Elsevier

Food Control

Volume 47, January 2015, Pages 231-236
Food Control

Antibacterial mechanism of lactic acid on physiological and morphological properties of Salmonella Enteritidis, Escherichia coli and Listeria monocytogenes

https://doi.org/10.1016/j.foodcont.2014.06.034Get rights and content

Highlights

  • Pathogens could be completely inactivated after exposure to lactic acid.

  • Lactic acid resulted in great leakage of protein of three pathogens.

  • Bacterial protein bands of lactic acid-treated cells got fainter or disappeared.

  • Z-Average sizes of pathogens were changed to smaller after lactic acid treatment.

  • Lactic acid caused collapsed or even broken cells with obvious pits and gaps.

Abstract

Lactic acid is widely used to inhibit the growth of important microbial pathogens, but its antibacterial mechanism is not yet fully understood. The objective of this study was to investigate the antibacterial mechanism of lactic acid on Salmonella Enteritidis, Escherichia coli and Listeria monocytogenes by size measurement, TEM, and SDS-PAGE analysis. The results indicated that 0.5% lactic acid could completely inhibit the growth of Salmonella Enteritidis, E. coli and L. monocytogenes cells. Meanwhile, lactic acid resulted in leakage of proteins of Salmonella, E. coli and Listeria cells, and the amount of leakage after 6 h exposure were up to 11.36, 11.76 and 16.29 μg/mL, respectively. 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.

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.

Section snippets

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)

Cited by (223)

View all citing articles on Scopus
1

Tel.: +86 27 87671045; fax: +86 27 87278373.

2

Tel.: +86 27 87270170; fax: +86 27 87270170.

View full text