Correlation between growth curve and killing curve of Escherichia coli after a brief exposure to suprainhibitory concentrations of ampicillin and piperacillin

Abstract

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 ...

Turgor pressure responses of a gram-negative bacterium to antibiotic treatment, measured by collapse of gas vesicles

“…Other instances appear in recent literature. For example, Escherichia coli, at a low pH and low ampicillin concentration after treatment for 1 to 4 h, exhibited a plateau at which more than 10% survived (33), but then dropped to only 1 in 104 surviving. The critical conclusion to be drawn from our new studies is that the division into the two-component classes had already been made by the bacteria during their prior growth; that is, cell commitment had occurred before we treated the culture with antibiotic.…”

Correlation between Growth Curves and Killing Curves of <i>Escherichia coli </i>in the Presence of Fleroxacin and Ampicillin

“…In our study, we used OD and one particular value, PIOD, for mass measurement. In an earlier study we showed that this value, obtained from con tinuous turbidimetric recording from a MS-2 system was reproducible and offers an interesting perspective for the measure ment of dose-effect relationship of [3-lac tams and of the morphological aspect of the antibacterial action [15,16]. On the other hand, the proportion of bacteria un dergoing filamentation observed hour by hour by phase contrast microscopy, as proposed by Greenwood [7], is time-con suming and does not enable many strains to be studied.…”

“…Growth curves describe the kinetic effect of an antibiotic's contact with a bacterial culture, but they must be interpreted with great caution because an increase or de crease in optical density (OD) may be due to very different microbiological effects. During the first 2 or 3 h after the introduc tion of a [3-lactam antibiotic into an Es cherichia coli culture, an increase in OD, which could be superimposed on the con trol, may be observed due to the formation of cell-wall-deficient bacteria [15,16]. Quinolones have a different mode of ac tion.…”

Antibacterial Effects of Meropenem and Imipenem against <i>Haemophilus influenzae</i>

“…The growth curve patterns [9,10] of bacteria briefly exposed to different concentrations of p-lactam antibiotics showed that during the period follow ing the addition of antibiotics, the OD continues to increase before lysis. The maximal value of prelytic increase in OD is the PIOD value.…”