Purpose: This study aimed to develop a method to determine the adhesion force of Salmonella at a large scale in a low moisture environment, and validate using individualized bacterium adhesion data.
Methods: Salmonella Enteritidis PT 30 cells were attached on square stainless steel coupons (#304, 5 cm2) of different surface finishes (984 nm and 9.34 nm root mean square (RMS) roughness) in five ml of phosphate buffer solution (PBS), with gentle stirring for two hours, after which coupons were removed and desiccated overnight. Samples were then centrifuged at relative centrifugal forces (RCF) varying from 0 to 40,000×g; the higher the centrifugal force, the more bacteria detach. Bacteria remaining on the coupons were recovered and enumerated on modified trypticase soy agar.
Results: The initial Salmonella populations on the rough and smooth stainless steel coupons were 4.01±0.05 log CFU/cm2 and 4.38±0.07 log CFU/cm2, respectively, which indicating that a greater (P<0.05) number of bacteria adhered to the smoother surface. After centrifuging at 2990×g, the rough and smooth surfaces resulted in Salmonella populations of 3.84±0.19 log CFU/cm2 and 3.62±0.32 log CFU/cm2 on the surface, respectively, with the latter being reduced significantly (P<0.05). These results suggest that surface roughness has a significant effect (~10 times) on the force required to detach bacteria (approximated value of 29.3 nN compared to 3.2 nN for rough and smooth surfaces, respectively).
Significance: Quantifying the adhesion forces at the individual bacterium scale can help develop and validate first-principle based cross-contamination models.