Previous studies have shown the efficacy of oral administration of selected lactobacilli strains to treat mastitis. The objective of this study was to find microbiological, biochemical and/or immunological biomarkers of the probiotic effect. Women with (n=23) and without (n=8) symptoms of mastitis received three daily doses (109 cfu) of Lactobacillus salivarius PS2 for 21 days. Samples of milk, blood and urine were collected before and after the probiotic intervention, and screened for a wide spectrum of microbiological, biochemical and immunological parameters. In the mastitis group, L. salivarius PS2 intake led to a reduction in milk bacterial counts, milk and blood leukocyte counts and interleukin (IL)-8 level in milk, an increase in those of immunoglobulin (Ig)E, IgG3, epidermal growth factor and IL-7, a modification of the milk electrolyte profile, and a reduction of some oxidative stress biomarkers. Such biomarkers will be useful in future clinical studies involving a larger cohort.

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