Short-term Adaptive Resistance in E. coli K-12 is not dependent on acrD, acrA and tolC

Previous studies have shown that pretreatment of Escherichia coli with a sub-inhibitory level of kanamycin can induce both short-term and long-term adaptive resistance based on the class of antibiotics, where short term adaptive resistance was observed for both non-aminoglycosides and aminoglycosides while long term adaptive resistance was specific for aminoglycosides. While acrD has been shown to be necessary for adaptive resistance to aminoglycosides in long term adaptive resistance, its involvement in short term adaptive resistance was not assessed. The current study examined the necessity for acrD, acrA and tolC –genes that encode for the three components of the AcrAD-TolC efflux pump that exports aminoglycosides out of the cell –in the physiological phenomenon of adaptive resistance. Using E. coli K-12 wild type, Δ acrD, Δ acrA and Δ tolC strains, either pretreated or non-pretreated with a sub-inhibitory level of kanamycin, growth was monitored over 2 hours using turbidity readings to assess the short-term protection to ampicillin, tetracycline, kanamycin and streptomycin, while changes in the minimum inhibitory concentrations of kanamycin and streptomycin were observed to assess long-term protection. When pretreated with kanamycin, all four E. coli K-12 strains had increased transient resistance to subsequent lethal exposures of ampicillin and tetracycline but showed increased transient susceptibility to subsequent lethal exposures of the aminoglycosides. In addition, the kanamycin pretreatment did not induce long-term adaptive resistance to aminoglycosides in any of the four E. coli K-12 strains. Our data suggest that acrD, acrA and tolC are not necessary for short-term adaptive resistance. The necessity of acrA and tolC for long-term adaptive resistance could not be conclusively determined because there was no long-term adaptive resistance observed in the E. coli parental strain. As such, it is inconclusive as to whether acr D, acr A, and tol C are involved in the long-term physiological phenomenon of adaptive resistance.