Closely related microorganisms that share common, measurable surface components are said to belong to a serogroup. Each serogroup of L. pneumophila contains further subtypes that have different surface structures on the cell membrane and can be distinguished by special tests.
- Evidence indicates that some L. pneumophila serogroups are more virulent than others.
- L. pneumophila serogroup 1 is the most frequently identified form of the bacterium isolated from patients with Legionnaires' disease, although other serogroups and subtypes of the bacterium are frequently isolated from water sources.
- Serogroups 4 and 6 are the second and third types most frequently linked with Legionnaires' disease.