Type III Secretion Genes in Aeromonas salmonicida subsp. salmonicida Are Located on a Large Thermolabile Virulence Plasmid

Abstract

Type III secretion genes in Aeromonas salmonicida subsp. salmonicida are located on a large plasmid of approximately 140 kb. Cultivation of this organism at elevated temperatures such as 25°C can, however, result in loss of this plasmid. This is accompanied by a loss of virulence for cultured fish cells.

A Type III Secretion System Is Required for Aeromonas hydrophila AH-1 Pathogenesis

“…These results indicate that the TTSS gene cluster is located on the chromosome of A. hydrophila AH-1, which is similar to the case for other pathogens such as P. aeruginosa (46) and P. luminescens (42). A. hydrophila AH-1 is unlike A. salmonicida, in which the TTSS is located on a large thermolabile plasmid of approximately 140 kb (39).…”

“…salmonicida (referred to here as A. salmonicida), has been reported to have a functional TTSS located on a large thermolabile virulence plasmid (5,39). Since motile aeromonads and A. salmonicida are in the same genus, it is possible that TTSSs are also present in other Aeromonas species, including A. hydrophila.…”

Type III Secretion System Genes in Clinical Aeromonas Isolates

“…In addition to several virulence factors' having been investigated in Aeromonas (7,8,16,19,24,27,31,32), genes for a putative type III secretion system (TTSS) were recently identified in this genus (4,5,25). The TTSS is common in pathogenic strains of gram-negative bacteria (enteropathogenic Escherichia coli, Salmonella enterica, Shigella flexneri, Yersinia spp., and Pseudomonas aeruginosa), and the cluster of genes encoding it is frequently included in genomic regions called pathogenicity islands (13,30).…”

Contribution of the type III secretion system (TTSS) to virulence of Aeromonas salmonicida subsp. salmonicida

“…Despite the presence of multiple virulence systems, until recently no single system appeared to contribute significantly to virulence, as shown by the retention of virulence by strains deficient in any given system (Ellis et al, 1988;Olivier, 1990;Vipond et al, 1998;Fernandez et al, 1998). A type III secretion system (TTSS) in A. salmonicida has been described recently (Burr et al, 2002(Burr et al, , 2003aStuber et al, 2003) and appears to be the exception to this rule (Burr et al, 2003b(Burr et al, , 2005.…”

“…In common with other bacteria, the A. salmonicida TTSS consists of bacterial innerand outer-membrane secretory pores, a host-cell translocation pore and a number of effector molecules. Unlike the yersiniae, where the TTSS is carried on a single 70 kb plasmid (pYV), the various genes of the TTSS of A. salmonicida are carried both on plasmids and chromosomally (Burr et al, 2002;Stuber et al, 2003). Two laboratory-derived TTSS-deficient strains of A. salmonicida JF2267 have been described as avirulent in a rainbow trout (Oncorhynchus mykiss) challenge model.…”