PROBIOTIC STRAINS IN YOGURT EFFECTIVE AGAINST FOOD-BORNE PATHOGENS

Most health-conscious consumers know that yogurt, as a dairy-based beverage, can be a good source of probiotic bacteria.  Functional yogurt can therefore be used as delivery vehicle to the target consumer of probiotic bacterial strains selected on the basis of their prophylactic and bio-therapeutic potential to combat intestinal pathogens. Development of such so-called functional yoghurt however, requires cognizance, consideration and conformance to the regulatory framework provided by South Africa’s draft regulation on labelling and advertising of foodstuffs (R.429/2014) and guidelines on probiotics by the European Food Safety Authority (EFSA).

The term probiotic is defined as “live microorganisms which upon ingestion in sufficient numbers exert health benefits beyond inherent general nutrition”. Potential health benefits include antimicrobial activity against pathogens, reduction in carcinogen-generating faecal enzymes in vivo; stimulation of the immune system; suppression of tumor formation; reduction of cholesterol levels; and protection against diarrhoea associated with antibiotic treatment or food allergies.

Most probiotic strains in this context belong to the Lactobacillus and Bifidobacterium genera, but it is important to carefully select strains of such organisms that will actively inhibit pathogens such as E. coli, Candida albicans and Listeria monocytogenes in the gastrointestinal tract of the consumer. In a previous study, antibacterial activities by strains of Lactobacillus rhamnosus, Lb. casei and Lb. plantarum were demonstrated against test strains of E. coli and Salmonella Typhimurium and to a lesser extent against Staphylococcus aureus. Against C. albicans, the extent of inhibition depended not only on the probiotic strain, but also on the specific C. albicans strains. In the study cited, a range of Lactobacillus and Bifidobacterium strains have been investigated for their potential to inhibit the growth of a range of clinical and reference indicator pathogens.

The results showed that the probiotic strains of dairy origin, i.e. Lactobacillus acidophilus D and L. rhamnosus V, and of bovine origin, i.e.  Lactobacillus pentosus LIP and Lactiplantibacillus plantarum VLL1, as well as L. acidophilus ATCC 4536 exhibit good inhibitory activities against the pathogenic E. coli species Enterohemorrhagic E. coli (EHEC), Enteroaggregative E. coli (EAEC), Enteroinvasive E. coli (EIEC), and Enterotoxigenic E. coli (ETEC). Furthermore, these probiotic strains exhibit good inhibitory activities against the C. albicans strains C. albicans ATCC 10231, C. albicans 1051255, and C. albicans M0826. Of significance is that these probiotic strains remain viable in yogurt during normal shelf-life. For example, the selected probiotic strains L. acidophilus, D, L. rhamnosus V, and L. plantarum VLL1 maintained viability above the minimum therapeutic levels (10⁶ CFU/g) during normal shelf-life in the presence of yoghurt starter cultures. In addition, the antimicrobial properties of L. acidophilus and L. rhamnosus were further evaluated on two pathogens (commonly known to contaminate dairy products) each. L. acidophilus showed the ability to inhibit the growth of E. coli and Salmonella Typhimurium during in vitro dual culture assay and the soft agar overlay method as well as in yoghurt containing unusually higher levels (around 7 log₁₀ CFU/ml) of the two pathogens during 21-day shelf-life under refrigeration. Similarly, L. rhamnosus also inhibited the growth of S. aureus and L. monocytogenes during in vitro dual culture assay and the soft agar overlay method as well as in yoghurt during normal shelf-life.

In conclusion: These findings show that the incorporation of probiotic strains into yoghurt could help the dairy industry control commonly known pathogens such as E. coli, Salmonella, S. aureus, L. monocytogenes, and C. albicans, thereby minimizing the risk of food contamination and subsequent illnesses and thus reducing the risks of product recalls. In addition, these probiotics can potentially serve as natural preservatives, reducing the need for artificial preservatives. The mechanism behind the antimicrobial activity of probiotics is likely due to the production of organic acids, bacteriocins, hydrogen peroxide, and bioactive metabolites such as peptides, short chain fatty acids and exopolysaccharides.