Bacterial communities are associated with intracellular neutrophils in mammary epithelial cells. Lactating C57BL/6 TLR2−/− (A) or C3H/HeJ (B–D) mice were challenge by approximately 1000 cfu via the teat canal. Mammary gland cryosections stained with DAPI (blue) and phalloidin-TRITC (red) (A), thin sections (1 µm) stained with toluidine blue (B) and TEM (C, D). Scale bars 10 µm (A), 20 µm (B), and 2000 nm (C, D). Intracellular neutrophils (white arrows in A) are demonstrated using confocal microscopy (showing a single Z-stack; see also Additional file 1), toluidine blue staining (yellow arrows in B) and TEM (black arrows in C) and many of these neutrophils are infected with bacteria. Inflammation is characterized by massive neutrophil recruitment into the alveolar spaces (yellow arrows in A and black arrows in B) interacting with free and phagocytosed bacteria. IBC of GFP-producing bacteria are visible in the alveolar epithelial cells (green arrow in A and white arrows in B). Free cytosolic bacteria (yellow arrow in D) are visible in comparison with bacteria still associated with remnants of hosting neutrophils (black and white arrows in D). All images are representative of the entire sample. The histological morphology and pathology results were very similar for each gland in a given mouse and between mice.