Rapid detection of Mycobacteria species, Nocardia species and other aerobic actinomycetes
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Mycobacteria species are responsible for significant morbidity and mortality in both immunocompromised and immunocompetent hosts. Mycobacterium tuberculosis is the causative agent of tuberculosis and it kills nearly 2 million people in the world each year. Nontuberculous mycobacteria such as Mycobacterium avium complex and Mycobacterium abscessus cause a variety of infections (eg, respiratory, skin, and soft tissue) and are important to detect and correctly identify in order to aid in clinical decision making. There are more than 150 recognized species of mycobacteria and identification of these organisms to the species level is required to help guide appropriate therapy. Although there are direct detection methods available for Mycobacterium tuberculosis, growth of the organism on culture media is still necessary to allow for antimicrobial susceptibility testing. At this time, direct molecular detection methods are lacking for the nontuberculous mycobacteria and growth in culture is critical for identification and antimicrobial susceptibility testing.
Nocardia species and other aerobic actinomycetes (eg, Tsukamurella species, Gordonia species, Rhodococcus species) are also important causes of disease and isolation on culture media is important to facilitate identification and antimicrobial susceptibility testing. Nocardia and the other aerobic actinomycetes grow well on mycobacterial medium and, therefore, ordering a mycobacterial culture is recommended when infection with this group of organisms is suspected.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
A final negative report is issued after 60 days incubation.
Positive cultures are reported as soon as detected.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Recovery of mycobacteria in the BBL MGIT tube is dependent on the number of organisms present in the specimen, specimen collection methods, methods of processing, and patient factors such as presence of symptoms prior to treatment.
The use of BBL MGIT PANTA antibiotic mixture, although necessary for all nonsterile specimens, may have inhibitory effects on some mycobacteria.
For over 20 years the gold standard detection system for propagating mycobacteria in vitro has been the Bactec 460 instrument. The radiometric systems uses carbon(14) radiolabeled palmitic acid as a substrate, which when metabolized by an organism, produces detectable amounts of carbon-labeled carbon dioxide. The MGIT system uses fluorescence technology to detect microbial growth. The Bactec 460 and MGIT systems were compared. A total of 1,963 patient specimens, including 1,519 respiratory tract specimens that required decontamination with sodium hydroxide, and 444 sterile specimens that did not need to be decontaminated, were cultured. A total of 168 cultures grew acid-fast bacilli in 1 or both systems (8.5% positivity rate). The contamination rate for respiratory tract specimens positive in the Bactec 460 was 3.8% and 7.9% in the MGIT. Contamination of sterile specimens was 6.3% in the Bactec 460 and 10.1% in the MGIT. Combined rates were 4.3% for the Bactec 460 and 8.4% for the MGIT. The overall recovery rates for mycobacterial species, excluding Mycobacterium gordonae, were 82.8%, 79.1%, and 78.4% for the Bactec 460, MGIT 960, and solid media respectively. Recovery rates for the Bactec 460 and MGIT 960 were considered to be equivalent.
Clinical Reference Provides recommendations for further in-depth reading of a clinical nature
1. Pfyffer GE, Palicova F: Mycobacterium: general characteristics; laboratory detection, and staining procedures. In Manual of Clinical Microbiology. 10th edition. Vol 1, Edited by J Versalovic, KC Carroll, G Funke, et.al: Washington, DC: ASM Press. 2011 pp 472-502
2. Tortoli E: Microbiological features and clinical relevance of new species of the genus Mycobacterium. Clin Microbiol Rev 2014;27(4):727-752. DOI:10.1128/CMR.00035-14
3. Wilson WW: Nocardiosis: updates and clinical overview. Mayo Clin Proc 2012;87(7):403-407