Wild-type strains of Providencia species were evaluated for their ability to invade HEp-2 monolayers based upon microscopic and semi-quantitative assays. Of 14 P. alcalifaciens strains tested, 3 (17%) were found to be highly invasive, 4 (22%) moderately invasive, and the remaining 61% weakly or noninvasive. HEp-2 invasion results were confirmed by thin-section electron microscopy. Invasive capabilities of P. alcalifaciens were greater at higher MOIs (100 to 1000) than at lower inocula (<10 MOI). No strain of P. stuartii or P. rettgeri tested invaded HEp-2 cells. Quantitative assays of Triton X-100-lysed, HEp-2-invaded cells indicated that between 0.001% and 0. 013% of the initial bacterial inoculum was gentamicin resistant. Further testing of select strains on various cell lines indicated the efficiency of invasion was Vero > Y1 > INT-407 > HEp-2. Two isolates recovered from a father and son with prolonged diarrhea after returning from Mexico were found to be identical on the basis of biotype, serotype, and genotype. These results provide additional evidence that some P. alcalifaciens strains cause gastroenteritis.

Two unusual cases of Aeromonas infection are described, one associated with bacteremia (Aeromonas schubertii) and another in which the organism was recovered from an infected gall bladder (Aeromonas veronii biotype veronii). These strains were initially identified as Vibrio damsela andVibrio cholerae by the Vitek and API 20E systems, respectively. Use of appropriate screening tests and familiarity with the newer Aeromonas species could prevent initial misidentifications and potential public health consequences.

Vibrio hollisae, one of the more recently described halophilic Vibrio species, is infrequently associated with gastrointestinal disease and only rarely recovered from individuals presenting with gram-negative sepsis. In this report we describe two cases of severe gastrointestinal disease associated with V. hollisae in otherwise healthy individuals. In one of these individuals, severe epigastric pain was apparently associated with signs of pseudoappendicitis, necessitating exploratory surgery. In both individuals, infection was associated with the ingestion of raw shellfish. These cases are discussed in light of previous reports on the epidemiology, pathogenesis, and spectrum of disease caused by this unusual pathogen.