Pulmonary clearance of encapsulated and unencapsulated Haemophilus influenzae strains

A mouse model system was employed to investigate the temporal pattern of pulmonary clearance of Haemophilus influenzae and to evaluate the effect of the type b polysaccharide capsule on this clearance pattern. The lungs of BALB/c mice were inoculated with boluses of several different H. influenzae strains via an endobronchial catheter. A fully encapsulated H. influenzae type b strain multiplied readily in the lungs for at least 6 h and then was eventually cleared from the lungs over the next 18 h. The pulmonary clearance pattern obtained with an unencapsulated variant of this H. influenzae type b strain was identical to that obtained with the fully encapsulated parent strain. Two nontypable H. influenzae strains isolated by transtracheal aspiration of patients with acute H. influenzae pneumonia also multiplied in the lung and resisted significant clearance for at least 6 hr after inoculation. Bolus deposition of either H. influenzae type b or nontypable H. influenzae in the lungs resulted in an eventual influx of polymorphonuclear leukocytes into the alveoli. The observed delay in clearance of all these strains suggests that resident host defense mechanisms must be augmented for clearance to occur. Furthermore, these data indicate that one or more factors other than the Haemophilus capsule are important bacterial determinants of pulmonary clearance of H. influenzae.