Fig. 4: Immunofluorescence staining of neutrophils in mouse lung with super-infection or single infection.

Representative 200× pictures show Ly6G neutrophils near bronchi of A mock-infected mice, B influenza A-infected bronchi, C influenza-infected bronchi with low bacterial infection, D influenza A-infected bronchi and parenchymal tissue with areas of profuse bacterial infection, and E parenchymal tissue infected with S. aureus. Bronchi are depicted with a white asterisk. Scale bars represent 100 µm. Morphometric analysis shows F neutrophils per mm² and G area covered by Ly6G stain in peri-bronchial areas of non-infected and infected mice. Mice were mock infected (filled circles) or infected with influenza A (empty circles), S. aureus (filled diamonds), or co-infected with influenza A and S. aureus: peribronchial areas with dominant influenza infection (filled squares) or with viral/bacterial co-infection (empty squares). n = 1–3 200× pictures per mice from 4 mice per group. Statistical significance was calculated by a two-tailed t-test. *p < 0.05; **p < 0.005; ****p ≤ 0.0001. The error bar indicates the standard deviation of the data from different mice.