Purpose: In this study, an outbreak of Salmonella Enteritidis involving 112 cases in Ningbo, China was investigated with a combination of genotypic sub-typing methods and phenotypic analysis.
Methods: All isolates from this outbreak were sub-typed by Pulsed-Field Gel Electrophoresis (PFGE) and Multiple-Locus Variable number of tandem repeat Analysis (MLVA), to track and to confirm that this outbreak was associated with contaminated mousse cakes. Low temperature stress tolerance was determined and these isolates were tested for their antibiotic resistance, virulence genes and plasmid replicon types, to identify the pathogenic factors and potential hazards of this outbreak-associated Salmonella Enteritidis.
Results: The Pulsed-Field Gel Electrophoresis (PFGE) and Multiple Locus Variable number tandem repeat Analysis (MLVA) profiles showed that most of the outbreak clinical isolates (22/23) were indistinguishable from each other and were identical to the isolates obtained from implicated mousse cakes, demonstrating that this outbreak of gastroenteritis was caused by Salmonella Enteritidis-contaminated mousse cakes. Moreover, all isolates, irrespective of source, had an identical antibiotic susceptibility pattern. Five virulence-associated genes in Salmonella pathogenicity islands (SPI) and the plasmid-associated virulence genes spvB/C were present in both the food and clinical isolates.
Significance: All of these isolates can survive well under low temperature treatment, indicating that manufacturers of foodstuffs with raw ingredients (not subjected to thermal process) should use an effective approach to prevent or eliminate the microbial hazards to public health.