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A factor (CPFI) that caused a cytopathic effect on cultures of pig-thyroid cells was detected in bacteria-free lysates of K88-positive porcine enteropathogenic strains of Escherichia coli. CPFI was produced only by strains that synthesised heat-labile enterotoxin (LT) and was present in lysates that contained LT but was not associated with heat-stable enterotoxin, K88 antigens or haemolytic substances. CPFI was transmitted with LT from a donor to recipient strains; the factor was heat labile, non-dialysable, precipitated by ammonium sulphate and excluded from Sephadex G-200. CPFI was neutralised by some adult pig sera. Another factor (CPFII) that caused a morphologically different cytopathic effect on cultures of pig-thyroid cells was detected in bacteria-free lysates of six apparently non-pathogenic strains of E. coli isolated from pigs. CPFII was not related to the above products of E. coli and was not neutralised by pig sera. It is suggested that CPFI is similar to LT and that the cytopathic effect of CPFI could provide a simple, sensitive assay that may be of value in the characterisation of LT.
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