Evil or just Hungry?

This paper is a review of how pathogenic metabolic pathways and virulence factors are key in pathogenicity. It also describes very nicely the problems which many pathogens have to overcome in the host environment. Animals are an incredibly good source of nutrients for bacteria and this is the main reason why the symbiotic relationship between hosts and their microbiota exist. It is also however the reason for host-pathogen relationships, after all they don’t cause harm and disease just for the sake of being evil.

Pathogens face many problems when trying to inhabit its host. The first problem is trying to out-compete the natural microbiota which may also be producing antimicrobial compounds. The second problem is the highly variable environmental conditions. Oxygen levels, pH and free iron levels are highly variable and many pathogens must move between these conditions. It is also thought that environmental conditions may trigger the production of virulence proteins.

The main point of the review is to highlight the importance of the metabolic pathways used in pathogenicity. The genes for these pathways are coded on pathogenicity islands along with virulence factors. The paper gives three good detailed examples of this including Vibrio cholerae. The pathogenicity island VP12 is only found in toxic strains of Vibrio cholerae. It encodes a neuraminidase (from the nan-nag cluster) that converts host cell surface polysialogangliosides to GM1 monoganglioside which specifically binds cholera toxin by releasing sialic acid. This sialic acid could then be used as an energy source to VP12-carrying Vibrios. The VP12 pathogenicity island also contains genes that are thought to be involved in the scavenging, transport and catabolism of sialic acids. It has been shown that inactivation of these catabolic genes greatly reduces the Vibrios virulence. The paper also mentions the nan cluster seems to be more or less exclusive to mammalian pathogens.

When bacteria move between different environments within the host they are exposed to highly variable nutrient sources. For this reason the metabolic pathways which pathogens need will depend on the host and route of infection. Pathogens therefore will require a versatile range of metabolic pathways some of which may not even be used. This may explain the common occurrence of genome reduction in pathogens once they reach their target area. When pathways are no longer needed they may be lost which would increase virulence by wasting less energy on metabolic pathways.

The paper also puts forward the idea of targeting metabolic pathways of pathogens as an antimicrobial therapy however very little research has been done concerning this idea. Overall I thought the paper was very interesting and brings up a good point. We seem to naturally in our heads think that pathogens are intentionally evil when in fact they’re just hungry. I like the idea of not only looking at how pathogens cause harm but also how they survive themselves and how metabolic pathways seem to be surprisingly closely linked to virulence. Their ability to move through such differing environments and out-compete bacteria so successfully is seriously impressive especially when you think of the diversity of environments they originally came from and have to move through.

A review of: Rohmer, L., Hocquet, D., Miller, SI. (2011) Are pathogenic bacteria just looking for food? Trends in Microbiology, Vol. 19, No. 7.