In this study, the investigators will try to confirm if application of probiotic strains isolated from breast milk actually have a beneficial effect on women suffering lactational mastitis. This project has been design to offer an integrated vision of the effects of probiotherapy (Lactobacillus salivarius PS2) on the human host. Therefore, the investigators propose a multidisciplinary approach involving the application of microbiological, immunological, genomic, metagenomic, transcriptomic and metabolomic techniques. The hypothesis is that probiotherapy will cause different effects on the host, and the objective is the finding of markers that may support the beneficial effect of the strain in such condition.

Inclusion Criteria:

• Normal term pregnancy
• Lactating women
• Healthy breastfed infant

Women with mastitis:

• Clinical symptoms of mastitis
• Painful breastfeeding
• Count of staphylococci, streptococci and/or corynebacteria in milk higher than 3,000 colony-forming units/mL
• Leukocyte count in milk higher 6 log10/mL

Women without mastitis:

• No clinical symptoms of mastitis
• No painful breastfeeding
• Count of staphylococci, streptococci and/or corynebacteria in milk lower than 500 colony-forming units/mL
• Leukocyte count in milk lower 5 log10/mL

Exclusion Criteria:

• Allergy to cow's milk protein
• Intolerance to lactose
• Antibiotic treatment
• Breast abscess
• Raynaud syndrome
• Any parallel disease

This study was designed to evaluate a high number of microbiological, biochemical or immunological parameters before and after the administration of a probiotic to women with mastitis and to elucidate which of them can be useful as biomarkers in future studies. The number of participants was considered suitable for such purpose.