Severe tissue damage and uncontrolled bacterial growth and invasion after E. coli P4 infection in neutrophil-depleted C3H/HeN mice. H&E staining of paraffin sections of mammary tissues 24 (A–B) and 48 h (C–D) after E. coli P4 infection of C3H/HeN mice rendered neutropenic by intraperitoneal injection of anti Gr-1 antibodies 24 h before infection. Normal tissue architecture is maintained and some congestion (blue arrows) is visible 24 h after infection (A–B), and neutrophils are not seen in the alveolar spaces (black arrows in A–B). Fatal septic mastitis 48 h after infection is associated with severe hemorrhages (blue arrows in C–D), massive bacterial growth in the alveolar spaces (black arrows in D) and invasion into alveolar epithelial cells which are undergoing severe swelling, cytoplasmic vacuolization and nuclear condensation (yellow arrow in D). Scale bars 200 μm (A and C), 50 μm (B) and 20 μm (D). All images are representative of the entire sample and the histological morphology and pathology results were very similar for each gland in a given mouse and between mice. (For a color version of this figure please consult www.vetres.org.)