strains Recent clinical isolates of E. coli, Klebsiella sp., P. aeruginosa, and a.-and /1-streptococcus (nonenterococcal) were used. Media Heart infusion broth (Difco Laboratories) and heart infusion agar (Difco) were used. Defibrinated horse blood (5°0) was supplemented to the agar for the study of a-and /-streptococcus.

Assessment of Timed Bacteriostatic and Bactericidal Activities of Cephalothin against Gram-Positive and Gram-Negative Bacteria

“…Each tube was in oculated with a number of CFU adjusted to con tain N X 106 (N; a few to 30, the number of colo nies usually being countable at one spot of inocu lation on agar plates) per milliliter of broth (re ferred to as 'standardized suspension') [5]. After incubation of these tubes at 35 °C for a certain number of hours, a 0.001 ml sample was removed from each tube and was placed into a 1 ml phos phate buffer solution.…”

“…After incubation of these tubes at 35 °C for a certain number of hours, a 0.001 ml sample was removed from each tube and was placed into a 1 ml phos phate buffer solution. After blending on a vortex mixer to obtain a uniform suspension of CFU, a 0.001 ml sample taken from the phosphate buffer suspension was transferred onto /9-lactamasetreated agar plates [5] (the number of CFU should be N per inoculation spot if no change in the number had occurred during incubation in the 'standardized suspension'). These plates were incu bated at 35 °C overnight.…”

Bacteriostatic and bactericidal activities of beta-lactam antibiotics enhanced by the addition of low concentrations of gentamicin

“…The minimum concentration E of the f)-lactam drug producing a 99.9% kill was defined as the 3-h MLC (11). The concentrations E' and F' of the ,B-lactam drug and gentamicin in combination which produced the best result in terms of a 99.9% kill were also determined.…”