Colors Galore: Microbial pigments and virulence

Staphylococcus aureus

 Chromobacterium violaceum

Don't the above pictures look pretty? Specially the hue of colors. These are various bacterial cultures which form colored colonies. Though the colors look pleasing to the eye, the actual function of the pigments responsible for these colors is not so pleasant- Many of these pigments are responsible for the virulence possessed by the bacteria.

Take the case of Staphylococcus aureus- the left most picture. The pigment carotenoid moves through a pathway producing the golden colored staphyloxanthin. Staphyloxanthin comprises of alternate single and double carbon bonds in its backbone which help absorb the reactive oxygen species (ROS). In case there is a mutation and the ability to produce staphyloxanthin is lost, the bacteria forms colorless colonies. These colorless staph are more susceptible to killing  by superoxides , oxygen radicals, hydrogen peroxide. It is hence not difficult to assume that the golden color is the "armor" against all these weapons.

Another pigment, Melanin, is produced by the fungus Cryptococcus neoformans. The strains producing the dark brown-black melanin were found to be more adept at resisting phagocytic invasions from the host cells as compared to the lightly or non-pigmented forms. In addition, they were also found to be more resistant to antimicrobial drugs. The melanin has also been said to have the ability to stabilize ROS.

Pseudomonas aeruginosa produces the greenish colored pyocyanin. In contrast to others, it actually promotes formation of reactive oxygen species which cause damage to the host cells. It is a zwitter ion and can easily penetrate the cell membrane. It acts as an electron acceptor thus interfering with the respiratory chain and increasing oxidative stress in the cell.

Chromobacterium violaceum gets its name because it forms purple colored colonies- courtesy the pigment violacein. Experiments with violacein have demonstrated that it brings about apoptosis in leukocyte cell lines. It is believed that this property could play a role in immune evasion during severe human infections. In addition, violacein has shown antimicrobial activities against some bacteria and protozoan and may help the bacterium survive in the environment.

Thus, apart from imparting virulence, protection may be another important function of microbial pigments. These pigments may protect the organism from UV light, oxidative species, other micro-organisms and extremes of heat and cold among others.


References:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2743764/