Ultrastructure of Pasteurella haemolytica-induced changes in gnotobiotic calf lung

Gnotobiotic calves born and maintained in a germ-free environment until inoculated with a pathogen are model animals for studying the progression of a specific disease, such as pneumonic pasteurellosis. To investigate early progression of pneumonic pasteurellosis, we compared the ultrastructure of regions of gas-exchange in the lungs of three challenge-exposed and three uninoculated control gnotobiotic calves. Three calves were inoculated endobronchially with a bolus of 7.9 x 10(10) CFU of Pasteurella haemolytica A1 and studied in a specific pathogen-free environment until severe respiratory distress occurred, at which time they were euthanized. Slices of lung tissue from the midregion of the right dorsal caudal lobe (area of lesion) of infected and control calves were fixed in glutaraldehyde and prepared for scanning (SEM) and transmission (TEM) electron microscopy. SEM revealed bacteria among long tangled strands of fibrin in pulmonary alveoli that became obliterated with cellular debris. TEM revealed areas of encapsulated and/or nonencapsulated bacteria among the cellular debris and patches of fibrin. Many neutrophils and macrophages that infiltrated the alveoli had phagocytosed bacteria and undergone degradation. Less cellular damage was present when encapsulated bacteria bordered the interalveolar septa than when nonencapsulated lysed bacteria were present. Where lysed bacteria were present, the pulmonary capillaries were dilated because of swollen, degranulated neutrophils, fibrin clots, and cellular necrosis. Both encapsulated and nonencapsulated bacteria were present in the lung tissue of gnotobiotic calves within the first 24 h after endobronchial inoculation of early log phase P. haemolytica. Loss of capsular material around individual and divided pairs of bacteria led to their consequential aggregation, lysing, and severe damage to the adjacent pulmonary capillaries and interalveolar septa.