Antibacterial mechanism of lactic acid on physiological and morphological properties of Salmonella Enteritidis, Escherichia coli and Listeria monocytogenes
Introduction
As reported by the Chinese Center for Disease Control and Prevention (CCDC), the food poisoning events in 2012 that accounts for 56.1% of the total resulted from microorganisms including Salmonella, Bacillus cereus, Vibrio parahaemolyticus, pathogenic Escherichia coli, etc (CCDC, 2013). As a human restricted pathogen, Salmonella typhi causes 21 million cases of typhoid fever and 200,000 deaths each year, particularly in the Indian subcontinent, Southeast Asia, Africa and Central America (Devi, Nisha, Sakthivel, & Pandian, 2010). Pathogenic E. coli normally inhabits the intestines of humans, causing illnesses, such as hemorrhagic colitis and hemolytic uremic syndrome (Ibrahim et al., 2006, Jo et al., 2007, Smigic et al., 2009, Velazquez et al., 2009). Listeria monocytogenes also causes much concern to the food industry, since it was associated with large-scale outbreaks (Byelashov et al., 2010). Listeriosis is a prominent foodborne infection in several South African outbreaks (Borges, Ferreira, Saavedra, & Simoes, 2013). In this regard, the control for pathogens has become a major concern for manufactures of ready-to-eat (RTE) foods, especially for products with extended shelf-lives (Borges et al., 2013, Lues and Theron, 2012).
In recent years, antimicrobial resistance has become a serious public health problem, which can be attributed to the use and overuse of antibiotics and transmission of resistance within and between individuals of microorganisms (Andersson, 2003, Andersson and Levin, 1999, Baquero et al., 1998, Borges et al., 2013, Guillemot, 1999, Monroe and Polk, 2000). During the last decade, antibiotics are losing their effectiveness due to the rapid appearance of antibiotic-resistant strains (Saleem et al., 2010). Therefore, efforts are being made to discover efficient natural antimicrobials. As natural and traditional antimicrobials, organic acids have long been used as food additives and preservatives to prevent food deterioration, to extend shelf lives of perishable foodstuffs, even to control microbial contamination and dissemination of foodborne pathogens during unsatisfactory producing and processing (Borges et al., 2013, Hsiao and Siebert, 1999, Ricke, 2003, Wang et al., 2013). The antibacterial mechanism for organic acids are not fully understood, while their antibacterial activity may vary depending on physiological status of the organism and the physicochemical characteristics of the external environment (Ricke, 2003).
In the present study, the antibacterial activity of lactic acid was assessed against both gram-negative (S. Enteritidis and E. coli) and gram-positive (L. monocytogenes) pathogens. The objective was to investigate, using physiological and morphological indices, the mechanisms of specific antimicrobial action of lactic acid against S. Enteritidis, E. coli and L. monocytogenes. It would further illustrate the antibacterial activity of lactic acid on those pathogens.
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Bacterial strains and growth conditions
Salmonella Enteritidis (ATCC 14028) and L. monocytogenes (ATCC 19115) were obtained from the State Key Laboratory of Agricultural Microbiology at Huazhong Agricultural University (Wuhan, China), while pathogenic E. coli (CGMCC 1.2385) (Li et al., 2012) was obtained from the China General Microbiological Culture Collection Center (CGMCC) (Beijing, China). For convenience, the cultures were preserved at −25 °C with 50% (v/v) glycerin solution at a ratio of 1:1. Working cultures of three pathogens
Antimicrobial activity of lactic acid
The effects of lactic acid on the growth of Salmonella, E. coli and Listeria were investigated by performing in vivo antimicrobial susceptibility experiments. Bacteria, at initial concentrations of ∼107 CFU/mL, were incubated in lactic acid for 6 h prior to plating to determine viability. Except for Listeria (Fig. 1c), exposure to 0.25% lactic acid completely killed Salmonella and E. coli after 2 h incubations. Also, 0.5% lactic acid had the potential to inactivate the Salmonella and E. coli
Conclusions
In general, the results indicated that S. Enteritidis, E. coli and L. monocytogenes 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
Acknowledgments
The authors would like to thank the financial support: 1) National Natural Science Foundation of China (Grant No. 31172294); 2) Fundamental Research Funds for the Central Universities (Project 2013PY096), China; 3) Ministry of Scientific and Technology (Grant No. 2012BAD28B06), China.
References (33)
Persistence of antibiotic resistant bacteria
Current Opinion in Microbiology
(2003)- et al.
The biological cost of antibiotic resistance
Current Opinion in Microbiology
(1999) A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding
Analytical Biochemistry
(1976)- et al.
Reduction of Listeria monocytogenes on frankfurters treated with lactic acid solutions of various temperatures
Food Microbiology
(2010) - et al.
Eugenol (an essential oil of clove) acts as an antibacterial agent against Salmonella typhi by disrupting the cellular membrane
Journal of Ethnopharmacology
(2010) Antibiotic use in humans and bacterial resistance
Current Opinion in Microbiology
(1999)- et al.
Inhibition of Listeria monocytogenes by pomegranate (Punica granatum) peel extract in meat paté at different temperatures
Food Control
(2012) - et al.
Modeling the inhibitory effects of organic acids on bacteria
International Journal of Food Microbiology
(1999) - et al.
Application of caffeine, 1,3,7-trimethylxanthine, to control Escherichia coli O157:H7
Food Chemistry
(2006) - et al.
Combined treatment with silver ions and organic acid enhances growth-inhibition of Escherichia coli O157: H7
Food Control
(2007)