In vitro analytical system for determining the ability of antibiotics at residue levels to select for resistance in bacteria.
An analytical procedure, based on the concept that exposure of bacteria to antibiotics will result in the selection of a resistant population, was developed. Two strains of enteric bacteria, Escherichia coli CS-1 and Enterobacter cloacae B520, which are sensitive to a wide variety of antibiotics, were used as the test organisms. E. coli CS-1 were exposed to 1.00 micrograms antibiotic or antimicrobial/mL; E. cloacae B520 were exposed to 0.01, 0.10, 0.50, 1.00, and 5.00 micrograms/mL. Both organisms developed increased resistance to other antibiotics after exposure to chlortetracycline and oxytetracycline, as measured by the minimum inhibitory concentration (MIC). E. cloacae B520 showed increased resistance to ampicillin, oxytetracycline, and chloramphenicol after exposure to levels as low as 0.10 microgram/mL. Exposure to streptomycin, sulfamethazine, tylosin, bacitracin, flavomycin, virginiamycin, and monensin at levels of 1.00 microgram/mL did not increase the MIC. Exposure to 5.00 micrograms streptomycin, sulfamethazine, tylosin, and monensin/mL increased the MIC of E. cloacae to one of the antibiotic markers. These increased MICs exceeded the 95% confidence limits of the MIC values of the unexposed organisms.
Brady
M S
MS
Rutgers University, Cook College, Department of Biochemistry and Microbiology, New Brunswick, NJ 08903.
Strobel
R J
RJ
Katz
S E
SE
D000900:Anti-Bacterial Agents / Q000032:analysis / Q000494:pharmacology*; D001419:Bacteria / Q000187:drug effects*; D003470:Culture Media; D004350:Drug Residues / Q000032:analysis; D004352:Drug Resistance, Microbial; D004754:Enterobacter / Q000187:drug effects; D004926:Escherichia coli / Q000187:drug effects; D008826:Microbial Sensitivity Tests
D000900:Anti-Bacterial Agents; D003470:Culture Media