ABSTRACT
This article summarizes the most recent (since 2012) taxonomic changes in the genus Mycobacterium. Only those mycobacteria that have been isolated from human specimens are included in this summary.
INTRODUCTION
Currently, there are over 170 recognized species of Mycobacterium, the only genus in the family Mycobacteriaceae. Organisms belonging to this genus are quite diverse with respect to their ability to cause disease in humans; some are strict pathogens, while others are opportunistic pathogens or nonpathogenic. Similar to other major groups of bacteria, the mycobacteria have undergone an extraordinary expansion in the number of different species over the last 2 decades, due in large part to the discriminatory power of gene sequencing, which phenotypic methods cannot achieve. This discriminatory power is such that phenotypic traits, i.e., biochemical and cultural characteristics, are no longer acceptable for the identification of mycobacteria. Paralleling the increasing number of mycobacterial species, infections caused by the nontuberculous mycobacteria (NTM), in particular, are also increasing on a worldwide level (1). For laboratories identifying mycobacteria isolated from clinical specimens, it is essential that these organisms are accurately identified for proper clinical management. As more clinical laboratories use molecular or other methods, such as mass spectrometry, for identification, our understanding of clinical significance will change and evolve as the number of case reports regarding the “new” species increase; it may well be that the role of many of the newly described mycobacterial species has been underestimated either because of misidentification or because the species were unrecognized. Toward this end, it is imperative for clinical microbiologists to keep up to date with the rapidly evolving taxonomy of the mycobacteria. The intent of this minireview is to provide a single reference for diagnostic laboratories regarding new, clinically relevant changes in taxonomy pertaining to the genus Mycobacterium.
METHODS
Only those newly recognized mycobacterial species recovered from human clinical material and reported between January 2012 and December 2015 are summarized here. These new species were identified using a combination of reference materials. A valuable resource was the List of Prokaryotic Names with Standing in Nomenclature (http://www.bacterio.net/-allnamesmr.html ). In addition, a combination of reference materials was used, including the Manual of Clinical Microbiology, 11th edition, chapters 30 through 32; the International Journal of Systematic and Evolutionary Microbiology; and the PubMed database (http://www.ncbi.nlm.nih.gov/pubmed ), using “nov. sp. Mycobacterium” as the search term.
RESULTS
Table 1 summarizes new Mycobacterium species identified between January 2012 and December 2015. For each species, the human source(s) from which the new species was isolated, its clinical relevance, and noteworthy growth characteristics are noted. In addition, information on whether 16S rRNA gene sequencing identified the species is included. In addition, brief summaries of case reports for some of the new species are provided in Table 2.
New mycobacterial species (family: Mycobacteriaceae) recovered between January 2012 and December 2015
Brief summaries of clinical case reports for new Mycobacterium species
DISCUSSION
Accurate identification of the mycobacteria is a challenge, particularly in light of the extraordinary number of species in the genus. It has become clear that identification of mycobacteria based on phenotypic and culture traits is poorly reproducible, time consuming, and lacking in sufficient discriminatory power (21). As a result, this approach to mycobacterial identification has been abandoned for the most part. Although the introduction of numerous new species over recent years might seem daunting to clinical laboratories that identify mycobacteria, accurate identification is critical for proper diagnosis and management of infections and for outbreak investigation.
This need was illustrated in recent reports of serious infections in patients who had undergone open cardiac surgery in which a contaminated heater-cooler device was used during extracorporeal circulation. These infections, occurring in patients in Europe and the United States, were caused by Mycobacterium chimaera, a nontuberculous mycobacterium ubiquitous in soil and water (22). This organism is a slow-growing NTM species included in the M. avium complex (MAC). Initially, strains of this organism were identified by a commercial probe assay as Mycobacterium intracellulare. In-depth investigation of these strains in 2004 revealed that the organisms were in fact distinct from M. intracellulare and belonged to a previously unidentified species within the MAC, namely, M. chimaera (23). Without delineation of this new species, this organism might have been misidentified and recognition of its association with heater-cooler devices in the operating room possibly delayed. Despite the already large number of species of this genus, there is little doubt that this number will continue to increase and that some newly described species will be recognized to cause infection in humans.
- Copyright © 2017 American Society for Microbiology.
REFERENCES
Author Bios
Betty A. Forbes, Ph.D, is a professor of pathology and internal medicine and director of the clinical microbiology laboratory at Virginia Commonwealth University (VCU) Medical Center in Richmond, Virginia. She is a diplomate of the American Board of Medical Microbiology and a fellow of the American Academy of Microbiology. She has been at VCU Medical Center since 2003. Previously, she served for 20 years as director of clinical microbiology at Upstate Medical University in Syracuse, New York. Dr. Forbes graduated from Northwestern University in 1970. She received her Ph.D. in medical microbiology and immunology from the University of Oklahoma Health Science Center in Oklahoma City, Oklahoma, and then completed a 2-year American Board of Medical Microbiology-approved postdoctoral fellowship program at the University of Michigan, St. Joseph Mercy Hospital, in Ann Arbor, Michigan. Following her formal fellowship training, she held a joint appointment with the Division of Infectious Disease, Department of Internal Medicine, and the Clinical Microbiology Laboratory in the Department of Pathology, University of Michigan as a research associate for 2 years. Dr. Forbes has a special interest in mycobacteria and has published extensively in diagnostics for clinical microbiology. She was an editor for Bailey and Scott's Diagnostic Microbiology and an editor in chief for Clinical Microbiology Reviews, and she has served as an editor for the Journal of Clinical Microbiology since 2009.
