Dietary influence of kefir on microbial activities in the mouse bowel D. Marquina 1 , A. Santos 1 , I. Corpas 2 , J. Mun ˜ oz 1 , J. Zazo 1 and J.M. Peinado 1 1 Department of Microbiology III and 2 Department of Animal Physiology, Biology Faculty, Complutense University of Madrid, Madrid, Spain 339/01: received 20 November 2001, revised 8 April 2002 and accepted 15 April 2002 D. MARQUINA, A. SANTOS, I. CORPAS, J. MUN ˜ OZ, J. ZAZO AND J.M. PEINADO. 2002. Aims: In this work the microflora present in kefir, a fermented milk product, was studied together with the effect of kefir administration on different groups of indigenous bacteria of mouse bowel. Methods and Results: Kefir microflora was composed of lactic acid bacteria, acetic acid bacteria and yeasts. Yeast population was composed of Saccharomyces cerevisiae, S. unisporus, Candida kefir, Kluyveromyces marxianus and K. lactis. The streptococci levels in kefir treated mice increased by 10-fold and the levels of sulfite-reducing clostridia decreased by 100-fold. The number of lactic acid bacteria increased significantly. Conclusions: The administration of kefir significantly increased the lactic acid bacteria counts in the mucosa of the bowel. Ingestion of kefir specifically lowered microbial populations of Enterobacteriaceae and clostridia. Significance and Impact of the Study: This is the first long-term study about the effects of the kefir administration on the intestinal microflora of mice. INTRODUCTION The term probiotic refers to live microorganisms that reside in the gastrointestinal tract and have beneficial effects on the host (De Simone et al. 1991; Lee and Salminen 1995). More than four hundred species of bacteria are estimated to survive in the gastrointestinal tract and these endogenous bacteria comprise the intestinal microflora. Even more species may be present but not culturable by the traditional plate count methods. After passage through the stomach and the small intestine, some probiotics survive and become established transiently in the large bowel. In order to survive in and colonize the gastrointestinal tract, probiotic bacteria need to express high tolerance to acid and bile and to have the ability to adhere to intestinal surfaces (Lee and Salminen 1995; Kirjavainen et al. 1998; Fujiwara et al. 2001). Possible health effects of probiotics include immune system stimulation, alleviation of lactose intolerance, hypo- cholesterolaemic effect, and prevention of cancer recurrence (De Simone et al. 1991; Reddy and Rivenson 1993; Lee and Salminen 1995). Probiotics have been shown to possess inhibitory activity toward the growth of pathogenic bacteria such as Escherichia coli, Listeria monocytogenes, Salmonella spp., and others (Harris et al. 1989; Ashenafi 1991; Chateau et al. 1993; Letellier et al. 2001). This inhibition is suppo- sedly due to the production of inhibitory compounds such as bacteriocins, hydrogen peroxide, organic acids and compet- itive adhesion to the epithelium. Kefir is a fermented milk which originated in the Caucasus countries. This acidic fermented milk is slightly carbonated and has the presence of small amounts of alcohol. Kefir distinguishes itself from the traditional fermented milks (yogurt) in that it is made only from kefir grains, which are composed of bacteria and yeasts (Marshall and Cole 1985). Kefir beverage contains live microflora from the grains, which are removed by filtration after the fermentation process. Kefir grains are groups of lactic acid bacteria (lactobacilli, lactococci and Leuconostoc), acetic acid bacteria (Acetobacter aceti) and yeasts (Saccharo- myces cerevisiae, Candida kefir, Kluyveromyces marxianus) held together by a matrix of different exopolysaccharides (kefiran). Correspondence to: Domingo Marquina, Department of Microbiology III, Biology Faculty, Complutense University of Madrid, 28040 Madrid, Spain (e-mail: dommarq@bio.ucm.es). ª 2002 The Society for Applied Microbiology Letters in Applied Microbiology 2002, 35, 136–140