Effects of Ultra-high Pressure Processing (UHP) on Biofilm Formation of Listeria monocytogenes

Introduction: Ultra-high pressure processing (UHP) provides a valuable alternative to thermal pasteurization during food processing, especially for Ready-to-Eat products. Among pathogenic microorganisms, Listeria monocytogenes is a major concern of Ready-to-eat products. Although previous studies proved that the inactivation of L. monocytogenes is effective after ultra-high pressure processing, a small amount of the L. monocytogenes may still live and the formation of its biofilm on the surface induces serious food safety issues.

Purpose: The aim of the work was to investigate the effect of ultra-high pressure processing (UHP) on the biofilm formation of L. monocytogenes.

Methods: L. monocytogenes (initial count of 10⁹ CFU/ml) was subjected to ultra-high pressure treatment at 100 to 500 MPa for 15 min at room temperature (20°C), and then was incubated in BHI for 24 h, 48 h and 72 h at 37°C. Microbial survival curves and growth curves were measured and compared with the amount of the biofilm after the ultra-high pressure treatment. Crystal violet staining assay was used to measure the biofilm production of L. monocytogenes in different time periods.

Results: After ultra-high pressure treatment, L. monocytogenes counts reduced from 9 log CFU/ml to 2.4 log CFU/ml at 500 Mpa. During subsequent culturing, although L. monocytogenes grew fast and the count increased to 8.4 log CFU/ml in 24 h as the same count as the control group (without ultra-high pressure treatment), the biofilm formation was still low after ultra-high pressure treatment. The absorbance value (OD_{595 nm}) of biofilm from the treated samples was 0.24, 0.15, and 0.12 after 24 h, 48 h and 72 h, respectively, while the control group was 0.27, 0.29, and 0.37, respectively.

Significance: This study suggests that ultra-high pressure has effects on the biofilm formation of L. monocytogenes, and it may weaken the biofilm formation capacity of L. monocytogenes and reduce the food safety problems.