Submitted to: National Antimicrobial Resistance Monitoring System Scientific Meeting
Publication Type: Abstract Only
Publication Acceptance Date: June 27, 2002
Publication Date: N/A
Technical Abstract: Background: The paradoxical effect between certain bacteria and some antibiotics has been known to occur since 1948. The paradox results from the production of a large proportion of surviving cells when the concentration of antibiotic increases beyond the MIC. The work described here focuses on the evaluation of Escherichia coli isolates obtained from a veterinary diagnostic laboratory for the paradoxical effect with some benzyl quaternary amine disinfectant components used in animal production. Method: Eighty-seven E. coli isolates obtained from neonatal pigs with diarrhea on five farms in Oklahoma were investigated by broth microdilution susceptibility testing on 96-well microplates with U-shaped wells and various quaternary amine disinfectant components. The individual components tested were the benzyl quaternary amine mixture Barquat MB-50, and didecyldimethylammonium chloride, benzyldimethyldodecylammonium chloride, benzyldimethyltetradecylammonium chloride (A), and benzyldimethylhexadecylammonium chloride (B). Results: Early studies for paradoxical behavior of 9 isolates with chemicals A and B and the benzyl amine mixture Barquat MB-50 resulted in MICs that varied from 3.8-15 ug/mL, but these same chemicals showed paradoxical growth at 140-480, 96, and 240-480 ug/mL, respectively. Further studies on 18 isolates with higher concentrations of chemicals A and B revealed MICs that range from 3.8-7.8 ug/mL in general, and these isolates showed paradoxical growth at 120-960 and 62-499 ug/mL for chemicals A and B, respectively. Mixtures of benzyl quaternary amines also show these same paradoxical effects. Of the 75 isolates evaluated at this time, all show paradoxical behavior between E. coli and the chemicals A and B. Conclusion: This work demonstrates for the first time that some benzyl quaternary amine disinfectant components used in animal production exhibit the paradoxical effect with E. coli. Benzyldimethyltetradecylammonium chloride, benzyldimethylhexadecylammonium chloride, and mixtures of these two disinfectant components produce paradoxical growth in all E. coli isolates tested. The increased use of non-antibiotic antimicrobial agents is a possible selection factor for antibiotic resistant strains in clinical and domestic environments.