Legionnaire's disease is caused most commonly by-

Ans. is 'a' i.e., Legionella pneumophila * Most common species infecting man is L pneumophila (most common serogroup 1). Other important species is L. micdadei (Pittsburgh pneumonia agent) which is partial acid fast.* At least 14 serogroups of L. pneumophila have been identified.* L. pneumophila serogroup 1 (SG 1) accounts nearly all severe infections.* L. pneumophila serogroup 6 is more commonly involved in hospital - acquired legionnaire's disease.