Analysis of virulence of clinical isolates of Salmonella enteritidis in vivo and in vitro

Salmonella enterica serotype Enteritidis (S. enteritidis) is a major food-b orne pathogen, and its incidence among all Salmonella serotypes has increas ed dramatically in the last two decades. To study the virulence characteris tics of clinical isolates of S. enteritidis, we determined the 50% lethal d oses (LD50) in mice of isolates of two major phage types (4 and 8), Isolate s of both phage types showed a wide range of LD50 after oral inoculation, v arying from under 10(2) organisms to over 10(8) organisms. No significant d ifference in LD50 was observed between the phage types. These observations indicated that clinical isolates of S, enteritidis are highly heterogeneous in their ability to cause death in mice. We compared the LD(50)s of these isolates to the results observed from in vitro pathogenicity assays. We als o analyzed these isolates for recognized Salmonella virulence loci (spv, so dCI, sopE, and sef), The in vitro phenotypes of the isolates showed no obvi ous correlation with their LD50 in any given assay, and the virulence genes tested were present in all isolates. However, the isolate with the lowest LD50 (isolate 97A 2472) was resistant to acidified sodium nitrite (ASN), Mo reover, the most acid-susceptible, macrophage-susceptible, and ASN-suscepti ble isolates were attenuated for virulence in mice. These results, based on extensive analysis of clinical isolates of S, enteritidis, demonstrate the complex nature of Salmonella pathogenesis in mice. Our results also indica te the limitation of in vitro assays in predicting in vivo virulence.