Nontypable Haemophilus influenzae are Unencapsulated both in Vivo and in Vitro

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Abstract
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, nontypable H influenzae appeared to be unencapsulated after cultivation in vitro or when directly visualized in bronchopulmonary secretions of infected patients.