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Thiry-Vella loops were prepared on the anterior small intestine of pigs. When the animals were fully recovered the loops were repeatedly inoculated with a porcine strain ofEscheri-chia coli in an attempt to study the protective mechanism operating in the gut. There was no evidence to indicate the development of an anti-enterotoxic immunity since fluid accumulation in the loops did not decrease as a result of daily exposure of the loops to approximately 1010E. coli over a period of 10 days. However, there was clear indication of a bactericidal mechanism operating in the gut since the number ofE. coli in the loops decreased by a factor of 1000 to 10,000 during the 24 h following inoculation.

Nursing litters of vaccinated (K88 antigen) and nonvaccinated gilts as well as weaned piglets, which had recovered from natural colibacillosis, were exposed to Escherichia coli0149:K91(B), K88ac(L) orogastrically and 3–4 h after exposure a proportion of each group was killed for adhesion studies; others were kept for clinical observations. From killed piglets duodenal contents were collected and after washing of the duodenum by standardised techniques it was homogenised. Viable counts of E coliO149 in these specimens were carried out and compared. Coli- counts in the duodenal contents and in the homogenate of washed duodenum of naturally infected piglets were also compared.

Unsupplemented porcine colostrum and milk exhibited a powerful bactericidal effect for porcine strains of E coliincubated in vitroat 37°C. This activity was independent of complement but was susceptible to acid pH, to the presence of soluble iron and to the selective immunoprecipitation of IgG, IgA and IgM. Manifestation of bactericidal activity required bacteria in an active state of metabolism and the length of incubation was an important factor in demonstrating the quality of the anticoli activity, ie, proliferation-inhibitory, bacteriostatic or bactericidal. Whey obtained by acid precipitation or by the application of rennin was devoid of bactericidal activity but was capable of slowing down proliferation of E coli. There was no correlation between lysozyme and anticoli activity although the complete removal of lysozyme by adsorption on to bentonite reduced bactericidal titres. With very few exceptions the highest bactericidal titres were recorded for colostrum, but even 28 days post partumabout one half of 22 undiluted milk samples exhibited bactericidal activity.

Piglets were exposed orogastrically to Escherichia colito enable study of the duration of anti-adhesive and bactericidal activities of milk of sows vaccinated with a K88 enriched E colivaccine. There was a marked increase in the number of the challenge strain in the digestive tract of weaned piglets of all ages (between 888 and 2144 per cent). In contrast, there was a decrease in their number (75 per cent) in the day-old colostrum-fed piglets. When the piglets were two weeks old milk was still capable of reducing the rate of proliferation of the pathogen but at five weeks it proliferated at equal rates in the digestive tract of both suckling and weaned litter-mates. The rate of adhesion of the K88-positivc E colito the small intestine of colostrum deprived piglets was high (5 × 107/g). Rate of adhesion fell gradually in weaned piglets from 5·4 × 107/g at two weeks to 2·0 × 106/g at four to five weeks of age. In contrast, resistance of the small intestine of suckling pigs to adhesion by K88-positive E coliremained relatively stable through the five week period of nursing bacterial counts ranging from 5 × 104/g to 8 × 104/g of tissue.