Nontypeable

Nontypeable is a microbiological term used to describe strains that cannot be assigned to a defined serotype using standard typing methods, usually because they lack the capsule or other surface antigens used for typing. The term is most commonly used with Haemophilus influenzae, where capsule-based serotyping identifies types a through f. Strains that do not possess a polysaccharide capsule are nonencapsulated and are referred to as nontypeable Haemophilus influenzae (NTHi).

NTHi are common inhabitants of the human upper respiratory tract and are a frequent cause of mucosal infections, including acute otitis media (ear infections), sinusitis, and bronchitis, and they contribute to exacerbations of chronic obstructive pulmonary disease. Invasive disease due to NTHi is less common than disease caused by encapsulated Hib, but can occur in vulnerable individuals.

Biology and typing: Because NTHi lack a capsule, capsule-based serotyping cannot classify them. Typing and epidemiological studies rely on molecular methods such as multilocus sequence typing or whole-genome sequencing, and on analyses of outer membrane proteins and lipooligosaccharide profiles. The absence of a capsule and variation in other surface components influence virulence, colonization, and antibiotic resistance, as well as biofilm formation.

Clinical implications: The Hib vaccine protects against disease caused by encapsulated Hib strains but does not protect against NTHi, so NTHi infections remain clinically important. Diagnosis involves culture on enriched media (often chocolate agar with X and V factors) and identification of H. influenzae, with further typing by molecular methods. Treatment follows guidelines for the site of infection and local resistance patterns; resistance to beta-lactams and other antibiotics is increasingly reported. Ongoing research aims to develop vaccines and therapies targeting NTHi-specific antigens.