UNDERSTANDING PATTERNS OF ESCHERICHIA COLI O157:H7 SHEDDING AND COLONISATION IN CATTLE AND THEIR ROLE IN TRANSMISSION

Escherichia coli O157:H7 is a human pathogen capable of causing severe disease due to the release of the phage-encoded exotoxin Shiga toxin (Stx). The primary
reservoir of E. coli O157:H7 is cattle from which the organism is shed asymptomatically and colonises specifically at the terminal rectum (TR). Prevalence
rates in cattle vary and shedding is transient making determination of transmission routes difficult. This thesis aims to gain further understanding of shedding patterns
and transmission of E. coli O157:H7 in cattle to inform future control strategies.
Rates of E. coli O157:H7 replication in gut contents and exceptional replication rates in TR mucus revealed that passive shedding could explain both low and high faecal
counts observed in epidemiological studies and that replication in TR mucus coincides with high rates of attachment to bovine terminal rectum epithelial (BTRE)
cells. The rumen was identified as a critical point of control of bacterial numbers which could be exploited in future control strategies which should also consider the
potential for passive shedding and environmental replication to maintain E. coli O157:H7 populations on farms in the absence of a colonised animal.
Transmission studies showed that while useful for studying colonisation of cattle, Stx- strains of E. coli O157:H7 are unable to transmit effectively compared to Stx+ strains and are not appropriate for use in experimental transmission studies.
Differences observed between shedding and transmission of phage type (PT) 21/28 and PT 32 strains could explain why PT 21/28 is more common in cattle and humans.
Studies of replication and colonisation of the strains from the transmission studies revealed that PT 21/28 is better able to replicate, attach and increase in number on BTRE cells compared to PT 32 and Stx- W3. These advantages to survival and colonisation indicate how PT 21/28 strains could out-compete other strains to persist in cattle populations.