Nontypable

Nontypable is a term used in microbiology to describe strains or isolates that cannot be assigned to a serotype using standard typing methods. It is most commonly applied to Haemophilus influenzae, where nontypable refers to non-encapsulated strains that lack the polysaccharide capsule used for serotyping and therefore cannot be typed as Hib or other serotypes.

In Haemophilus influenzae, nontypable strains (NTHi) do not produce a capsule and are not classified within the serotypes a through f. They are part of the normal flora of the upper respiratory tract but can cause mucosal infections, particularly in children and individuals with underlying conditions. Common illnesses associated with NTHi include otitis media, sinusitis, bronchitis, and exacerbations of chronic obstructive pulmonary disease. Invasive disease caused by NTHi is less common than by encapsulated Hib but can occur, especially in young children, the elderly, or immunocompromised patients.

Laboratory identification of nontypable H. influenzae relies on the absence of a capsule and the use of molecular methods for confirmation, since capsule-based serotyping cannot classify these strains. Typing of NTHi typically involves non-capsular genetic markers or whole-genome analysis rather than antisera against capsules. Antibiotic resistance among NTHi strains can occur, including beta-lactamase production, which influences treatment choices. Clinically, infections caused by NTHi are often treated with antibiotics effective against non-encapsulated strains, with therapy guided by local resistance patterns and susceptibility testing.

The concept of nontypable extends to organisms other than H. influenzae when a serotype cannot be determined by standard typing methods, but the term is most widely used in reference to non-encapsulated H. influenzae in medical microbiology.