SUMMARY: Seven phages highly active and against one or other of seven bovine enteropathogenic strains of belonging to six different serotypes were isolated from sewage. Severe experimentally induced diarrhoea in calves could be cured by a single dose of 10 phage organisms. It could be prevented by doses as low as 10, by spraying the litter in the calf rooms with aqueous phage suspensions or simply by keeping the calves in uncleaned rooms previously occupied by calves whose infections had been treated with phage. Microbiological examinations of calves used in these experiments revealed that the phage organisms multiplied rapidly and profusely after gaining entry to the -infected small intestine, quickly reducing the to numbers that were virtually harmless. The only phage-resistant that emerged in the studies on calves infected with one or other of the seven strains were K~. These organisms were much less virulent than the K organisms from which they were derived and did not present a serious problem in calves given adequate amounts of colostrum. Infections produced by oral inoculation of a mixture of six strains of the could be controlled by administration of a pool of the six phages that were active against them but, in general, the control was less complete than that observed in the single-strain infections. K phage-resistant bacteria emerged in some of the calves used in these mixed infections and they were as virulent as their parent organisms; evidence from studies suggested that they might have arisen by genetic transfer between organisms of the different infecting strains. Infections produced by these K mutants and their parents could be controlled by the use of mutant phages derived from phages that were active on their parents. During the experiments with mixed infection, an extraneous phage active against one of the six strains suddenly appeared in calves kept in the same rooms. Microbiological examinations revealed that this phage was effectively controlling the multiplication of organisms of that particular strain of in the small intestines of the calves.


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