Antibiotic Susceptibility Test to Characterize the Antimicrobial Dynamics of Antibiotic Concentration by Isobiologram

The antibiotic susceptibility test (AST) method was improved by evaluating the synergy of antimicrobial concentration and time to antimicrobial rate. The concentration of E. coli BBZ6-3 suspension in the MHB was detected with turbidity ( A 600) by automatic growth curve analyzer (Bioscreen) in continuous time while exposed to various concentrations of antibiotics. Based on this testing system of antibacterial dynamics, isobiologram was built by Sigma plot through relative bacteriostatic rate (RBR=1- At - i / At -0) to antibiotic concentration and treating time. It could perfectly suggest that the process and trend of bacteriostatic action was affected with the two factors, which possessed compatible advantage of both quantitative character of two-dimensional curve and visual character of three-dimensional curve. With the normalized processing, RBR could eliminate the influence of some factors, such as the background effect of bacteria growth, inoculums concentration and variety of antibiotics. So the bacteriostatic rate of different antibiotics, specially the effect of concentration and time, could directly be compared through it. The essence of bacteriostaticaction was that the nutriment utilization efficiency and degree were inhibited by antibiotics when bacteria grew in media, which was different form direct sterilization but was a correlative course. Isobiologram was divided into three parts by 0.5 RBR (traditional visual MIC by the naked eye): differentiation area (left area of lengthways isobiologram 0.5 RBR, all concentrations), growth area (above isobiologram 0.5 RBR), bacteriostasis area (right area of lengthways isobiologram 0.5 RBR and below isobiologram 0.5 RBR). The periphery of differentiation area was 100–300 min, so it was credible to determine the MIC at 8–12 h in traditional AST. In fact, the isobiologram density of growth area and bacteriostasis area was the evidence of concentration dependence and time dependence. Bacteriostatic dynamics testing system and RBR isobiologram, which reformed the traditional AST method, would be used to analysis the effect of different inoculation quantity on antibacterial rate and the bacterial population variation, post antibiotic effect (PAE) of different concentrations of antibiotics, and compare bacteriostatic dynamics of different strains with different level of resistance.