The effect of physico-chemical treatment in reducing <i>Listeria monocytogenes</i> biofilms on lettuce leaf surfaces

The purpose of this research was to characterize <i>Listeria monocytogenes</i> from several environmental and clinical sources and assess the efficacy of single and combined physico-chemical treatments in reducing biofilm on lettuce leaves. PCR analysis of <i>L. monocytogenes</i> isolates collected from different clinical (10 strains) and environmental sources (12 strains) was used to look for the presence of one <i>Listeria</i>-specific gene and five virulence genes. Biofilms of <i>L. monocytogenes</i> were developed on lettuce leaves over 24 h. A 5-min ultrasound and a 300-ppm sodium hypochlorite (NaOCl) wash resulted in similar reductions in cell numbers of 0.82 log CFU cm⁻². For chlorine dioxide (ClO₂) at 60 ppm, the cell numbers were reduced by ∼5.45 log CFU cm⁻². A combined treatment of 5 min of ultrasound plus 300 ppm NaOCl or 40 ppm ClO₂, provided maximal efficacy, reducing the number of <i>L. monocytogenes</i> on the lettuce surface to non-detectable levels. Therefore, ClO₂ has the potential to replace NaOCl for the disinfection of food products in the food industry.