Comparison of virulence states between hyperinfectious salmonellae and other clinical isolates following laboratory culture and animal passage.

aAll (184) Salmonella human and animal isolates tested were recovered from different outbreaks or individual cases submitted to diagnostic laboratories, or from surveillance submissions of on-farm healthy animals [20]. Eighty-one of these strains harbored the virulence plasmid necessary for systemic disease [46] but exhibited an i.p. virulence defect in a mouse model of typhoid fever; of these isolates, 14 were virulent by the oral route of infection. Conventionally virulent S. Typhimurium reference strain 14028 was used in all studies for comparison.bIn vitro/in vivo passage. In vitro passage. Bacteria derived from overnight stationary phase cultures containing LB medium were used to infect BALB/c mice via the oral or i.p. route of infection as described in Materials and Methods. In vivo passage. Bacteria (109 cells) derived from stationary phase cultures containing LB medium were used to orally infect mice. Five to seven days post-infection, bacteria derived from spleens harvested from acutely infected animals (108 to 109 CFU/g of spleen determined by direct colony count) were used, without ex vivo growth, to infect naïve mice via the oral or i.p. route of infection as described in Materials and Methods.cLD50 virulence assay. The dose required to kill 50% of infected animals (LD50) was determined via the oral (via gastrointubation) and i.p. routes by infecting at least 10 mice as described in Materials and Methods.dCompetitive Index (CI) virulence assay. An equivalent dose (500 bacterial cells) of a test strain and a Lac+ derivative of S. Typhimurium reference strain 14028 (MT2057) was co-administered i.p. to at least 5 mice; the CI value is the ratio of test strain/reference wild-type strain recovered from target tissue (spleen) divided by the input ratio [28] as described in Materials and Methods.