Presence of bacteria in milk does not indicate infection, non-infective bacterial colonization can be distinguished by bacterial infection by an immune reaction of detecting specific antibodies IgA or IgG on the surface of the bacteria

Bacterial colonization of infant and breast occurs soon after birth which is mostly infected by Staphylococcus aureus

Commensal /non-pathogenic organisms enter the infant from the mother which is helpful in defense mechanism of the body, which is called bacterial interference. 

Epidemic puerperal mastitis occurs when the infant and the mother is exposed to unfamiliar or virulent microorganisms, it is an hospital acquired disease

Infection of microorganisms may occur by any one of the route

• Hematogenous spread by lactiferous ducts into lobe.
• Nipple fissures by entering the lymphatic system.