Effect of physiological heterogeneity of E. coli popula-tion 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+expr·(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. Supported by the Natural Science Foundation of Shandong Province, China (Grant No. Y2005C58), the Natural Key Technology R&D Program of China (Grant No. 2006BAK02A03-6) and the Youth Scientific Research Foundation of Shandong Academy of Agricultural Science (2005YQ035)