Kefir is an acidic milk product in which some bacteria and yeasts can live in harmony and weak alcoholic fermentation occurs. In addition to the positive impact of yoghurt on the pathogenic microorganisms, there are also some studies on the ability of reducing the negative effects of antibiotics on intestinal flora ( Coşkun, 2006). It has been reported that antibacterial effect of yoghurt results from the lactic acid produced by cultures ( Kilic, 1990 Gülmez et al., 2003). It has been revealed that the antibacterial effect of bacteria is used in yoghurt production against various food pathogens. thermophilus cultures are used in its production. Yoghurt is a dairy product consumed extensively in Turkey. Early in the 20th century, the Nobel-winner Russian scientist, Elie Metchnikoff (1845-1916) laid the foundation for the studies in this field by attributing the longevity of Bulgarian peasants who consume a lot of fermented dairy products ( Hung et al., 2008 Sanders, 2003).
Probiotics are known as some beneficial yeasts and bacteria, especially lactic acid bacteria, Fermented dairy products (yoghurt, kefir etc.) have played an important role in human nutrition for centuries, when administered in adequate amounts, confer a health benefit on the host. Comparing to the antimicrobial effect of 0.9% lactic acid which was used as positive control, our results concluded that the bacteria found in fermented dairy products have an antibacterial activity and this activity is not caused by only lactic acid. It was determined that the most sensitive pathogenic bacteria to dairy products were Salmonella typhimurium whereas the least sensitive pathogen was Pseudomonas aeruginosa.
Homemade yoghurt showed the maximum antibacterial effect against the pathogens. Antibaterial effects of the samples were tested at 24, 48 h and the 7 day intervals. In this study, the antibacterial effects of homemade yoghurt, commercial yoghurt, commercial kefir and probiotic yoghurt on viability of Salmonella typhimurium (ATCC 14028), Escherichia coli (ATCC 25922), Pseudomonas aeruginosa (ATCC 27853) and Staphylococcus aureus (ATCC 25923) was investigated by using disc diffusion method.
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