Bacterial degradation pathways
Microbes have the capacity to completely degrade all naturally occurring compounds. This is called the 'principle of microbial infallibility' proposed by Martin Alexander in 1965. However, also the complete degradation of all anthropogenic, xenobiotic compounds by microbes is crucial to prevent negative impact in the environment.
An important part of the biogeochemical sulfur cycle:
Complete degradation of the plant sulfolipid SQDG and its sulfo-sugar sulfoquinovose by bacterial communities
The naturally occurring, sulfonated sugar sulfoquinovose (SQ, 6-deoxy-6-sulfo-glucose) is produced by all phototrophic organisms as part of the sulfolipid SQDG (sulfoquinovosly diacylglycerol) in their photosynthetic membranes. Hence, SQ comprises a major proportion of the organosulfur in nature, where it is degraded by bacteria: its complete degradation inclusive a recycling of the organo-sulfur in SQ/SQDG in form of inorganic sulfate (or sulfide) represents an important part of the biogeochemical sulfur cycle. Moreover, SQDG and SQ catabolism is relevant in soil, freshwater and marine bacteria, in plant pathogens and plant-growth-promoting bacteria (e.g., Rhizobia), and also in gut microbiomes of all herbivores and omnivores, through the vegetable diet.
We discovered three different degradation pathways for SQ in bacteria, as published in nature , PNAS , iScience.
The first known pathway, which is termed 'sulfoglycolysis' (or more precisely, 'SQ Embden-Meyerhoff-Parnas' pathway), was discovered in the most well-known model bacterium, Escherichia coli K-12. This pathway proceeds in direct analogy to the classical glycolysis (Embden-Meyerhof-Parnas) pathway for glucose, however, it is catalyzed by a completely different set of enzymes (see EcoCyc pathway PWY-7446). The second pathway was discovered in a Pseudomonas putida strain, and this pathway (see MetaCyc pathway PWY-7722) proceeds in direct analogy to the classical Entner-Doudoroff pathway for glucose; also here, a completely separate set enzymes is employed, which also suggests convergent evolution. The third SQ pathway, termed 'transaldolase pathway', was discovered in an aerobic Bacillus strain as well as in anaerobic, SQ-fermenting human-gut Firmicutes, Enterococcus gilvus, Clostridium symbiosum, and Eubacterium rectale.
Importantly, all SQ-utilizing bacterial strains known thus far, are unable to cleave the stable carbon-sulfur bond on the hexose SQ, and therefore only utilize the non-sulfonated C3-half of SQ for energy conservation and growth (in form of GAP or pyruvate), while the sulfonated C3-half is excreted in form of 2,3-dihydroxypropanesulfonate (DHPS) or 3-sulfolactate (SL), respectively. Hence, it seems as if complete degradation of SQ inclusive sulfur-release from DHPS or SL, is accomplished by bacterial communities in the environment.
In that respect, we described also a first two-step degradation pathway for anaerobic degradation of SQ to H2S, as demonstrated using a defined two-member bacterial co-culture constituted of E. coli K-12 and a newly isolated Desulfovibrio sp. strain DF1. The SQ-degrading E. coli catalyzes a fermentation of SQ to 2,3-dihydroxypropane-1-sulfonate (DHPS), succinate, acetate and formate, thus, a novel type of mixed-acid fermentation. The DHPS-degrading Desulfovibrio strain catalyzes another novel fermentation, of the DHPS to acetate and H2S.
Xenobiotic LAS-surfactant degradation by bacterial communities
The major surfactant in laundry detergents (washing agent) in world-wide use is synthetic, xenobiotic linear-alkylbenzenesulfonate (LAS) of petrochemical origin. Surprisingly, LAS is completely biodegradable, for example in sewage treatment plants, as known since 1957.
However, until today not much is known on how these xenobiotica are degraded, and by which microbes. In particular, we are interested in the bacterial communities whose members have 'learned' to utilise these chemicals as novel growth substrates, and in the enzymes and genes that have been recruited in these organisms in order to assemble such novel degradation pathways. More...