Nontypable Haemophilus influenzae are unencapsulated both in vivo and in vitro.

Some investigators have suggested that nontypable Haemophilus influenzae isolated from sputum of adults with pneumonia are variant forms of typable H influenzae that have lost their capsule during passage in vitro. We examined colonies of both typable and nontypable H influenzae after they had been grown in vitro, as well as bronchopulmonary secretions from patients with pneumonia or acute, purulent tracheobronchitis due to H influenzae; electron microscopy combined with ruthenium-red staining was used to detect the presence of capsular glycocalyx. H influenzae types a, b, and e', whether grown in vitro or observed directly in bronchopulmonary secretions, had readily detectable capsular glycocalyx external to the cell membrane. In contrast, non-typable H influenzae appeared to be unencapsulated after cultivation in vitro or when directly visualized in bronchopulmonary secretions of infected patients.