A review of Jagmann N, Styp von Rekowski K, Philipp B. 2011. Interactions of bacterian with different mechanisms for chitin degradation result in the formation of a mixed-specie biofilm. FEMS Microbiology Letters. 326:69-75.
Within biofilms, bacteria need to breakdown polymers into oligomers and monomers before uptake into the cells. Two methods exist, both producing extra cellular hydrolytic enzymes, however whilst one method releases enzymes into the environment, the other remains associated with the cells. The advantage of releasing enzymes is the ability to hydrolyse polymers which are more often than not embedded in complex organic aggregates or gels, but with the risk of losing the poly and monomers to the environment or scavenging “cheater” bacteria. Whilst bacteria which remain associated to their enzymes reduce this risk via the close attachment, they are unable to access many sources of polymers.
Chitin is one of the most abundant polymers found in aquatic marine environments. The process of chitin degradation via chitinases release has been well described. What is not well understood is the process of chitin degradation via cell associated chitinases. The aim of this paper was to investigate the differences between the two strategies and how each overcome their respective disadvantages.
Using a reductionist laboratory model experiment, two strains, Aeromonas hydrophila strain AH-1N and Flavobacterium sp. Strain 4D9, representing both strategies were investigated both separately and in a co-culture. These two strains have both been described living together in the same environments so the co-culture was used. As chitin is rarely found on its own naturally, it was embedded into an agarose bed in the experiment.
The results of chitin usage embedded within agarose agreed with previous work. When tested separately strain AH-1N increased in number and was able to access and degrade the chitin, suggesting freely released enzymes, whereas 4D9 increased only slightly and were unable to degrade the chitin, suggesting the enzymes are associated with the cells. This demonstrates the disadvantage of this method.
In co-culture, the time before the all chitin had disappeared was 32 days, much longer that AH-1N on its own. AH-1N experienced 8.7 fold lower counts whereas 4D9 experienced 13,700 fold increases in number from its single culture. They suggest two theories for this. Either the 4D9 had been able to access the chitin via cavities left by the freely released enzymes of the AH-1N, or they were obtaining organic substrates released by AH-1N such as chitin degradation products. As the 4D9 only grew on periphery of the agarose it is likely the second theory is true.
To confirm this, the substrates produced by AH-1N in single culture were analysed. It was found that acetate and ammonium were transiently released but GlcNAc was not. However 4D9 grew very poorly with acetate ruling this substance out. When the products of AH-1N’s enzymes on the chitin were analysed separately from the bacterial cells, it was identified that GlcNAc was produced from the degradation of chitin. 4D9 is able to utilise this and it is this that is likely the growth medium used and therefore responsible for the growth shown in the co-culture.
However this doesn’t explain how when GlcNAc was not produced by AH-1N. As it was shown GlcNAc was produced as a product of chitin degradation, AH-1N clearly has tight coupling between polymer hydrolysis and GlcNAc uptake. To overcome this, 4D9 had to establish close contact zone of chitin hydrolysis to intercept the GlcNAc and uptake it for itself. This was supported by the active integration of 4D9 into the biofilm in co-culture, but with no such biofilm formation in single culture. They suggest that 4D9 is more efficient in the uptake of GlcNAc than AH-1N, which accounted for the lower growth numbers of AH-1N in co-culture and the slower degradation rates of chitin due to lower rates of AH-1N enzyme production.
In conclusion this paper demonstrates that bacterium with cell associated enzymes are highly dependent on those with freely releasing enzymes. Those that freely release enzymes need to do so to be able to utilise polymers in the natural environment. However they experience a trade-off between this and the risk of being vulnerable to exploitation. Bacterium which have cell associated enzymes have a trade-off between having limited direct access to polymers but at the benefit of not being exploited. These results demonstrate some of the reasons that cause mixed species biofilms that are observed in many environments.
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