nonserogroupable

Nonserogroupable is a term used in microbiology to describe bacterial isolates that cannot be assigned to any standard serogroup by routine serological methods. It is most often applied to Neisseria meningitidis, where serogroups are defined by the structure of the capsular polysaccharide. An isolate is nonserogroupable when it does not react with available antisera against known capsule types, either because it lacks a capsule (unencapsulated) or because the capsule is present but not detected due to antigenic variation or alterations in capsule biosynthesis.

In meningococci, serogrouping relies on recognition of capsule polysaccharides. Nonserogroupable strains may result from mutations or deletions in capsule biosynthesis genes, phase variation that suppresses capsule expression, or the presence of a capsule that is poorly expressed under testing conditions. Although encapsulated strains are typically associated with invasive disease and broader serogroup classifications, nonserogroupable strains can be colonizers and, less commonly, cause disease.

Detection and significance: Nonserogroupable isolates complicate epidemiological surveillance and vaccine impact assessments because serogroup distribution guides public health responses. When standard serogrouping fails, molecular methods—such as PCR targeting capsule synthesis genes or whole-genome sequencing—can determine capsule presence and help refine typing beyond serogroup designations.

See also: Capsule (bacteria), Serogrouping, Neisseria meningitidis, Bacterial typing.