TY - JOUR
T1 - Comparison of two methods for determining in vitro postantibiotic effects of three antibiotics on Escherichia coli
AU - Rescott, D. L.
AU - Nix, D. E.
AU - Holden, P.
AU - Schentag, J. J.
PY - 1988
Y1 - 1988
N2 - 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 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.
AB - 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 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.
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U2 - 10.1128/AAC.32.4.450
DO - 10.1128/AAC.32.4.450
M3 - Article
C2 - 3288112
AN - SCOPUS:0023849615
SN - 0066-4804
VL - 32
SP - 450
EP - 453
JO - Antimicrobial Agents and Chemotherapy
JF - Antimicrobial Agents and Chemotherapy
IS - 4
ER -