Summary.A total of 63 Aeromonas strains isolated from diarrhoeal faeces or water samples were tested for adhesion to HEp-2 cells. An association between diarrhoea and high level adhesion was observed in that 12 of the 34 faecal isolates and none of the 29 environmental isolates yielded >20 bacteria per HEp-2 cell in the adhesion assay. The proportion of high adherers was significantly greater for A . sobria (57%) than for A . hydrophila isolates (19%). Three of the eight faecal A . caviae isolates were also found to be high adherers. All of the environmental isolates were heavily pilated with pili having a mean diameter of 5 nm and a mean length of 420 nm; these were termed type-S pili. Of the 34 faecal isolates, 32% possessed S pili, and 68% were lightly pilated with up to 15 thin, flexible type-L pili, of mean diameter 2-5 nm and mean length 960 nm. Type-L pilation was associated with a high level of HEp-2 cell adhesion, and was more common in A . sobria and A . caviae than in A . hydrophila isolates. These results suggest that adherence to HEp-2 cells is a useful model for the investigation of Aeromonas enteropathogenicity, and that adhesion may be pilusmediated in this organism.

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Disease progression in human immunodeficiency virus type-1 (HIV-1)-infected individuals is frequently accompanied by declining CD4 cell numbers and the acquisition of a T-tropic (X4) or dual tropic (R5X4) phenotype. Understanding the mechanism of CD4 cell loss in HIV-1 infection is essential for the development of effective therapeutic strategies. In this study, donor populations of peripheral blood mononuclear cells (PBMCs) were selected for their ability to support an equivalent acute infection by both R5 and X4 virus phenotypes. This demonstrated that CD4+ T-lymphocyte loss was due to the gp120 region of Env and was replication independent. Furthermore, apoptosis was only detected in cells infected with an X4 virus after the majority of CD4+ T-lymphocyte loss had occurred. These observations indicate that the CD4+ T-lymphocyte loss in an X4 HIV-1 infection is not directly mediated by apoptosis, although apoptosis may be induced in the remaining cell population as a consequence of this CD4+ T-lymphocyte loss.

Contrary to earlier reports, we have found that tri- and hexapeptides analogous or homologous with segments of the 23-residue N-terminal fusion sequence (FS) of the viral transmembrane glycoprotein gp41 (residues 517-539) did not significantly inhibit HIV-1-induced syncytium formation, using an uninfected cell-infected cell fusion assay. In contrast, we found that the high molecular weight apolipoprotein A-1 and a 23-residue analog of the FS, with the phenylalanine residues at positions 524 and 527 replaced with alanine residues, were effective inhibitors. Although the tripeptides were ineffective as inhibitors of syncytium formation, we found a number of them inhibited red cell lysis induced by the synthetic peptide AVGIGALFLGFLGAAGSTMGARS (based on the HIV-1 gp41 FS). This effect was also seen with apolipoprotein A-1. The Ala524,527 analog of the fusion sequence could not be tested in this system because it was hemolytic. We concluded that the smaller peptides were effective inhibitors of hemolysis because they interfered with pore formation by the fusion sequence peptide, either by disrupting the pores or by preventing the peptide from adopting the alpha-helical conformation found in the pores. On the other hand, membrane fusion, which is a prelude to syncytium formation, has been shown to require the fusion sequence in the beta-strand conformation. We argue that small peptides would be unable to block interaction between such strands, although larger molecules, such as apolipoprotein A-1 and the Ala524,527 analog, would be able to do so and thus inhibit fusion. It seems, therefore, that a successful drug directed against the FS-cell membrane interaction stage of syncytium formation would need to be of relatively high molecular weight and complexity.