Yersinia enterocolitica is a bacterium capable of growth at 48C in donated blood and has been responsible for many deaths following transfusion. Interaction of Y. enterocolitica with blood cells is of interest in understanding the mechanisms of survival and growth in blood. The closely related organism Y. pseudotuberculosis is known to invade platelets and cause platelet aggregation by a mechanism that involves expression of the chromosomal inv gene. Yersinia isolates were made to express green¯uorescent protein (GFP) and their interaction with platelets was studied by¯ow cytometry. Y. enterocolitica did not cause platelet aggregation or activation, not even when grown at 228C to maximise inv expression. Attachment of Y. enterocolitica O:9 to platelets occurred with virulence plasmid-bearing (pYV ) strains grown at 378C but not with pYV 2 strains nor with strains grown at 228C. Y. pseudotuberculosis containing inv did cause platelet activation and aggregation when grown at 228C, as has been shown before, but also showed enhanced attachment to platelets when grown at 378C. Electron microscopy studies con®rmed that inv-expressing Y. pseudotuberculosis invaded platelets but Y. enterocolitica attached only to the outer surface of platelets. Interaction of Y. enterocolitica O:9 with platelets provided a modest protection against bacterial killing by human serum. Interaction of Y. enterocolitica O:9 with platelets does not lead to platelet invasion or activation, and is mediated through plasmid-coded factors, not inv.

tion-inhibition, neutralization, and complementfixation tests were performed on sera of rabbits inoculated with Venezuelan equine encephalomyelitis (VEE) virus in combination with Freund's adjuvants and in Hank's salt solution. This study, indicated that the complete adjuvants (i.e., with mycobacteria) considerably increased the antibody response to VEE virus. MIycobacterium butyricum (M. smegmatis) appeared to be more effective than MI. tuberculosis H37Ra. In the absence of mycobacteria, the response was much less pronounced. Paper electrophoretic

Hemagglutination and fluorescent antibody (FA) are compared for the direct detection of virus devoid of host cells. A determination was made of the minimal number of tissue plaque-forming units of Venezuelan equine encephalomyelitis virus that could be detected by the hemagglutination technique. Similar concentrations of the virus in bovine albumin borate saline, Brain Heart Infusion broth (Difco), and demineralized water were tested by the FA technique. Somewhat higher concentrations of the virus in bovine albumin borate saline were used in the hemagglutination-inhibition test. The quantitative hemagglutination procedure employed for these studies was carried out at 37 C for 75 min with variations in concentration of goose red cells. As a result of lowering the red cell concentration, smaller concentrations of virus were detected. The direct FA staining procedure applied to slide preparations containing known numbers of tissue culture plaque-forming units of virus was negative. Adsorbed viral antigen on agglutinated goose erythrocytes was visualized by direct and indirect FA techniques.

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