The effect of a brief beta-lactam action on a bacterial culture has been studied by combining the use of a high performance photometer with phase contrast microscopic examination. After exposure of the culture with the antibiotic, the growth curves presented in logn OD have shown the importance of the pre-lytic increase in OD (PIOD). Using Escherichia coli a study of the PIOD values allowed us to separate the beta-lactam antibiotics studied into two groups: those whose PIOD were concentration-dependent (ampicillin, carbenicillin, ticarcillin, moxalactam, ceftazidime, cefsulodin, cefotaxime) and those whose PIOD were concentration-independent (or weakly dependent) (azlocillin, mezlocillin, piperacillin). When the PIOD was independent of the concentrations, long filaments were observed by phase contrast microscopy. Using Pseudomonas aeruginosa, all the antibiotics studied produced PIOD values that were barely dependent of the concentrations, and long filaments were observed by phase contrast microscopy. Study of the lag of regrowth in the post antibiotic period using a beta-lactamase technique showed that, if the PIOD was hardly concentration dependent, so was the lag of regrowth.

Methicillin- and gentamicin-resistantStaphylococcus aureusmay remain sensitive to minocycline and to rifampicin. A study of growth curves has shown that at inhibitory concentrations (0·4 μg/ml), minocycline prevents the development of mutants resistant to rifampicin.

Escherichia coli strains that were susceptible to multiple antibiotics were exposed to suprainhibitory concentrations of ampicillin and piperacillin. As with the majority of beta-lactam antibiotics, the growth curves showed an increase in optical density (OD) before lysis during the first hours. This increase in OD depended on the concentration of ampicillin and was independent of the concentration of piperacillin. A good correlation was found between the prelytic increase in OD and the killing curve. During the prelytic increase in OD, the number of CFU per milliliter remained constant. The decrease in the number of CFU per milliliter depended on the concentration of ampicillin and was independent of the concentration of piperacillin. pH variations gave rise to similar effects on growth curves and killing curves.The antimicrobial activity of antibiotics is usually defined by the MIC and the MBC. This usually involves a single value obtained after one night of incubation. Growth curves allow an approach which documents the effect of a brief contact of an antibiotic with a bacterial culture (4). However, growth curves must be interpreted with great caution. Indeed, an increase or a decrease in optical density (OD) may be due to very different microbiological effects. Our previous study (12) showed the value of measuring the prelytic increase in OD. During the first 2 or 3 h after the introduction of beta-lactam antibiotics into an Escherichia coli culture, an increase in OD which could be superimposed on the control curve was observed. This increase in OD was due to the formation of cells with deficient cell walls.The aim of the present study was (i) to verify whether bacteria remain viable during the prelytic increase in OD and, if so, in what proportion, and (ii) to determine whether variations in culture media (e.g., of pH) influence growth curves and the number of CFU per milliliter in the same way. Since it was a matter of verifying the agreement between growth curves and killing curves, we limited our study to strains of E. coli that were susceptible to multiple antibiotics. Only the early response (the first 6 h of contact between the antibiotic and the culture) were taken into consideration. We chose ampicillin and piperacillin at three suprainhibitory concentrations, 16, 64, and 256 ,ug/ml. Indeed, our previous study (12) [Oxoid Ltd., London, England]; macrodilution method; inoculum, 106 CFU/ml) was 1.56 ,ug/ml, with a possible variation of one dilution after 24 h of incubation at 35°C.Culture media. Growth curves and killing curves were determined with Iso-Sensitest medium. Modifications of pH were obtained by adding an acid or a base. The osmolarities of the media so modified were 342 mOsm/k at pH 7.3, 441 mOsm/k at pH 6.3, and 433 mOsm/k at pH 8.3. Colony counts were made on the same media containing 1.5% agar.Growth curves. The MS-2 system (Abbott Laboratories, Diagnostics Div., Irving, Tex.) (8) and open multichamber cuvettes were used to determine growth curves. Ampicillin and piperacillin were ...

Using a triple agar layer technique and enzymatic inactivation of penicillin, the occurrence of a paradoxical zone phenomenon (illustrated by a typical ‘target’ image around the reservoir of antibiotic) was determined for several bacterial species specially chosen with regards to their taxonomic position, clinical importance and penicillin susceptibility. Among gram-positive bacteria, a paradoxical zone was obtained for approximately 43% of the strains studied here (all 10 strains of Staphylococcus aureus, all 10 strains of Streptococcus faecalis, 7 of 10 strains of group B β-hemolytic streptococcci, 1 of 10 group A strains, 3 of 10 strains of α-hemolytic streptococci, 3 of 10 strains of Clostridium perfringens but for none of 10 strains each of Streptococcus pneumoniae, and Listeria monocytogenes). Among gram-negative bacteria, a target image was regularly obtained with Haemophilus influenzae (all 10 strains tested) and Proteus species (9 of 10 strains) but with none of the following species: Escherichia coli, Salmonella, Shigella, Pseudomonas aeruginosa, Neisseria gonorrhoeae, Neisseria meningitidis and Bacteroides fragilis. Therapeutic implications of these observations are difficult to assess, and need further investigation.

Ten strains of Pseudomonas aeruginosa that were susceptible to imipenem (MICs 2 mg/l) were exposed to a new parenteral carbapenem, meropenem (MIC 0.25 mg/l). Kinetic turbidometry showed that, as with other beta-lactam antibiotics, there was a prelytic increase in the culture OD following exposure to meropenem. The maximal value of the prelytic increase in the OD was higher for meropenem than for imipenem at concentrations 0.5, 1, 2, 4 and 8 x MIC. This corresponded to the formation of short filaments during exposure to low concentrations of meropenem. These filaments remained viable for 1-2 h, according to the drug concentration. For this reason, the killing began later with meropenem than with imipenem. After this delay, the killing rate for meropenem was the same as with imipenem, but occurred with lower concentrations of meropenem.