Axenic bacterial cultures

Axenic bacterial cultures that grow on HMW PAHs of four or more fused rings have been reported. Many studies also suggest that this breadth includes cometabolism or fortuitous metabolism. This is a process in which enzymes responsible for the initial oxidation of a particular PAH fortuitously oxidize other, higher-molecular-weight PAHs, even though these latter substrates may not ultimately be used as a source of carbon and energy for the cells. In addition,... 

Table 1 Selection of axenic bacterial cultures with the capacity to grow on MTBE and/or TBA as sole source of carbon and energy... 

The authors pointed out that the cobalt-dependenq of this enzyme may explain why several axenic bacterial cultures require Co for degradation of TEA and HIBA [21,24,25,60]. Most remarkably, the same gene (nearly 100% identity) was foimd in the finished genome of M. petroleiphilum PMl [22], suggesting a horizontal gene transfer between both cultures. 

Axenic and Mixed Bacterial Cultures Capable of Growth... 

In addition to axenic bacterial strains, mixed bacterial cultures capable of MTBE and/or TBA-degradation have been reported (Table 2). Bacteria closely related to some of the above-mentioned axenic cultures where found present in these mixed bacterial cultures, i.e., M. petroleiphilum [40,41], Hydrogenophaga [40,42] and Methylobactium [42]. For example, M. petroleiphilum PMl was isolated from the mixed bacterial culture enriched by Eweis et al. [28]. 

Isolation of stable axenic strains from mixed bacterial cultures is not always feasible [ 17,46]. The following reasons have been proposed to explain the observed difficulties in isolating axenic MTBE-degrading microorganisms ... 

Reported specific MTBE degradation rates for both axenic and mixed aerobic bacterial cultures are in the range of 8.5-52 mg MTBE g dry weight h (Table 3), with the exception of 250.2 mg MTBE gdw h, reported for axenic culture Hydrogenophaga ENV735 [23,26]. However, this bacterial species requires yeast extract (0.01%) in order to efficiently degrade MTBE. 

Use of axenic cultures is attractive in view of the high degree of reproducibility, and avoids potentially serious problems due to the possible occurrence of bacterial transformation of the toxicant during tests with unialgal cultures containing bacteria. 

Axenic cultures of lichen thalli are not possible because of the bacterial flora on the thalli. Bacterial cells are found lodged in chinks and cracks of the lichen thalli and embedded within the extracellular polysaccharide material found throughout the thalli (Jacobs and Ahmadjian, 1971). Attempts to sterilize thalli with plasmolyzing agents or radiation have not been successful. The only possible way to achieve lichen thalli that are free from contaminants is to begin with the separate symbionts and recombine them under sterile conditions. Such a possibility now exists with pusillum (Ahmadjian and Heikkila, 1970). 

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