Virulence in the chick model and stress tolerance of Salmonella enterica serovar Orion var. 15+

Three Salmonella enterica serovar Orion var. 15+ isolates of distinct prove nance were tested for survival in various stress assays. All were less able to survive desiccation than a virulent S. Enreritidis strain, with levels of survival similar to a rpoS mutant of the S. Enteritidis strain, whereas one isolate (F3720) was significantly more acid tolerant. The S. Orion var. 15+ isolates were motile by flagellae and elaborated type-1 and curli-like fimbriae; surface organelles that are considered virulence determinants in Salmonella pathogenesis. Each adhered and invaded HEp-2 tissue culture cel ls with similar proficiency to the S. Enteritidis control but were signific antly less virulent than S. En teritidis in the one-day-old and seven-day-o ld chick model. Given an oral dose of 1 x 10(3) cfu to one-day-old chicken, S. Orion var. 15+ isolates colonised 25% of liver and spleens examined at 24 h whereas S. Enteritidis colonised 100% of organs by the same with the s ame dose. Given an oral dose of 1 x 10(7) cfu at seven-day old, S. Orion va r. 15+ failed to colonise livers and spleens in any bird examined at 24 h w hereas S. Enteritidis colonised 50% of organs by the same with the same dos e. Based on the number of internal organs colonised, one of the three S. Or ion var. 15+ isolates tested (strain F3720) was significantly more invasive than the other two (B1 and B7). Also, strain F3720 was shed less than eith er B1 or B7 supporting the concept that there may be an inverse relationshi p between the ability to colonise deep tissues and to persist in the gut. T hese data are discussed in the light that S. Orion var. 15+ is associated w ith sporadic outbreaks of human infection rather than epidemics.