Antimicrobial Activity And Mode Of Action Of Lactic Acid Against Common Food-borne Pathogens

Microbial safety of fresh produce products remains serious public-health concern. Consumption of fresh and fresh-cut fruits and vegetables is highly recommended due to nutritional and health-related benefits, unfortunately, with the increase in produce consumption the number of produce-related outbreaks of foodborne illnesses also increased. Food-borne outbreaks associated with consumption of produce can be very costly to growers, processors and consumers. Currently, besides the sensory and nutritional qualities, consumers are paying more attention not only to the risk of food-borne pathogens but also to the safety of artificial chemical preservatives that are used to control those foodborne pathogens.At the beginning, the effectiveness of lactic acid on Salmonella, E. coli and Listeria inoculated on minimally processed lotus sprouts was studied to compare its efficacy with tap water and sodium hypochlorite solution. Also, the color of minimally processed lotus sprout was determined to evaluate the antibrowning effect of lactic acid on lotus sprouts. Then, the mechanisms of specific antimicrobial action of lactic acid against those three pathogens were investigated using physiological and morphological indices. In addition, proteomic method was used to evaluate the effect of lactic acid on the physiological function and metabolism changes of pathogens. The specific findings of this study were as follows:Results indicated that the LA concentration of 0.5% or higher was effective to reduce foodborne pathogens in lotus sprouts. When treated with 0.5% and 2.0% lactic acid,1.5 and 2.3 log reductions were achieved, respectively. The effectiveness of LA treatment on pathogens increased with the increase of its concentration. The L* values of LA treatment decreased slightly during storage. Furthermore, LA treatment contributed to slow accumulation of red color on lotus sprouts, which was more effective than NaCIO treatment to improve the color of minimally processed lotus sprouts. Results indicated that LA treatment had significantly effective antibrowning on lotus sprout (p<0.05).Three pathogens could grow well in normal media at mildly acidic environment. Additionally, pH 5.8 and 6.5 are typical pH values of many ready-to-eat (RTE) foods, and thus they are chosed to contact the following study. For three pathogens, lactic acid generally inhibited their growth more than HC1 and the control, among which 1.0% and 2.0% lactic acid could almost inhibit the growth of pathogens. The growth of pathogens in HC1 at pH 5.8 and 6.5 was similar to that of their respective control, while 0.5% and higher lactic acid content was found much more effective to inhibit the bacterial growth. Lactic acid was able to inhibit the growth of three pathogens more than HC1 at the same pH. In general, bacterial cells were damaged by several mechanisms rather than just the alteration of pH (proton concentration).As the first barrier against antimicrobials, the surface characteristics and membrane integrity of bacterial cells can be greatly affected by antimicrobials. From the zeta potential measurements,0.5% lactic acid could alter the surface charge of bacterial cells to less negative or even positive. Also, the surface hydrophobicity of Salmonella, E. coli and Listeria cells was significantly increased after lactic acid treatment (p<0.05). In addition, the results of released K+ and SEM also suggested cytoplasmic membrane damage, while the result of released K+ was consistent with that of zeta potential. Lactic acid exhibited different ways to destroy the target pathogens, including the leakage of intracellular K+, the damage of membrane permeability and integrity, as well as the changes of surface charge and hydrophobicity.Cell morphology and intracellular material changes of pathogens were used to further study the mechanism of lactic acid. The results indicated that three bacterial cells could be completely inactivated after exposure to 0.5% lactic acid for 2 h. Meanwhile, lactic acid could result in great leakage of proteins of Salmonella, E. coli and Listeria cells. Measurements of the release of proteins and SDS-PAGE confirmed the disruptive action of lactic acid on cytoplasmic membrane, as well as the content and activity of bacterial proteins. The Z-Average sizes of three pathogens were changed to smaller after lactic acid treatment. The damaged membrane structure and intracellular structure induced by lactic acid could be observed from TEM images. The results suggested that the antimicrobial effect was probably caused by physiological and morphological changes in bacterial cells.Proteomic method was used to evaluate the effect of lactic acid on the physiological function and metabolism changes of three pathogens. For Salmonella, E. coli and Listeria, 15,19 and 47 proteins were resppectively found to express differentially, and among them,10,15 and 43 proteins were successfully identified by mass spectrometry, respectively. In combination with GO and KEGG pathway analysis, it was found that energy and organic compounds metabolism or even gene expression of three pathogens were affected by lactic acid, leading to the loss or change of some physiology function and the cell death eventually.