The Role of Mycobacteria in Biodegradation

Crude oiland petrochemical-induced contamination of the environment has gained considerable attention in recent decades. Today, leakage and spread of oil contamination due to inefficient extraction, transmission and crude oil refinery processes along with occurrence of different incidents in various sections of oil industry like oil production site, oil refineries and transmission lines are inevitable (1). Crude oil product may enter into the environmental resources and exert harmful effects such as health risks, contamination of natural resources and underground water. In addition to their effects on the environment, these compounds have harmful effects on human. Studies have shown that oil compounds can cause mutation and cancer, and negatively affect human neonates (1,2). Therefore, cleaning the oil-contaminated areas is one of the main environmental concerns. For this purpose, different physical, chemical, and biological methods have been proposed (1,3). Biodegradation is one of the most important solutions to clean these compounds from the environment. During this natural process, microorganisms break the organic chemicals into simpler forms. Chemical and mechanical methods, in addition to being costly, do not clean these compounds effectively and can transmit the contamination to other areas. While biodegradation is a cost-effective and efficient method in comparison with the two mentioned methods, it is now recognized as one of the important strategies in environmental protection (4,5). Presence of microorganisms in contaminated site is another advantage of this method. So far, more than 200 microorganisms with such capabilities have been identified among which Pseudomonas, Actinomyces (especially mycobacteria), Flavobacterium, Bacillus, Alcaligenes and Micrococcus can be mentioned (6). Non-tuberculous mycobacteria live in the environmental resources including soil, water, dust, milk, and animals. Today, this group of bacteria are known as one of the important biodegrading bacteria and can degrade pollutants. Based on the literature, various species of environmental mycobacteria have been reported with biodegradation properties (4,7). The most important molecular method used for the identification of mycobacterial species is gene sequence investigation of 16S rRNA in A (125-270), B (408-502) positions and rpoB gene in V (2581-3300) position and also the sequence of hsp65 in 624-664 and 683-725 positions (8). Among nontuberculous mycobacteria, the species with profound biodegradation properties are: M. fredriksbergense, M. austroafricanum, M. obuense, M. phocaicum, M. paragordonae, M. lentiflavum, M. ratisbonense and M. fortuitum (4,7,8). Presence of fatty acid-saturated cell wall and also presence and expression of biodegradation genes such as nidA, ABC transporter, alkB and so on have enabled the mycobacteria to biodegrade the pollutants (9,10). Among the methods used to evaluate the biodegradation, methods such as chromatography (i.e. HPLC), enrichment technique, spray plate technique and gibbs and turbidometric methods can be mentioned. Among them, enrichment method is one of the most accurate ones in which a salt-based varied medium of methylsulfonylmethane (MSM), which contains the minimum mineral and the studied pollutant, is used as the only source of carbon (9-13). Recently, by advancement in genetic sciences and achievements in getting information of genomics of organisms, researchers are now able to identify the biodegrading bacteria by means of whole genome sequencing (14). In general, previous studies have shown that mycobacteria are one of the bacterial species which possess biodegradation properties. Moreover, presence of fatty acid-saturated cell wall has enhanced the resistance of this bacterium against toxic and harsh environmental conditions; which helps this microorganism to survive in contaminated ecosystems. As non-tuberculous mycobacteria exist in various environments such as water and soil, these bacteria can be used for contamination removal from water and soil.