Plasmid-mediated serum resistance and alterations in the composition of lipopolysaccharides in Salmonella dublin

Mechanism of resistance to complement-mediated killing of bacteria encoded by the Salmonella typhimurium virulence plasmid gene rck

We find that pADEO16, a recombinant cosmid carrying the rck gene of the Salmonella typhimurium virulence plasmid, when cloned into either rough or smooth Escherichia coli and Salmonella strains, confers high level resistance to the bactericidal activity of pooled normal human serum. The rck gene encodes a 17-kD outer membrane protein that is homologous to a family of virulence-associated outer membrane proteins, including pagC and Ail. Complement depletion, C3 and C5 binding, and membrane-bound C3 cleavage products are similar in strains with and without rck. Although a large difference in C9 binding was not seen, trypsin cleaved 55.7% of bound 125I-C9 counts from rough S. typhimurium with pADEO16, whereas only 26.4% were released from S. typhimurium with K2011, containing a mutation in rck. The majority of C9 extracted from rck strain membranes sediments at a lower molecular weight than in strains without rck, suggesting less C9 polymerization. Furthermore, SDS-PAGE analysis of gradient peak fractions indicated that the slower sedimenting C9-containing complexes in rck strains did not contain polymerized C9 typical of the tubular membrane attack complex. These results indicate that complement resistance mediated by Rck is associated with a failure to form fully polymerized tubular membrane attack complexes.

Non-typhoid Salmonella bacteraemia in Greater Copenhagen 1984 to 1988

A retrospective survey of non-typhoid Salmonella bacteraemia in the period 1984 to 1988 was carried out by the five departments of clinical microbiology in Greater Copenhagen. A total of 168 patients were identified. A gradual increase was observed from 11 cases in 1984 to 58 cases in 1988. The corresponding incidence per 100,000 inhabitants in Copenhagen rose from 0.9 in 1984 to 5.0 in 1988. During the same period the total registered incidence of human Salmonella infections in Denmark increased from 17.6 to 67.4 per 100,000 inhabitants. The serotype most often isolated from bacteraemic patients was Salmonella dublin followed by Salmonella enteritidis and Salmonella typhimurium. Salmonella dublin demonstrated enhanced invasive and pathogenic properties. Predisposing factors were present in 56% of the patients; the most common was malignant disease. A fatal or complicated course of the bacteraemia was observed more frequently in patients with underlying diseases than in persons who had previously been healthy. A total of 17% of the patients died; one-fifth of these had a ruptured aortic aneurysm. It is concluded that the substantial increase in the number of cases and the often serious course taken by the infection demonstrate a need for increased efforts at prophylaxis.

Isolation of the replication and partitioning regions of the Salmonella typhimurium virulence plasmid and stabilization of heterologous replicons

Although the virulence plasmid of Salmonella typhimurium has a copy number of one to two per chromosome, plasmid-free segregants are produced at a rate less than 10(-7) per cell per generation. Three regions appear to be involved in the maintenance of this virulence plasmid. The first two, repB and repC, are functional replicons hybridizing with IncFII and IncFI plasmids, respectively, neither exhibiting the segregational stability of the parent virulence plasmid. The third region, par, cloned as a 3.9-kilobase Sau3A fragment, is not a functional replicon but exhibits incompatibility with the virulence plasmid. Subsequent tests revealed the ability of this 3.9-kilobase par insert to increase the stability of pACYC184 in S. typhimurium from less than 34% to 99% plasmid-containing cells after 50 generations. In addition, the par region increased the stability of oriC, R388, and repC replicons in both S. typhimurium and Escherichia coli hosts. The par region encodes 44,000- and 40,000-molecular-weight proteins essential for the Par+ phenotype but not for the Inc+ phenotype. Although actual sequestering of plasmids within the cell was not demonstrated, all results indicate that the par region described is an actual partitioning locus, similar in organization to those described for plasmids F, P1, and NR1.