Actinomycetes | Mycobacteria
By Dr. Harriet Burge, EMLab P&K Chief Aerobiologist and Director of Scientific Advisory Board
What Are Actinomycetes?
Actinomycetes are a subgroup of the actinobacteria, which are Gram positive organisms with a high G+C ratio in their DNA. This group is very ancient, and branched off from the other groups very early in the evolutionary sequence.
The actinomycete group includes many familiar and important bacteria, including Mycobacterium (the causal agents of tuberculosis and leprosy), Corynebacterium (a common commensal on human skin, and therefore often recovered in bacterial air samples), and Streptomyces (the source of many antibiotics as well as the pleasant odor of freshly turned soil).
The actinomycetes are rod shaped or filamentous. Those that are rod shaped may form long, branching, chains of cells. Many actinomycetes form true filaments that branch and form colonies that look like fungi, although the diameter of the filaments is much smaller than that of the fungi. Filamentous forms produce spores that may be single, in short chains, or in very long chains that may form beautiful spirals.
There are both anaerobic and aerobic actinomycetes. Actinomyces is an important anaerobic genus (see below). The truly filamentous forms are predominantly aerobic.
Actinomycetes are abundant in soil, and are responsible for much of the digestion of resistant carbohydrates such as chitin and cellulose. They are responsible for the pleasant odor of freshly turned soil. Many actinomycetes and other actinobacteria are well known as degraders of toxic materials and are used in bioremediation. They are particularly well adapted to survival in harsh environments. Some are able to grow at elevated temperatures (>50°C) and are essential to the composting process.
Human Health Importance
Many of our best known and most valuable antibiotics are produced by actinomycetes. These include novobiocin, amphotericin, vancomycin, neomycin, gentimycin, chloramphenicol, tetracycline, erythromycin, nystatin, etc. Some of these antibiotics target bacterial ribosomes and are used in treating respiratory infections, including Legionnaires' disease (tetracycline, erythromycin). Vancomycin attacks bacterial cell walls and deadly organisms such as MRSA (methicillin-resistant Staphylococcus aureus) (multiply drug resistant staph). Rifamycin targets bacterial RNA polymerase, and is useful against tuberculosis and leprosy. Amphotericin is one of the few antibiotics that attacks fungal membranes. These antibiotics generally do not affect human cells and therefore have few side effects. However, actinomycete metabolites such as adriamycin, prevent DNA replication, and are used in the treatment of cancer, while rapamycin is used to suppress the immune system to enable organ transplants.
Members of the genus Actinomyces are normal commensal members of human oral cavities. They can cause serious infections when they invade tissues through breaks in the oral mucosa. The disease is becoming less common, but is still present in the USA, especially in inner city populations.
Nocardia asteroides is an actinomycete that is common in soil, and can cause infection via the respiratory route. Infection is opportunistic, and relies on deficits in cell-mediated immunity. Other species of Nocardia may also be involved.
Hypersensitivity pneumonitis (HP)
Thermophilic actinomycetes are the most common cause of HP. Farmer's lung disease is HP resulting from exposure to hay that has become colonized with thermophilic actinomycetes, which produce an abundance of airborne spores. Clouds of these spores are released when farmers (especially dairy farmers) handle stored hay in winter and early spring. The same fungi that cause molding of hay are common inhabitants of soil, and have also been documented to colonize ventilation systems, clothes dryers, refrigerator drip pans, and any other site that combines heat, cellulosic or other carbohydrate material, and water. Common species include Thermoactinomyces vulgaris, Saccharopolyspora rectivirgula, Thermoactinomyces viridis and others.
Volatile Organic Compounds (VOCs)
The odor of freshly turned soil is the result of geosmin, a volatile organic compound produced by actinomycetes. Geosmin is also produced by some cyanobacteria and produces an earthy taste in drinking water. Some fungi also produce geosmin, which can impart the same earthy taste to wine made from moldy grapes. In general, people find the geosmin odor pleasant in soil. However, one indoor air research group is investigating the possibility that exposure to geosmin is related to building-related symptoms. The data at present is too limited for conclusions. However, in the future, collection of samples that will reveal these organisms might be recommended.
Sampling and Analysis
Generally, cultural sampling is necessary for the recovery of actinomycetes. Non-spore forming actinomycetes such as Corynebacterium are commonly recovered from air in human-occupied environments. These bacteria are carried on skin scales, and thus are readily collected by most suction devices.
Spore-forming actinomycetes are released into the air from environmental reservoirs as single or short chains of spores, and an efficient collector must be used. The particle size may be as small as 1 micron or less. The spores produced by these organisms are similar to endospores of Bacillus species, and are resistant to environmental stresses. Actinomycete spores will germinate on the same culture media as other bacteria, and are readily identified to the genus level. It is important to note that the thermophilic actinomycetes, while they grow on the same culture media as other bacteria, must be incubated at very high temperatures. Incubation temperatures in excess of 45°C are essential for recovery of these organisms.
Clinical Microbiology Reviews. 1994. 7(3):357-417.
By Dr. Michael Berg, EMLab P&K Senior Molecular Biologist
Mycobacteria are commonly found in a wide variety of environmental reservoirs, including natural and municipal water, soil, aerosols, protozoans, animals and humans. Only a few mycobacteria are obligate pathogens such as Mycobacteria tuberculosis and M. leprae, which are known to cause tuberculosis and leprosy. It is important to differentiate between those highly infectious pathogens and other mycobacteria. Those other "environmental" mycobacteria are commonly referred to as "Nontuberculous Mycobacteria" (NTM), "atypical Mycobacteria" or "Mycobacteria other than tuberculosis (MOTT)." NTM are distinguished from the members of the M. tuberculosis complex (and M. leprae) by the fact that they survive in the environment as saprophytes, commensals, and symbionts. There has been no evidence of animal-to-human or human-to-human transmission of NTM. However, some environmental mycobacteria are opportunistic pathogens and may cause pulmonary and cutaneous diseases, lymphadenitis and disseminated infections. Infections of AIDS patients with nontuberculous mycobacteria of the M. avium complex (MAC) used to be frequent before antiretroviral therapy was available.
NTM are often overlooked as a potential threat and cause of disease because they are difficult to isolate and easily outgrown by other bacteria. Environmental sources such as municipal water supplies are rarely tested for the presence of mycobacteria. A study in France found that 72% of water samples from the water distribution system contained environmental mycobacteria. Their resistance to chemical treatment with common disinfectants and ability to tolerate wide ranges of pHs and temperatures as well as the formation of biofilms, allows environmental mycobacteria to persist in drinking water for long periods of time. Commercial potting soil and water damaged building material are other common environmental reservoirs for NTM that have been associated with pulmonary mycobacterial infections.
Pulmonary infections, in particular hypersensitivity pneumonitis, are the most common clinical manifestations of disease caused by NTM. Hypersensitivity pneumonitis has been reported in automobile workers exposed to aerosols generated from metal working fluid, in lifeguards exposed to aerosols generated in indoor swimming pools, and in individuals at home exposed to aerosols from aerated hot tubs, spas, humidifiers or showers, as well as water damaged building material. Reported incidence rates for NTM disease vary from 1.0 to 1.8 cases per 100,000 persons. However, mycobacteria infections are often misdiagnosed and many experts believe that incidence is much higher.
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2. De Groote MA, Pace NR, Fulton K, Falkinham JO 3rd. Relationships between Mycobacterium isolates from patients with pulmonary mycobacterial infection and potting soils. Appl Environ Microbiol. 2006 Dec;72(12):7602-6.
3. Le Dantec C, Duguet JP, Montiel A, Dumoutier N, Dubrou S, Vincent V. Occurrence of mycobacteria in water treatment lines and in water distribution systems. Appl Environ Microbiol. 2002 Nov;68(11):5318-2.
4. NTM Information and Research
This article was originally published on June 2008.