JOURNAL

OF

CLINICAL MICROBIOLOGY, Jan. 1991,

p.

Vol. 29, No. 1

200-201

0095-1137/91/010200-02$02.00/0 Copyright C) 1991, American Society for Microbiology

Serological Cross-Reaction between Legionella pneumophila and Citrobacter freundii in Indirect Immunofluorescence and Rapid Microagglutination Tests J. J. GRAY,* K. N. WARD, R. E. WARREN, AND M. FARRINGTON Clinical Microbiology and Public Health Laboratory, Addenbrooke's Hospital, Hills Road, Cambridge CB2 2QQ, United Kingdom Received 15 August 1990/Accepted 16 October 1990

We describe the first case in which a diagnostic rise in titers of antibody to formalinized Legionella pneumophila serogroup 1 yolk sac antigen was shown to be caused by a cross-reaction between L. pneumophila and another organism, Citrobacterfreundii.

phila indirect immunofluorescence (IF) (7) with formalinized yolk sac L. pneumophila serogroup 1 antigen and rapid microagglutination (RMAT) (3) antibody tests (Division of Microbiological Reagents and Quality Control) showed greater-than-fourfold rises in antibody titers from