Control of foodborne pathogens and their biofilms by levulinic acid and sodium dodecyl sulfate
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The efficacy of commonly used sanitizers in food processing facilities is reduced when organic matter is present. If the sanitizing procedures are inadequate, surviving microorganisms could attach and form biofilms given time and nutrients. The objective of this study was to validate the antimicrobial efficacy of levulinic acid (LVA) and sodium dodecyl sulfate (SDS) on deli slicers, and to determine the effectiveness of this treatment for inactivating foodborne pathogens in single- and mixed-species biofilms. LVA + SDS was applied at three concentrations (0.5% LVA + 0.05% SDS, 1% LVA + 0.1% SDS, and 2% LVA + 0.5% SDS) on slicers pre-contaminated by Listeria monocytogenes, Salmonella, and Escherichia coli O157:H7 (ca. 8.5 log CFU/blade) at 21oC. Sampling and enumeration were conducted after a treatment time of 0, 1, 2, 3, 5, 10, and 20 min. Single- and mixed-species biofilms were incubated at 21°C for 72 h before being treated with different concentrations of LVA plus SDS (0.5% LVA + 0.05% SDS, 1% LVA + 0.1% SDS, and 3% LVA + 2% SDS). The bactericidal activity of the LVA with SDS treatment was concentration dependent on both the slicers and in biofilms. Contaminated slicer surfaces sprayed with 1% LVA plus 0.1% SDS as a foam (45-55 psi) reduced within 1 min 6.0 to 8.0 log CFU of the pathogens/blade. Greater than 6.9 log CFU of pathogens/coupon in single-species biofilms were reduced within 10 min by 3% LVA plus 2% SDS. E. coli O157:H7 and Salmonella were antagonistic to each other, being significantly (P < 0.05) more sensitive to LVA plus SDS in mixed-species biofilms than in single-species biofilms. Microscopic images of LVA plus SDS-treated biofilms captured by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the cells were detached from the biofilm matrix and the integrity of cell envelopes was disrupted. Photomicrographs captured by confocal laser scanning microscopy (CLSM) confirmed that more pathogens in the biofilms were detached when the treatment concentrations were increased. Results of this study provide a better understanding of the antimicrobial behavior of LVA plus SDS to foodborne pathogens on slicers and in their biofilms.