Gram Negative Coccobacilli
A 21-yrs-old college student complained of malaise, low – grade fever, and a harsh cough, but not of muscle aches and pains. 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. Most likely diagnosis?
a. M. pneumoniae causes a respiratory infection known as primary atypical pneumonia, or walking pneumonia. Although disease caused by M. pneumoniae can be contracted year-round, thousands of cases occur during the winter months in all age groups.
b. The disease, if untreated, will persist for two weeks or longer. Rare but serious side effects include cardiomyopathies and CNS complications. Infection with M. pneumoniae may be treated with either erythromycin or tetracycline. The organism lacks a cell wall and so is resistant to the penicillin and the cephalosporin groups of antibiotics.
c. 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.
d. However, if cold agglutinins are present, a quick diagnosis can be made if signs and symptoms are characteristic. Complement fixation tests that measure an antibody to a glycolipid antigen of M. pneumoniae are useful but not routinely performed in most laboratories.
e. Also, cross-reactions may occur. Culture of M. pneumoniae, while not technically difficult, may take up to two weeks before visible growth is observed.
f. A DNA probe is available. It is a 1251 probe for the 16S ribosomal RNA of M. pneumoniae. 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.