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Subject: Francisella Tularensis Culture (Rule out)
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Francisella tularensis is a slow-growing, fastidious, gram-negative bacillus capable of producing severe infection, including localized and systemic disease. Infections have typically been acquired by zoonotic tick-borne transmission or direct contact. The common reservoir for organisms includes rabbits, rodents, deer, squirrels, and other wild mammals. Domestic animals may harbor the organism. This organism is easily transmissible, so it is critical that the laboratory be informed whenever tularemia is suspected. Typical disease syndromes include glandular, oculoglandular, and ulceroglandular tularemia; oropharyngeal tularemia; typhoidal tularemia; and pneumonic tularemia. There is great concern regarding the use of this organism for a bioterror-related attack.
This culture is used to isolate F. tularensis from clinical specimens.
Specimens for F. tularensis isolation should be inoculated onto cysteine-enriched agar media. Blood–cysteine–glucose agar is recommended; most clinical isolates will grow on chocolate, Thayer-Martin, and nonselective buffered charcoal–yeast extract (BCYE) agar. Enriched broth media, such as thioglycollate broth, should also be inoculated. Blood agar and MacConkey agar are typically inoculated for clinical specimens for isolation of other possible pathogens.
Because of the risk of laboratory-acquired infection and because isolation of F. tularensis may represent a sentinel event in a bioterror attack, most clinical microbiology laboratories limit the workup of suspected isolates to simple tests to rule out suspicious colonies, referring isolates that fail to “rule out” to their local public health laboratory for identification and further characterization. Final results for testing, therefore, may be delayed compared to common bacterial isolates.
Turnaround time: Isolation and a preliminary identification are usually available within 3–6 days. Additional time is required for transfer to the local public health laboratory, confirmation of identification, and further testing.
Lymph node aspirate, ulcerative lesions, sputum, BAL, or other localized specimens, depending on the clinical presentation, are usually submitted for diagnosis.
Culture of multiple specimens from different infected tissues may improve detection.
Blood cultures are recommended for evaluation of patients with suspected tularemia.
Serologic testing is recommended for diagnosis in patients with suspected tularemia.
Expected results: Negative. After the acute phase of infection, cultures may become negative. Tularemia cannot be confidently ruled out by negative cultures.
Positive: Isolation of F. tularensis is diagnostic of tularemia. Tularemia is a reportable disease; positive cultures must be reported to the local department of health.
Because F. tularensis organisms are tiny and faintly staining, direct detection by Gram stain of clinical specimens is uncommon. Cultures late in infection may be negative. Serologic diagnosis may be helpful in patients with disease consistent with tularemia, but negative cultures.
The diagnosis of tularemia may not be entertained until after the most acute phase of illness, a time when cultures are less likely to be positive.
Clinicians may fail to request specific cultures for tularemia or alert the laboratory of their clinical suspicion.