Replication kinetics of Salmonella enteritidis in internal organs of ducklings after oral challenge: a quantitative time-course study using real-time PCR

Spontaneous excision of the Salmonella enterica serovar Enteritidis-specific defective prophage-like element phiSE14

Salmonella enterica serovar Enteritidis has emerged as a major health problem worldwide in the last few decades. DNA loci unique to S. Enteritidis can provide markers for detection of this pathogen and may reveal pathogenic mechanisms restricted to this serovar. An in silico comparison of 16 Salmonella genomic sequences revealed the presence of an approximately 12.5-kb genomic island (GEI) specific to the sequenced S. Enteritidis strain NCTC13349. The GEI is inserted at the 5' end of gene ydaO (SEN1377), is flanked by 308-bp imperfect direct repeats (attL and attR), and includes 21 open reading frames (SEN1378 to SEN1398), encoding primarily phage-related proteins. Accordingly, this GEI has been annotated as the defective prophage SE14 in the genome of strain NCTC13349. The genetic structure and location of phiSE14 are conserved in 99 of 103 wild-type strains of S. Enteritidis studied here, including reference strains NCTC13349 and LK5. Notably, an extrachromosomal circular form of phiSE14 was detected in every strain carrying this island. The presence of attP sites in the circular forms detected in NCTC13349 and LK5 was confirmed. In addition, we observed spontaneous loss of a tetRA-tagged version of phiSE14, leaving an empty attB site in the genome of strain NCTC13349. Collectively, these results demonstrate that phiSE14 is an unstable genetic element that undergoes spontaneous excision under standard growth conditions. An internal fragment of phiSE14 designated Sdf I has been used as a serovar-specific genetic marker in PCR-based detection systems and as a tool to determine S. Enteritidis levels in experimental infections. The instability of this region may require a reassessment of its suitability for such applications.

Quantitative analysis of Salmonella Enteritidis loads in ducklings after nasal inoculation

The distribution patterns of Salmonella Enteritidis in systemic organs, which have not been described previously, should be studied to better understand its pathogenesis in vivo. We inoculated the ducklings with Salmonella Enteritidis via the nasal route and performed a real-time PCR assay for determining the concentration of Salmonella Enteritidis DNA and studied the histopathology of various tissues postinoculation. The results show that the Salmonella Enteritidis load in systemic organs has a close correlation with the progression of disease. Further, rapid dissemination and active replication of Salmonella Enteritidis in multiple systemic organs accelerated the progression of disease.