Gram Negative Coccobacilli
A 21-yrs-old student complained of malaise, low – grade fever, and a harsh cough. CXR revealed a diffuse interstitial pneumonia in the left lobes of the lung. The WBC count was normal. The student had been ill for a week. The following laboratory data were available within two days. Cold agglutinins – negative; complement fixation (Mycoplasma pneumoniae) – 1:64; viral culture – pending, but negative to date; bacterial culture of sputum on blood agar and MacConkey’s agar – normal oral florA. In order to confirm the diagnosis, which of the following procedures could be ordered to achieve a specific and sensitive diagnosis?
|A||A DNA probe to the 165 ribosomal RNA of an organism lacking a cell wall|
|B||Another viral culture in one week|
|C||A repeat CF test in five days|
|D||A repeat cold agglutinin test|
a. M. pneumoniae causes a respiratory infection known as primary atypical pneumonia, or walking pneumonia.
b. Until recently, diagnostic tests have been of limited value. Up to 50% of cases may not show cold agglutinins, an insensitive and nonspecific acute-phase reactant. However, if cold agglutinins are present, a quick diagnosis can be made if signs and symptoms are characteristic.
c. Complement fixation tests that measure an antibody to a glycolipid antigen of M. pneumoniae are useful but not routinely performed in most laboratories. Also, cross-reactions may occur.
d. Culture of M. pneumoniae, while not technically difficult, may take up to two weeks before visible growth is observed. A DNA probe is available. It is a 1251 probe for the 16S ribosomal RNA of M. pneumoniae.
e. Evaluations in a number of laboratories indicate that compared with culture it is highly sensitive and specific. Recently, DNA probes have been applied to the detection of M. pneumoniae in clinical specimens