A novel approach for estimating growth phases and parameters of bacterial population in batch culture

Accurate assessment of antibiotic susceptibility and screening resistant strains of a bacterial population by linear gradient plate

The dynamics of a bacterial population exposed to the minimum inhibitory concentration (MIC) of an antibiotic is an important issue in pharmacological research. Therefore, a novel antibiotic susceptibility test is urgently needed that can both precisely determine the MIC and accurately select antibiotic-resistant strains from clinical bacterial populations. For this purpose, we developed a method based on Fick's laws of diffusion using agar plates containing a linear gradient of antibiotic. The gradient plate contained two layers. The bottom layer consisted of 15 mL agar containing the appropriate concentration of enrofloxacin and allowed to harden in the form of a wedge with the plate slanted such that the entire bottom was just covered. The upper layer consisted of 15 mL plain nutrient agar added with the plate held in the horizontal position. After allowing vertical diffusion of the drug from the bottom agar layer for 12 h, the enrofloxacin concentration was diluted in proportion to the ratio of the agar layer thicknesses. The uniform linear concentration gradient was verified by measuring the enrofloxacin concentration on the agar surface. When heavy bacterial suspensions were spread on the agar surface and incubated for more than 12 h, only resistant cells were able to form colonies beyond the boundary of confluent growth of susceptible cells. In this way, the true MIC of enrofloxacin was determined. The MICs obtained using this linear gradient plate were consistent with those obtained using conventional antibiotic susceptibility tests. Discrete colonies were then spread onto a gradient plate with higher antibiotic concentrations; the boundary line increased significantly, and gene mutations conferring resistance were identified. This new method enables the rapid identification of resistant strains in the bacterial population. Use of the linear gradient plate can easily identify the precise MIC and reveal the dynamic differentiation of bacteria near the MIC. This method allows the study of genetic and physiological characteristics of individual strains, and may be useful for early warning of antibiotic resistance that may occur after use of certain antimicrobial agents, and guide clinical treatment.

A modified suspension test for estimating the mutagenicity of samples containing free and (or) protein-bound histidine

The Ames test has not been very effective in estimating the mutagenicity of histidine-containing samples because external free and (or) protein-bound histidine in these samples would allow the histidine auxotrophs in such test samples to grow more compared with the negative controls that were used as the reference. This could give rise to a false positive.n this study, a modified suspension mutagenicity assay (MS assay) was deveopled. The tester strains were incubated in Luria-Bertani (LB) broth containing different concentrations of traditional Chineses medicines (TCMs) until the declining phase, and the test samples were assayed to be mutagenic or not by observing whether statistically significant differences were demonstrated in the relative reversion frequencies (RRFs) between the negative control groups and the test groups. Collectively, using LB broth as the test medium and comparing the RRFs in the declining phase made this assay less influenced by the presence of histidine in the test samples.The mutagenicity of some TCMs was measured with the MS assay. The results in MS assay were consistent with those in the mammalian bone marrow chromosomal aberration test, which indicated that the MS assay was appropriate to estimate the mutagenicity of samples containing free and (or) protein-bound histidine.

Effect of physiological heterogeneity of E. coli population on antibiotic susceptivity test

According to the instantaneous growth rate (dN/dt) of E. coli CVCC249 growing in batch culture, the entire growth progress was distinguished into four phases: accelerating growth phase, constant growth phase, decelerating growth phase and declining phase, in each of which obvious variation in physiological and biochemical properties was detected, including total DNA, total protein, and MTT-dehydrogenase activity, etc., that led to difference in their antibiotic susceptivity. Antibiotic susceptivity of the population sampled from each phase was tested by Concentration-killing Curve (CKC) approach following the formula N=N (0)/{1+exp[r.(x-BC (50))]}, showing as normal distribution at the individual cell level for an internal population, in which the median bactericidal concentration BC (50) represents the mean level of susceptivity, while the bactericidal span BC (1-99)=(2lnN (0))/r indicates the variation degree of the antibiotic susceptivity. Furthermore, tested by CKC approach, the antibiotic susceptivity of E. coli CVCC249 population in each physiological phase to gentamicin or enoxacin was various: susceptivity of the population in the constant growth phase and declining phase all increased compared with that in the accelerating growth phase for gentamicin but declined for enoxacin. The primary investigations revealed that the physiological phase should be taken into account in the context of antibiotic susceptivity and research into antimicrobial mechanism. However there are few reports concerned with this study. Further research using different kinds of antibiotics with synchronized continuous culture of different bacterial strains is required.