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Mycobacterium species | Microorganism of the Month: Stenotrophomonas maltophilia

Mycobacterium species

By Tharanga Abeysekera, EMLab P&K Analyst

The mycobacteria are rod shaped, sometimes branching bacteria with a Gram positive acid fast cell wall. Acid fastness is determined by the ability of the cell to retain a dye when treated with acid, and, like the Gram reaction, is related to the nature of the cell wall. The Mycobacterium cell wall is thicker than in many bacteria, and is waxy, hydrophobic, and rich in mycolic acids. The mycolic acids are long chain fatty acids that comprise up to 60% of the cell wall. They play a role in isolating the cell from the environment, and protecting it from adverse conditions.

The mycobacteria are aerobic, non-motile organisms that grow at temperatures ranging from 20-50°C. They grow more slowly than many bacteria, and environmental isolation requires selective media such as Middlebrook 7H10 agar and Lowenstien Jensen agar. Mycobacteria that produce visible colonies within 7 days are considered rapid growers. Those that take longer than 7 days (some require weeks) to produce visible colonies are considered slow growers. Mycobacterium species can grow at temperatures ranging from 20°C to 50°C. Mycobacterium colonies on a culture plate can be rough to smooth in morphology and vary in color as well. Some colonies produce a yellow pigment when exposed to light.

Two deadly human diseases are caused by slow-growing Mycobacterium species. Tuberculosis is caused by Mycobacterium tuberculosis. This is an airborne disease that is readily transmitted from one person to another. Risk factors for the disease are exposure and lack of acquired immunity. While relatively uncommon in the US, this disease affects millions of people world wide, and is among the leading causes of death in many areas. Tuberculosis is treatable, but requires long-term antibiotic therapy. Leprosy is a deadly disease caused by Mycobacterium leprae. Leprosy is not airborne (as far as we know) and is not readily transmitted between people. Risk factors for the disease appear in part genetic. While rare in the U.S., leprosy remains a significant problem in India and parts of South America and Africa. Leprosy is also treatable using an antibiotic cocktail.

Faster growing Mycobacterium species (often called Nontuberculous Mycobacteria or (NTM) are common in the environment and can cause infections in immunocompromised individuals. Disease include pulmonary disease resembling tuberculosis, lymphadenitis, skin disease, or disseminated disease. These diseases have become of special concern because of the AIDS epidemic.

One of the fast-growing mycobacteria, Mycobacterium immunogenum, has learned to colonize machining systems containing water-miscible Metal Working Fluids (MWF). MWF are used to reduce heat and friction and to improve product quality in industrial machining and grinding operations. There are many formulations for MWF. The water based MWF formula supports microbial growth. Mycobacterium immunogenum exposure has been linked to outbreaks of hypersensitivity pneumonitis (HP) in machinists. HP is an inflammation in and around the lung resulting from repeated inhalation and sensitization to small particle antigens. It is initially treatable with avoidance of exposure to the offending antigen, and with steroids. Prolonged exposure can lead to permanent lung damage and death. MWF operations employ about 1.2 million workers. These workers are exposed to the fluids by breathing aerosols generated in the machining process. Why the organism grows in some coolant systems and not others remains unknown. The misuse of biocide in heavily contaminated systems can actually increase the Mycobacterium population by eliminating the competing bacteria while having a very slight effect on the Mycobacterium population.

Testing for Mycobacterium in MWF can be done using several methods. Generally, the bulk fluid is collected and cultured. Alternatively, the fluid can be evaluated using chemical analysis for mycolic acids, by PCR, or by microscopy and staining using the acid fast method. Air sampling could also be used, but is subject to a high rate of false negatives.

1. Bergey's Manual of Systematic Bacteriology, Vol 2.

2. CDC NIOSH: Metalworking Fluids

3. NALCO, bulletin B-654

4. The International Leprosy Association's Global Project on the History of Leprosy


Microorganism of the Month: Stenotrophomonas maltophilia

By Yamile Echemendia, EMLab P&K Analyst

Previously known as Pseudomonas maltophilia or Xanthomonas maltophilia, this microorganism has now been reclassified as Stenotrophomonas maltophilia and is the sole member of the genus. In culture, colonies are smooth, glistening, with entire margins and are white to pale yellow. Its cells are straight or slightly curved non-sporulating Gram-negative bacilli that are 0.5 to 1.5 µm long. They can be found singly or in pairs and are motile by means of several polar flagellae (slender tapering narrow outgrowths of the cells of many microorganisms that are a means of motility). This bacterium is an obligate aerobe (organism that needs oxygen for metabolism) that grows at temperatures between 5°C and 40°C and is optimal at 35°C.

Stenotrophomonas maltophilia is a ubiquitous free-living bacterium that is commonly found in soils and especially in plant rhizospheres (the area of soil that immediately surrounds and is affected by a plant's roots) where the high content of amino acids in root exudates constitutes a growth factor for this organism. Other food sources for the bacterium include frozen fish, milk, poultry, eggs and lamb carcasses. In addition, Stenotrophomonas maltophilia has been isolated from a number of water sources such as rivers, wells, bottle water and sewage.

Little is known about virulence factors associated with Stenotrophomonas maltophilia and considerable ambiguity exists about the route(s) of acquisition. Although it is not a part of the normal flora of healthy humans, it is frequently encountered as a commensal (a type of relationship between organisms of two different species in which one benefits from the association while the other remains unharmed) in the transient flora in hospitalized patients. Stenotrophomonas maltophilia is considered an opportunistic pathogen. Episodes of infection caused by this microorganism have become increasingly important in the hospital setting, the presence of a compromised immune system being the most important predisposing factor. Despite the fact that the majority of Stenotrophomonas maltophilia infections are nosocomial (originating, acquired or occurring in a hospital) some may be community-acquired.

Distinguished by a high degree of antibiotic resistance rather than by invasiveness and tissue destruction, Stenotrophomonas maltophilia is a major concern, primarily, in immunocompromized patients where it may cause a wide spectrum of diseases such as urinary tract infection, endocarditis and meningitis, serious postoperative wound infections and respiratory tract infections. Management of Stenotrophomonas maltophilia infections present problems for both the laboratory technician and the physician because clinical isolates are frequently resistant to many antimicrobial agents and the methods for determining the susceptibility of this organism to antibiotics are, at present, often unreliable.

Several hospital outbreaks of Stenotrophomonas maltophilia infection and/or colonization have been described. In several instances, environmental reservoirs for the bacterium have been identified, such as deionized water dispensers, ice-making machines, inhalation therapy equipments, blood sampling tubes, contact lenses care systems, dialysis machines, faucets aerators and the hands of the health personnel, among others.

Several strategies to prevent infections with Stenotrophomonas maltophilia have been proposed. These include avoidance of inappropriate antibiotic use, maintenance and, where appropriate, disinfection and/or sterilization of respiratory therapy equipment, hemodialyzers and ice-making machines. The fact that this organism has been associated with plumbing systems (e.g. water faucets and sink drains) within both the home and the hospital environments suggests that control directed towards these sources may be helpful. For example, the practice of rinsing reusable equipment for the delivery of aerosolized antibiotics in tap water should be avoided.

Much remains to be understood about the epidemiology of Stenotrophomonas maltophilia. In particular, more information about nosocomial reservoirs and routes of transmission of the bacterium is essential for the development of more effective strategies to prevent future outbreaks of infection within the growing immunocompromized population.

1. Berg, G. et. al. 1999. Genotipic and phenotypic relationships between clinical and environmental isolates of Stenotrophomonas maltophilia. Journal of Clinical Microbiology, Vol. 37, No. 11:3594-3600.

2. Denton, M. and Kerr, K.G. 1998. Microbiological and clinical aspect of infection associated with Stenotrophomonas maltophilia. Clinical Microbiology Reviews,Vol. 11, No.1:57-80.

3. Felicia P. Y. Laing, et. al. 1995. Molecular epidemiology of Xanthomonas maltophilia colonization and infection in the hospital environment. Journal of Clinical Microbiology, Vol. 33, No. 3:513-518.

4. Qureshi, A. et. al. 2005. Stenotrophomonas maltophilia in salad. Emerging Infectious Diseases, Vol. 11, No. 7: 1157-1158.

5. Weber, DJ. et. al. 1999. Faucet aerators: A source of patient colonization with Stenotrophomonas maltophilia. Am. J. Infect Control, Vol. 27, No. 1:59-63.


This article was originally published on July 2007.