<prompt> Legionnaires disease is caused most commonly by-Legionella pneumophilaLegionella micdadeiPittsburgh agentNone of the above  <response>: 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 legionnaires disease. 