White spot syndrome virus (WSSV) was specifically detected by PCR in Penaeus merguiensis hemocytes, hemolymph and plasma. This suggested a close association between the shrimp hemolymph and the virus. Three types of hemocyte from shrimp were isolated using flow cytometry. Dynamic changes of the hemocyte subpopulations in P. merguiensis at different times after infection were observed, indicating that the WSSV infection selectively affected specific subpopulations. Immunofluorescence assay (IFA) and a Wright-Giemsa double staining study of hemocyte types further confirmed the cellular localization of the virus in the infected hemocytes. Electron microscopy revealed virus particles in both vacuoles and the nucleus of the semigranular cells (SGC), as well as in the vacuoles of the granular cells (GC). However, no virus could be detected in the hyaline cells (HC). Our results suggest that the virus infects 2 types of shrimp hemocytes -GCs and SGCs. The SGC type contains higher virus loads and exhibits faster infection rates, and is apparently more susceptible to WSSV infection.

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Phage AR1, previously known to infect Escherichia coli O157:H7 with high specificity, was further characterized for its genetic properties. The phage DNA sequences including capsid genes and a putative α-glucosyltransferase gene (α-gt) have been deduced. These sequences are conservative but not identical to those of T4 phage. However, a nonessential gene, SegD, organized within the capsid gene cluster of T4 is missing in the corresponding region of AR1 genome, and this characteristic has not been observed among T-even related phages. The difference between AR1 and T4 was further exemplified by their distinct host ranges. Strains of E. coli O157:H7 collected from different sources were permissive to AR1 but resistant to T4 that normally infects K-12 strains of E. coli through contact with the outer membrane protein OmpC. Thus, the O157:H7 strains must have a varied OmpC. Indeed, the OmpC sequence of O157:H7 strains was proved to differ from that of K-12 strains by a total of 15 amino acid substitutions and two gaps (a five-residue deletion and a four-residue insertion). The OmpC molecules are relatively conserved across the gram-negative bacteria, and this is the first time OmpC divergence has been found within the same E. coli species. Since OmpC is located in the outer membrane and its expression is regulated by environmental conditions, alteration of the structure in pathogenic O157:H7 strains may have biological significance.

We expressed three Actinobacillus pleuropneumoniae ApxI deletion derivatives to map the domain that could induce protective immunity. Antiserum to ApxI N-terminal covered by residues 40 to 380 was found to neutralize ApxI hemolytic activity but not ApxIII cytotoxicity. When used as a subunit vaccine in mice, this recombinant N-terminal fragment elicited protection against lethal infection with heterologous A. pleuropneumoniae serovars