Immunologic Diagnosis of Leglonnaires' Disease: Cross-Reactions with Anaerobic and Microaerophilic Organisms and Infections Caused by Them

Abstract

Three hundred twenty-four isolates of anaerobic and microaerophilic organisms were examined with a polyvalent direct immunofluorescent antibody (DFA) reagent directed against Legionella pneumophila, serogroups (Sa) I–IV. Three of 53 isolates of Bacteroides fragilis cross-reacted with the sa I component of the reagent. Rabbit antisera to three sa I strains of L. pneumophila cross-reacted with these three B. fragilis isolates, and cross-reactivity was not due to preexisting rabbit antibodies; antisera to a fourth sa I strain did not cross-react with these isolates. Indirect fluorescent antibody (lFA) examinations of convalescent-phase sera from eight of 41 patients with B. fragilis group infections showed titers of ⩾1:128 or a fourfold or greater seroconversion against L. pneumophila, sa I. All eight patients had bacteremia due to B. fragilis, but five B. fragilis isolates available from these patients were DFA-negative. Thus, false-positive reactions may result from DFA as well as IFA examinations of sera from patients with B. fragilis infections.