Asbestos: Forms and Nomenclature | Microorganism of the Month: Thermophilic actinomycetes
By Dr. Harriet Burge, EMLab P&K Chief Aerobiologist and Director of Scientific Advisory Board
Asbestos is the name used for a group of minerals that are fibrous in nature. Chemically, they may be identical to non-fibrous forms, which are not called asbestos. Asbestos minerals are unique in that, when disturbed, the fibrous strands split into thinner and thinner fibers until they are visible only with the aid of a microscope. It is important to note that these are natural minerals. According to the USEPA, non-occupationally exposed people have tens of thousands to hundreds of thousands of asbestos fibers in each gram of dry lung tissue.
Asbestos has two general forms: Serpentine and Amphibole. Characteristics of these forms and their subforms are summarized in the following table.
|Asbestos type||Non-asbestiform name||Chemical formula|
|Anthophyllite asbestos||Amphibole||Anthophyllite||(Mg, Fe)7Si8O22(OH)2|
|Actinolite asbestos||Amphibole||Actinolite||Ca2(Mg, Fe)5Si8O22(OH)2|
Serpentine is a rock-forming mineral with five different forms: Antigorite, Clinochrysotile, Lizardite, Orthochrysotile, Parachrysotile. Of these forms, the chrysotiles are most commonly asbestiform. The fibers of the chrysotile are curly and are bendable. Chrysotile is called "white asbestos", although the natural fibers are generally green or gold.
Chrysotile with penny.
Source: U.S. Geological Survey (USGS)
Chrysotile is the only form of asbestos currently used in manufacturing in the US. It is banned in Japan and Australia. Although less widely used than in the past, chrysotile is still used by US manufacturers. Chrysotile cement is the most important product, using at least 90% of the world's production. This cement is used to form pipes, shingles, and sheets. Chrysotile fibers are also widely used in friction materials (brake shoes, elevator brakes, clutches, etc.) because of its strength and fire resistance. Chrysotile is still a part of roof sealants, door seals for furnaces, some plastics, paper, and other uses for which it's flame resistance properties are important.
The World Health Organization does not consider moderate exposure to chrysotile to be a health hazard. The fibers are relatively large, and are efficiently removed from the lung. Even those who work in manufacturing that uses chrysotile are at limited risk provided exposure controls are in place.
The general formula for the amphiboles is (Si4O11)-6. The different amphibolites are distinguished by the amount and positioning of metal atoms including: sodium, calcium, manganese, magnesium, iron and aluminum. The amphibole fibers are needle-like and rigid, and are not readily bent. Amosite, crocidolite, tremolite, actinolite and anthophyllite are amphibole forms.
The Amphiboles are no longer used in manufacturing in the western world and Japan. They are still widely used in India. They are considered to present a severe health hazard to workers, and even to occupants of towns near mining sites.
By Griselda Y. Hernandez, EMLab P&K Analyst
Actinomycetes are Gram positive bacteria that tend to form filaments and may also produce airborne spores. They have been called "actinomycetes" because of a superficial resemblance to fungi. However, the actinomycetes are prokaryotic (have no organized nucleus) and are physiologically and chemically related to bacteria. They are also much smaller than fungi. Actinobacteria is the up-to-date name for these organisms, although you will still see the term "actinomycete" in the recent literature.
Actinobacteria are unicellular organisms that fail to separate into individual cells following division. The cells remain organized into long chains some of which even branch. They can take on many different branching patterns (Madigan, 2006) that help to define species, although identification of thermophilic actinobacteria continues to pose a challenge. In some species, the terminal cells in a chain turn into spores and become specialized for airborne dispersal. Heat resistance of these spores varies depending on the species of actinomycete (Fergus, 1967). For example, Thermoactinomyces vulgaris has spores that can withstand 100°C for up to 4 hours!
Actinobacteria may be aerobic or anaerobic, although the thermophilic forms are primarily aerobic. The majority of actinobacteria are mesophilic, growing at temperatures ranging from 18°C to 40°C. A few actinobacteria are thermophilic, which means that they have an optimal range for growth between 40°C and 80°C (Figure 1: Tortora, 2007). Common species of thermophilic actinobacteria include Thermoactinomyces vulgaris and Saccharopolyspora rectivirgula (formerly known as Micropolyspora faenae).
Actinobacteria, including thermophilic species, are ubiquitous. Both mesophilic and thermophilic species are abundant in soil. (Madigan, 2006) In fact, the odor we commonly associate with freshly turned soil is a volatile organic compound produced by actinobacteria. In soil, although they are much slower growers compared to other bacteria and fungi, the actinobacteria are effective decomposers, breaking down organic matter such as lignin and cellulose at elevated temperatures. This makes them highly useful for composting since the elevated temperatures speed up microbial activity thereby catalyzing the decomposition process. Interestingly the elevated temperatures have been shown to denature viruses, kill pathogenic bacteria (i.e. coliforms) and weed seeds.
Figure 1: Temperature ranges of organisms (Tortora, 2007).
Figure 2: Streptomyces with closed spiral spore-forming structures.
Figure 3: Streptomyces with monoverticillate spore-bearing structures.
|Source: Images used with permission. Madigan, M.T. and J.M. Martinko (2006). Brock Biology of Microorganisms, 11th edition, Prentice Hall, Upper Saddle River NJ|
The thermophilic actinobacteria are responsible for the decay of self-heating compost, and can be abundant in baled hay that has become wet.
In addition, they can be abundant in silos, corn mills, closed stables, bagasses (sugar cane remnants), house dust and home and industrial air conditioning systems (Fink, 1971). They have also been recently found in clothes dryers (Cox, 1995).
The most important human health effect of exposure to the thermophilic actinobacteria is hypersensitivity pneumonitis (HP). Originally described as Farmer's lung disease, HP is a disease of the lower respiratory tract caused by an immune response to inhaled highly reactive antigens. Antigens associated with thermophilic actinobacterial spores are often associated with HP, although other antigen sources also have been reported (e.g., pigeon serum, some fungal spore antigens, some highly reactive chemicals, etc.). HP is a serious disease that can become chronic and eventually fatal if exposure to the offending antigen continues.
In a laboratory setting, thermophilic actinomycetes can be easily isolated and cultured from air samples, soil, water, dust and sludge and can be easily identified. To the trained eye, they are not easily mistaken for fungi due to their much smaller size. Samples are commonly analyzed for these organisms since they are found in HVAC systems in homes, schools, industrial and non-industrial indoor settings were bacteria are greater in numbers than in outdoor samples due to mechanically ventilated systems (Flannigan, 2001). Overcrowding may also elevate and aerosolize bacteria due to dust-raising activities.
Actinobacterial spores are not recognizable on spore traps. Therefore, sampling for thermophilic actinomycetes involves collecting air, dust, bulk, or swab samples for culturable analysis. For air samples, an efficient collector is required because the spores are very small (1µm), for example Andersen N6 sampler with TSA plates. Filtration sampling such as dust collection is used because spores are resistant to drying and dust are set up for culture.
Laboratory isolation of thermophilic actinobacteria is usually done on trypticase soy agar with 5% blood (TSA), which is incubated at temperatures greater than 50°C. The organisms do not appear on bacterial air samples incubated at 37°C, so that separate air samples must be collected if these organisms are to be recovered.
1. Cross T. "Thermophilic Actinomycetes". The Journal of Applied Bacteriology. Mar 1968, v. 31, no 1
2. Fergus, CL. 1967. "Resistance of spores of some thermophilic actinomycetes to high temperature". Mycopathologia. Sep. 1967, v. 32, no 3 p. 205-208.
3. Finks JN, Resnick AJ and Salvaggio J. "Presence of Thermophilic Actinomycetes in Residential Heating Systems". Applied Microbiology. Oct. 1971, v.22, no 4. p. 730-1.
4. Flannigan B, Sampson RA and Miller JD. 2001. Microorganisms in home and indoor work environments: diversity, health impacts, investigation and control. Taylor & Francis. London.
5. Forbes BA, Sahm Df and Weissfeld AS. 2002. Bailey and Scott's Diagnostic Microbiology, 11th Edition. Mosby, Inc., St. Louis.
6. Kurup VP and Fink JN. 1975. "A Scheme for the Identification of Thermophilic Actinomycetes Associated with Hypersensitivity Pneumonitis". Journal of Clinical Microbiology. Jul 1975, v. 2, no 1 p. 55-61.
7. Madigan MT and Martinko JM. 2006. Brock Biology of Microorganisms, 11th Edition. Pearson Prentice Hall, Upper Saddle River.
8. Tortora GJ, Funke BR and Case CL. 2007. Microbiology: An Introduction. Pearson Benjamin Cummings, San Francisco.
9. Institute for Applied Micobiology of the Justus-Liebig University: Actinomycetes
This article was originally published on August 2007.