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Phages encoding production of Vero cytotoxins VT1 or VT2 were isolated from strains of Escherichia coli of human and bovine origin. Two human strains of serotype 0 1 57 : H7 produced both VT1 and VT2 and each carried two separate phages encoding either VT1 or VT2. The phages were morphologically similar to each other and to a VT2 phage previously isolated from a strain of serotype 0157 : H-; all had regular hexagonal heads and short tails. The phages had similar genome sizes and DNA hybridization and restriction enzyme digestion showed that the DNAs were very closely related. This contrasts with another report that one of the strains tested (933) released two clearly distinguishable phages separately encoding VTl and VT2. The 0157 phages differed from a VT1 phage isolated from a bovine E. coli strain belonging to serotype 026 : H11 and from the reference VT1 phage isolated previously from a human strain, H19, of serotype 026 : H 1 1. The two 026 phages were morphologically similar with elongated heads and long tails. They had similar genome sizes and DNA hybridization indicated a high level of homology between them. Hybridization of an 0157 phage DNA probe to DNA of the 026 phages, and vice versa, showed there was some cross-hybridization between the two types of phage. A phage from a bovine strain of serotype 029 : H34 had a regular hexagonal head and short tail resembling those of the 0157 phages. The DNA was distinguishable from that of all the other phages tested in restriction digest patterns but hybridized significantly to that of an 0157 phage. Hybridization of the phage genomes with VTl and VT2 gene probes showed that sequences encoding these toxins were highly conserved in the different phages from strains belonging to the three serogroups.

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We evaluated the reliability of a rapid human immunodeficiency virus type 1 test for quick clinical decision making, such as in needle-stick accidents. The test was evaluated with 1,160 patients. It proved to be a simple and useful test with 99.6% specificity and 99.4% sensitivity. One patient with late-stage AIDS had a falsenegative result.