Bacterial counts in mammary tissue homogenates after intramammary infection with E. coli P4 in C3H/HeN (A), iNOS −/− (B) and IL1R −/− (C–D) mice. Data represent individual mouse bacterial counts and horizontal bars represent mean bacterial counts (A–B). Total bacterial counts 24 and 48 h after challenge of normal, macrophage depleted and neutrophil-depleted C3H/HeN mice (A) demonstrating the unrestricted overgrowth of bacteria after neutrophil depletion. Differential counts of total, extracellular and intracellular bacteria in mammary tissues of iNOS −/− mice 24 and 48 h after challenged (B) demonstrating epithelial invasion of bacteria 48 h after challenge. Horizontal bars represent the means, n ≥ 6/group and p values were calculated using the non-parametric Mann–Whitney two-independent-samples test for comparison of means. Epithelial invasion in IL1R −/− mice is reflected by differential counts of total (T), extracellular (E) and intracellular (I) bacteria in mammary tissue of IL-1βR −/− mice 24 (C) and 48 h (D) after challenge using drop plating of the dilutions indicated of tissue homogenates (1:10 to 1:10⁷). The challenge dose (C) of the experiment is presented on drop plate C. (For a color version of this figure please consult www.vetres.org.)