The postantibiotic effect (PAE) for 10 isolates of Escherichia coli was measured by two methods after 1 h of exposure to ampicillin, ciprofloxacin, or tobramycin. The reference method involved serial colony counting to determine growth after antibiotic exposure in relation to control growth. A spectrophotometric procedure was developed with the Abbott MS-2 research system. This method measured the time to detection of growth after exposure and compared this with the time for growth detection in control chambers having the same initial colony count. A reference curve of time to growth versus log initial CFU per milliliter was used to standardize control growth. PAE was determined after exposure to antibiotic at two and six times the MIC and with inocula ranging from 103 to 109 CFU/ml. There was a statistically significant correlation between PAE measured by the spectrophotometric and the reference methods, and the residuals about the regression line were normally distributed. The mean PAE determined by both methods was statistically different for tobramycin-exposed, but not for ampicillin-or ciprofloxacin-exposed, organisms. There was a concentration-dependent PAE for ciprofloxacin and tobramycin. The PAEs for ciprofloxacin (151 min) and tobramycin (108 min) at concentrations six times the MIC were prolonged compared with those measured at two times the MIC (69 and 66 min, respectively). PAE was inversely related to the exposed inoculum for ciprofloxacin and tobramycin. The PAE for E. coli exposed to ampicillin was minimal and was not affected by either concentration or inoculum. The MS-2 method for determining PAE yields similar results, but is less laborious than the reference method.Postantibiotic effect (PAE) is the lag phase or recovery period of bacterial growth after brief exposure to an antibiotic. The presence of PAE may be an important consideration in designing antibiotic dosage regimens (4, 7, 9). A prolonged PAE should allow extension of antibiotic dosing intervals beyond the time that antibiotic concentrations fall below the MIC (8). Theoretically, antimicrobial agents with minimal PAEs may require serum concentrations above MICs for an entire dosing interval.The reference method for PAE measurement is the broth technique, involving serial colony counting and broth subculture of growth (4). Consequently, the technique is very time consuming and tedious, because numerous colony counts must be performed after exposure to the antibiotic. A more rapid and less tedious method would greatly facilitate the clinical study of PAE as a factor in design of antibiotic dosage regimens.A procedure was developed to measure PAE with the Abbott MS-2 research system (MS-2). The MS-2 method was considerably less time consuming than the broth technique, but required validation because it utilizes optical endpoints rather than survival. The MS-2 method and the broth technique were compared by using three antibiotics against 10 Escherichia coli isolates. Variables known to alter the duration of the PAE, including antimic...
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