In animals, the study of antibiotic resistance in bacteria has been focused on
organisms that are pathogenic in human or animal hosts. The development of
antibiotic resistance in commensal bacteria is also of concern because they may act
as a reservoir ofresistance genes. This thesis aimed to determine levels of resistance
to veterinary and medical antibiotics in the commensal Escherichia coli of calves, to
explore the genotypic diversity of isolates, and to study the molecular mechanism
and transfer dynamics ofresistance to apramycin.
The antibiotic sensitivity testing of calf faecal E. coli, obtained by weekly sampling,
demonstrated that there was resistance to beta-lactams, cephalosporins, streptomycin,
trimethoprim, chloramphenicol, tetracycline and sulphamethoxazole. These
resistance phenotypes had not been selected for on antibiotic-containing media,
indicating a high prevalence of the corresponding resistance determinants.
Five hundred and forty three isolates were genotyped by pulsed-field gel
electrophoresis. Examination of the patterns generated by restriction with Xbal and
analysed with BioNumerics software revealed a total of 55 different genotypes.
Ampicillin resistant isolates were more diverse (24 genotypes) than apramycin or
nalidixic acid resistant isolates (5 and 2 genotypes respectively). Apramycin
resistance (aprR) was conferred by three 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 (4.12x10" during 7
hrs mating), horizontal spread to three different genotypes, and an apparent fitness
advantage in vitro.
This thesis shows a very high prevalence of antibiotic resistance genes in the
commensal faecal flora of food-producing calves. This may have significant
implications for the transmission ofresistance genes to human clinical bacteria