serotypes

A serotype is a distinct variation within a species of microorganisms or viruses, defined by differences in surface antigens that can be detected by serological methods. Serotyping classifies isolates to reflect immune recognition of those antigens and is widely used in epidemiology, vaccine design, and clinical microbiology.

In bacteria, serotypes arise from variation in capsule polysaccharides, lipopolysaccharide O antigens, and flagellar H antigens. Examples include Escherichia coli with many O and H serotypes; Salmonella enterica serotypes such as Typhimurium and Enteritidis; Streptococcus pneumoniae with multiple capsular serotypes; and Neisseria meningitidis serogroups A, B, C, W, and Y. In viruses, serotypes (or serogroups) are defined by antigenic differences in surface proteins, such as influenza A subtypes defined by hemagglutinin and neuraminidase (H1N1, H3N2), poliovirus serotypes 1–3, and dengue virus serotypes 1–4.

Serotyping uses serological methods such as agglutination with specific antisera, neutralization tests, and enzyme-linked immunosorbent assays, as well as molecular methods that infer serotype from genes encoding surface antigens. Cross-reactivity and convergent antigenicity can complicate interpretation, and a serotype does not always reflect the organism's overall genetic relatedness.

Applications include outbreak investigation and surveillance, vaccine design and evaluation, and tracking of antigenic shift and replacement. For example, influenza vaccine composition is guided by circulating serotypes, and Salmonella serotyping aids source attribution during outbreaks.

Terminology varies: serotype and serovar are often used interchangeably for bacteria, while serogroup is common for some pathogens. Serotyping remains a fundamental tool in microbiology and public health.