Pasteurized whole milk confers reduced susceptibilities to the antimicrobial agents trimethoprim, gatifloxacin, cefotaxime and tetracycline via themarRABlocus inEscherichia coli

Abstract

We inoculated pasteurized whole milk withEscherichia colistrains GC4468 (intactmarRABlocus), JHC1096 (ΔmarRAB), or AG112 (ΔmarR), and incubated each overnight at 37°C. All strains were then recovered from the milk cultures, and susceptibilities to antimicrobial agents were determined by the E-test strip method (CLSI). Cells of strain GC4468, prior to culturing in milk, were susceptible to trimethoprim, gatifloxacin, cefotaxime and tetracycline. After culturing GC4468 in pasteurized milk, however, the minimal inhibitory concentrations (MICs) increased 1·4-fold for trimethoprim (P⩽0·05), 1·5-fold for gatifloxacin (P⩽0·05), 2·0-fold for cefotaxime (P=0·008), and 1·4-fold for tetracycline (P⩾0·05). After culturing GC4468 on milk count agar the MICs were enhanced 3·4-fold for trimethoprim (P⩽0·05), 10-fold for gatifloxacin (P=0·001), 7·1-fold for cefotaxime (P=0·011), and 40·5-fold for tetracycline (P=0·074), but exhibiting tetracycline resistance with a mean MIC of 74·7±18·47 μg/ml (CLSI). The MICs of the antimicrobial agents for JHC1096 cells after culturing in pasteurized whole milk were indistinguishable (P⩾0·05) from baseline MICs measured before culturing in the same type of milk. Thus,Esch. colicells harbouring themarRABlocus exhibit reduced susceptibilities to multiple antimicrobial agents after culturing in pasteurized whole milk.