More than just a pretty face
It was recognized almost 25 years ago that the S. aureus yellow pigment consists of a number of carotenoids, similar to those produced in carrots and other fruits and vegetables. Studies of these carotenoid pigments have revealed their free-radical scavenging properties, protecting cells and tissues from the damaging effects of free radicals and singlet oxygen. (In other words, they’re antioxidants). Interestingly, one mechanism by which phagocytic cells of the host immune system destroy pathogenic invaders is via release of reactive oxygen species. Do these bacterial carotenoids protect S. aureus against damage initiated by the host immune system?
This question was investigated in a paper published this summer in the Journal of Experimental Medicine. Here, they investigated a mutant in the crtM gene, a critical gene in the carotenoid biosynthesis process. crtM mutants lack pigment and do not produce carotenoid, but other than that, were similar to the wild-type bacterium. In vitro, the mutants were more easily killed by oxygen and hydrogen peroxide—but when the crtM gene was added back in on a plasmid, the bacterium could again resist oxidative killing at a level similar to the wild type.
Looking at this mutant in a mouse model of infection, they also saw that the mutant did not cause skin lesions as the wild-type isolate did, suggesting the carotenoid is important in vivo. Finally, they also transformed an isolate of group A streptococcus (Streptococcus pyogenes) not known to produce carotenoids with the plasmid carrying the crtM gene—causing the isolate to produce a faint yellow pigment. When they examined the lesion pathogenesis in mice injected with the wild-type and the crtM-positive isolates of S. pyogenes, they found that lesions formed were larger with the crtM-positive isolates. So, while the wild-type was already invasive, addition of the carotenoid gene made them even nastier.
I wrote previously about pili being discovered in gram-positive bacteria: once it’s found in one species, it begins a search in other, related, species—and indeed, pili have now been found in groups A and B streptococci, and will probably be found in other gram-positives. Likewise, carotenoid pigments were first investigated as an antioxidant virulence factor last year in the group B streptococcus (GBS, Streptococcus agalactiae). This is another gram-positive bacterium; clinically, it is an important cause of neonatal meningitis and other invasive infections. This bacterium also produces a carotenoid pigment that can give the colonies a slightly orange tinge. Interestingly, it was found that knocking out the gene that produces the GBS hemolysin cylE (a protein that breaks open blood cells, producing a characteristic clearing around a colony on a blood agar plate) renders the colonies both non-hemolytic and non-pigmented, though it’s not yet known whether this is a direct or indirect effect. Additionally, these cylE knockouts were reduced in virulence in a mouse model of infection and more easily cleared by the host. They were also more susceptible to oxidative damage in a macrophage killing assay—so, similar to S. aureus, presence of the carotenoid increases the virulence of the bacterium.
Previous studies examining the role that the GBS hemolysin plays in disease pathogenesis has focused on damage the hemolysin is able to inflict upon host cells. Investigating it from the angle of protection of the bacterium, rather than damage to the host, represents a shift in thinking about bacterial virulence. It also opens up a new avenue for antimicrobial targets—creating drugs to render the bacteria’s defenses inadequate, thereby allowing the host’s own immune clearance mechanisms to operate more effectively. Finally, it suggests that other microbes that produce antioxidant carotenoids or melanin, such as cystic fibrosis pathogen Burkholderia cepacia--an organism that is often notoriously difficult to kill once patients are colonized—and fungus Aspergillus fumigatus, the leading cause of fungal infections worldwide. The challenge here, of course, will be to design drugs that target these microbial carotenoids but are safe to humans. With this research, an observation that has been more of a curiosity and diagnostic determinant has become a possible way to get a handle on these important pathogens.