CHARACTERIZATION OF 2 UNUSUAL CLINICALLY SIGNIFICANT FRANCISELLA STRAINS

We have isolated two phenotypically distinct nonfastidious Francisella strains (Fx1 and Fx2) from the blood of compromised patients with pne umonia and compared them with eight other Francisella strains, includi ng Francisella tularensis biovar tularensis, F. tularensis biovar novi cida, and F. philomiragia. Our isolates grew well on sheep blood agar, chocolate agar, modified Thayer-Martin agar, and Trypticase soy agar. Fx1 and Fx2 were determined to be within the Francisella genus by cel lular fatty acid analysis and by the utilization of glucose, productio n of H2S and catalase, and lack of motility, oxidase, nitrate reductas e, and gelatinase. They were additionally shown to belong to the speci es F. tularensis by sequencing of two variable regions comprising appr oximately 500 nucleotides of the 16S rRNA gene. Also, RNA probe hybrid ization confirmed their belonging to the species F. tularensis. Howeve r, the new strains, which are not identical, are distinguished from ot her F. tularensis strains by growth characteristics, repetitive extrag enic palindromic PCR fragment pattern, and some biochemical tests. Key biochemical differences included tire findings that Fx1 was positive for beta-galactosidase and arabinose hydrolysis and that both strains were citrulline ureidase positive and glycerol negative. Commercial F. tularensis antiserum agglutinated stock F. tularensis strains but not Fx1, Fx2, F. tularensis biovar novicida, or F. philomiragia; serum fr om either patient failed to agglutinate or only weakly agglutinated co mmercial antigen but showed agglutination when tested against each pat ient's respective isolate. Fx1 and Fx2 produced beta-lactamase. Becaus e of their good growth, negative serology, and biochemical profile, th e organisms could be misidentified in the clinical laboratory if stand ard strategies or commercial identification systems are used.