Human ingestion of a very small number of Escherichia coli 0157 cells may be sufficient to cause serious illness, or even death. Although most human E. coli 0157 infections are foodbome, contact with the organism in the environment is an important vector of initiatingoutbreaks. Consequently, persistence of the organism in different environments needs to beelucidated. This thesis details the observed persistence of E. coli O 157 under a range of environments and environmental conditions simulated in various laboratory experiments.
The first experimental chapter (Article I) investigated the survival of E. coli O157:H7 in cattle faeces on the surfaces of materials commonly found on farms. Persistence was greatest on moist wood samples under cooler temperatures with large numbers remaining after 28 days. It was established that substantial numbers of cells may also be transferred to human hands from such surfaces during brief contact. In chapter 4 (Article II), the effect that the traditional practice of applying sawdust to floor tiles in meat-processing areas has
on pathogen persistence was examined. Viable E. coli O157:H7 persisted on all tiles over 3 days, although desiccation of surfaces resulted in a more rapid decline in numbers, while cleaning of the tiles with bleach prior to contamination with meat juice enhanced recovery of the pathogen. The presence of sawdust was only found to reduce survival of E. coli O157:H7 on tiles under dry conditions. Chapter 5 (Article III) investigated the effect of earthworm digestion on the survival and dissipation of E. coli O 157 :H7 in compost and soil.
Deep-burrowing eaithw01ms (Lumbricus terrestris) significantly aided vertical movement of E. coli 0157 in soil, whilst litter-dwelling earthworms (Dendrobaena veneta) significantly aided lateral movement within compost. Although the presence of earthworms in soil and compost may aid proliferation of E. coli O157:H7 in early stages of contamination, long-term persistence of the pathogen appears to be unaffected. In chapter 6 (Article JV), cattle slurry and ovine stomach content waste inoculated with E. coli O 157 :H7
were applied to soil cores ± established maize plants. The pathogen survived in soil for over 5 weeks, though at significantly greater numbers in soil receiving stomach content waste in comparison to cattle slurry. In contrast to published work, persistence of the pathogen in soil was unaffected by the presence of a rhizosphere. In chapter 7 (Article V), cow and sheep faeces were inoculated with either a high (ca. 108 CFU mr 1) or low (ca. 105 CFU mr 1) load
of a lu.x-marked strain of E. coli O157:H7, and subjected to four simulated heavy rainfall events. Greater densities of pathogenic and generic E. coli cells were recovered in the leachates from sheep faeces compared to cattle faeces. Pathogen activity (luminescence) was also greater in sheep faeces. The technique employed may be useful to depict realistic pathogen dispersal from animal waste for use in microcosm studies. In chapter 8 (Article VI), we inoculated microcosms of lake, puddle, river, and animal-drinking trough waters with E.coli O157:H7 and incubated them at 10°C. Cells were still detected in 45% of non-sterile
samples after 2 months. Persistence of the organism was enhanced by water aeration and by prior sterilisation; however there was no correlation between water chemistry and mean E. coli O157:H7 die-off times/rates in any water type. Lastly, we applied slurry inoculated with E. coli O157:H7 to sands; which were then maintained in a dry state (non-tidal), or subjected to a repeated seawater tidal simulation (Article VII). Pathogen activity declined with increasing seawater concentration; however, cells remained viable in all sand and
water treatments over the 5 d period. In all treatments, cell activity rapidly increased upon addition of available nutrients to the water. In summary, this work shows that E. coli 0157 is a highly resilient bacterium, capable of persisting in a range of matrices under varied environmental stresses.