Effect of NaCl on Enterotoxin Production and Invasion Efficiency of Staphylococcus aureus

Introduction: Although many processed foods contain NaCl and the foods are occasionally contaminated with Staphylococcus aureus, which results in foodborne illness, the effect of NaCl on the pathogenicity of Staphylococcus aureus has not been studied.

Purpose: The objective of this study was to evaluate the effect of NaCl on the enterotoxin production and Caco-2 cell invasion efficiency of S. aureus.

Methods: Staphylococcus aureus strains NCCP10826, ATCC27664, ATCC13565, ATCC23235, and ATCC14458 were inoculated in to 10 ml tryptic soy broth plus 0, 2, 4, and 6% NaCl at 35°C  for 24h, followed by plating 0.1ml of the culture on tryptic soy agar plus 0, 2, 4, and 6% NaCl. After incubation at 35°C for 24h, the colonies of S. aureus on plates were suspended in phosphate buffered saline and diluted to OD₆₀₀=0.1. The diluents were further used to measure a staphylococcal enterotoxin by a immunoassay and to evaluate Caco-2 cell invasion efficiency.

Results: For enterotoxin production, there were no correlations between NaCl concentration and enterotoxin production for all S. aureus strains. Of five strains, the adhesion efficiency of S. aureus ATCC14458 was significantly increased (P < 0.05) up to 6.44% at 2% NaCl. In addition, the invasion efficiency of S. aureus ATCC14458 significantly increased (P < 0.05) up to 9.1% at 6% NaCl, which was 2.3 times higher (P < 0.05) than that of 0% NaCl.  

Significance: These results suggest that NaCl in food may increase the cell invasion efficiency of Staphylococcus aureus ATCC14458.