Molecular epidemiology of antibiotic resistance in the commensal Escherichia coli of calves

• Main Author: Yates, Catherine M.
• Published: University of Edinburgh 2005
• Subjects: 636.089
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.664117

Weekly faecal samples from 11 calves on a non-organic Scottish farm, were screened for <i>E. coli</i> by overnight culture at 44°C on Chromocult TBX (typtone bile X-glucuronide) agar, with and without antibiotics (16mg/L ampicillin, 8mg/L apramycin, or 8mg/L nalidixic acid). Multiple picks were taken from each plate showing growth. Isolates were characterised by breakpoint susceptibility testing with 25 antibiotics, and genotyped by pulsed-field gel electrophoresis (PFGE). Apramycin resistance (aprR) plasmids were characterised by PCR of the resistance gene, restriction profiling, resistance phenotyping, and measurements of transfer frequency and competitive fitness costs of carriage. Plasmid transfer dynamics were explored with a mathematical model. Resistance to beta-lactams, cephalosporins, streptomycin, trimethoprim, chloramphenicol, tetracycline and sulphamethoxazole was detected in the unselected population. These resistance phenotypes were not screened for, indicating high prevalences of the resistance determinants. The unselected (n=139) and ampicillin resistant (n=208) populations were the most diverse (30 and 24 genotypes respectively). Nalidixic acid resistant isolates (n=151) comprised only 2 genotypes. Of 45 aprR isolates, 5 genotypes were identified. Apramycin resistance was conferred by 3 conjugative plasmids, pUK2001, pUK2002 and pUK2003, of sizes 91, 115 and 181Kb respectively. All aprR plasmids conferred cross-resistance to the medical antibiotics tobramycin and gentamicin. Plasmids pUK2002 and pUK2003 also carried tetracycline and streptomycin resistance. Plasmid pUK2001 demonstrated very high transfer frequencies (1.15x10^-2h^-1), horizontal spread to three different genotypes, and an apparent fitness advantage <i>in vitro</i>. The detection of resistance to medical antibiotics in the commensal <i>E. coli</i> of food animals, and the spread of aprR plasmids under unknown selective pressures, poses important implications for the persistence of resistance genes in farm animals and the transmission of apramycin resistance into clinical bacteria.