Humans don't make chitin. So why do many bacterial pathogens require chitinases to maintain an infection? A study in mBio this week reveals that in Listeria monocytogenes, at least, chitinase helps the environmental pathogen live a double life, digesting chitin while the bacterium lives in the soil and attacking the innate immune system when it makes its way into the human host. This is the first study to demonstrate chitinase activity against a specific host immune response, say the authors.
The fact that chitinase is involved in the pathogenicity of certain environmental pathogens has been known for a while. Chitin binding proteins are now linked to pathogenicity of Serratia marcescens, Legionella pneumophila, and L. monocytogenes, and in L. monocytogenes, two chitinase-encoding genes and one chitin binding protein are known to contribute to virulence of L. monocytogenes in vivo. Freitag and her co-authors followed up on the ability of one chitinase, ChiA, to enhance virulence in Listeria.
"We found that while the Listeria chitinase mutant has no discernible defects in establishing infections in mice, it was cleared much more rapidly by the host immune response," says the study's lead author, Nancy Freitag of the University of Illinois at Chicago. Freitag says they pitted ΔchiA, a Listeria mutant that lacks a functioning ChiA chitinase, against mice. Although the mutant could establish an infection and replicate, by 72 to 96 hours postinfection, about the time when an immune response should kick in, the numbers of bacteria fell off. Without chitinase, the mice won out against Listeria.
But how does chitinase attack the innate immune system? Freitag says they don't have a firm answer, but their experiments suggest inducible nitric oxide synthase is involved. To find out whether the enzyme alters the host immune response by changing the expression of certain proinflammatory cytokines, they looked at the expression of tumor necrosis factor alpha, interleukin 1β, and of the inducible nitric oxide synthase (iNOS) in the mice livers 48 hours after infection. The levels of iNOS was four-fold higher in mice infected with the ΔchiA strain than in mice infected with the wild type strain, indicating that a functioning chitinase knocks down iNOS.
" We don't yet know the direct target of Listeria ChiA, but we speculate that the enzyme might target a carbohydrate-modified host factor linked to transcriptional induction of NOS2, thereby inhibiting iNOS expression," says Freitag.
The dual-use aspect of chitinase would make for a nice fitness advantage for environmental pathogens. In the environment, it enables the bacterium to exploit chitin as an energy source. In the host, the enzyme helps the pathogen sidle past the innate immune system and on to a progressively worse infection. And having one tool for both cuts down on the genetic luggage the bacterium has to carry around.