Microevolution of <em>E. coli </em>O157:H7 in a herd of Australian cattle — ASN Events
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Microevolution of E. coli O157:H7 in a herd of Australian cattle (#125)

Meghan Jones 1 , Geraldine Lammers 2 , Sophie Octavia 1 , Jane Heller 2 , Ruiting Lan 1
  1. School of Biotechnology and Biomolecular Sciences, University of New South Wales, Sydney, NSW, Australia
  2. School of Animal and Veterinary Sciences, Charles Sturt University, Wagga Wagga, NSW, Australia

Escherichia coli O157 can cause haemorrhagic colitis and haemolytic-uremic syndrome (HUS). They are often found as normal flora of ruminant animals such as sheep and cattle, allowing these organisms to be released into the environment. Single nucleotide polymorphisms (SNPs) have been widely used to determine genetic relatedness and evolutionary relationships of E. coli O157 populations1. However, SNPs are not very discriminatory as a previous study found that Australian O157 belonged to one SNP clade (clade 7). A more discriminating method, multilocus variable number tandem repeat (VNTR) analysis (MLVA) has been widely used and standardised for outbreak detection of O157. In this study, we analysed 169 E. coli O157 isolates from a herd of 23 cows obtained during a nine month period using both SNP typing and MLVA. SNP typing showed that 163/169 were clade 7, with six isolates belonging to a novel clade.

MLVA separated the 169 isolates into 32 unique MLVA profiles, with five predominant profiles. Isolates taken from the same cow did not always belong to the same profile, with all cows excreting isolates from at least two different MLVA profiles. The majority of profiles differed by either one repeat or one VNTR, with the most divergent profile differing by 5 VNTRs. The average number of alleles of the eight loci was 6 ranging from 2 to 19. By minimum spanning tree analysis three of the 5 dominant MLVA profiles were closely related with one repeat difference, likely to have evolved from one to another during the sampling period. Two MLVA profiles obtained during the late sampling period differed from the early sampled profiles by 2 or more alleles suggesting independent introductions. In conclusion, over a period of 7 months, the O157:H7 clone underwent microevolution and dynamic clone turnover.

  1. Manning, S. D., A. S. Motiwala, A. C. Springman, W. Qi, D. W. Lacher, L. M. Ouellette, J. M. Mladonicky, P. Somsel, J. T. Rudrik, S. E. Dietrich, W. Zhang, B. Swaminathan, D. Alland and T. S. Whittam (2008). "Variation in virulence among clades of Escherichia coli O157:H7 associated with disease outbreaks." Proceedings of the National Academy of Sciences 105(12): 4868-4873.
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