RECENT LABORATORY TECHNIQUES FOR THE DIAGNOSIS OF MYCOBACTERIAL INFECTIONS

More rapid and more sensitive techniques have replaced traditional methods of direct examination and culturing for diagnosing mycobacterial infections. Among the most recent methods are Isolator blood culture, radiometric detection, hybridization and amplification, each with its advantages and disadvantages. The Isolator blood culture system improves the sensitivity of mycobacteria detection, especially for Mycobacterium avium-intracellulare. In terms of cost and culture time, this system is similar to traditional methods, although it is limited to blood cultures. Radiometric detection, associated with culture in a liquid medium, greatly improves diagnostic power. The main advantage lies in the reduced delay before detection, 13.4 days compared with 21.7 days on solid media (for M. tuberculosis) and 5.1 days. instead of 18.6 days for other mycobacteria. A culture is considered to be negative after 6 weeks: without growth compared with 12 weeks for the Lowenstein-Jensen medium. The disadvantages of this system include the cost of equipment, personnel and radioactive products. In addition, as for all culture methods, a sufficient bacterial population is required in the sample. Hybridization techniques rely on DNA or RNA probes which recognize mycobacterial sequences. The Gen-Probe recognizes M. tuberculosis, M. avium, M. intracellulare and M. gordonae ribosomal RNA. Syngene probes recognize M. tuberculosis and M. avium-intracellulare DNA sequences. Rapid identification in less than 2 hours is a major advantage. At the present time 50 pg of DNA are required, a quantity which is not usually present in clinical samples. Combining this technique with primary culture in liquid medium or amplification techniques improves sensitivity. Polymerase chain reaction amplifies a specific sequence of double strand DNA. This method provides a means of identifying mycobacterial DNA in a clinical sample within 24-48 hours, a major improvement over traditional culture. Cost however may be a barrier although the cost/benefit ration remains to be determined.