Unencapsulated Haemophilus influenzae--what kind of pathogen?

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Haemophilus influenzae are small, gram-negative, rod-shaped bacteria. Because of their special growth requirements, they do not grow on usual blood agar media, but flourish on the mucosal membranes of the human respiratory tract where they adhere to the epithelial cells by fimbriae (a potential vaccine component). Nasopharyngeal carriage of Haemophilus influenzae is very common, and in healthy carriers the bacteria are usually unencapsulated. The outer membrane of Haemophilus influenzae contains lipopolysaccharide (of so called R form, without O antigen) and major outer membrane proteins. The lipopolysaccharide is a virulence determinant. An extracellular enzyme, IgA protease, is another potential virulence determinant. The outer membrane of Haemophilus influenzae is a rather ineffective barrier towards antibiotics, and thus the major determinants of antibacterial resistance in Haemophilus influenzae are plasmid-coded enzymes that inactivate the antibiotic, and changes in the target molecules.