A NEW METHOD TO DETERMINE POSTANTIBIOTIC EFFECT AND EFFECTS OF SUBINHIBITORY ANTIBIOTIC CONCENTRATIONS

It has been shown that bacteria in a postantibiotic (PA) phase exposed to subinhibitory concentrations (sub-MICs) of antibiotics show a long delay before regrowth. This effect has been named the PA sub-MIC effe ct (PA SME). In the present study, we have used a new method to demons trate this phenomenon. A computerized incubator for bacteria, Bioscree n C (Lab Systems, Helsinki, Finland), which incubates the bacteria, me asures growth continuously by vertical photometry, processes the data, and provides a printout of the results was used. With this method, on e may easily test several antibiotics against different bacteria for P A effects (PAEs), PA SMEs, and SMEs. In this study, the effects of ben zylpenicillin against beta-hemolytic streptococci and pneumococci were examined. The bacteria were exposed to 2, 10, or 50x MIC for 2 h, was hed and diluted, incubated in the Bioscreen C incubator, and then expo sed to 0.1 to 0.9x MIC. The regrowth was monitored for 20 h. The PAE w as calculated as the difference in the time required for the exposed a nd unexposed bacteria to grow to a defined point (A50) on the absorban ce curve. A50 was defined as 50% of the maximum absorbance for the con trol cultures. The PA SMEs were calculated as the difference in the ti me required for the reexposed cultures and the unexposed controls to r each A50. The PAEs ranged between 0.6 and 3.2 h and varied little with the concentration used for the induction of the PAEs. At 0.2x MIC, th e PA SMEs were 2 to 3 h longer than the PAEs. Higher sub-MICs increase d this delay before regrowth. Most cultures exposed to sub-MICs alone were only slightly affected compared with the controls.