F. tularensis on CHOC at 72 hrs/CDC
F.tularensis Gram stain/CDC
If tularemia is suspected, clinicians should notify the laboratory so that special precautions can be taken. Cultures of appropriate specimens should be collected before antimicrobial therapy is started. Appropriate specimens include aspirates of lymph nodes or ulcers, sputum, and pharyngeal washings. Growth is slow in broth, so blood cultures should be held up to three weeks if F. tularensis is suspected. Blood cultures are often negative.
Growth of Francisella tularensis is definitive for diagnosis. Cultures should be held for 14 days, but small colonies can usually be seen by 72 hours of incubation. This fastidious organism can grow poorly or not at all on sheep's blood agar (SBA). The organism prefers cystein-enriched media, such as chocolate (CHOC), Thayer-Martin (TM), buffered charcoal-yeast extract (BCYE), and thioglycollate (THIO), which will support good growth. The small gram-negative coccobacilli are oxidase and urease negative and weakly catalase positive.
Diagnosis can also be made by PCR, direct fluorescent antibody, or immunohistochemical staining. Diagnosis by serology is not clinically useful because acute and convalescent sera should be drawn four weeks apart.