Modulating Campylobacter jejuni virulence by changing its rRNA methylation pattern.

The ribosomal RNA molecules (rRNAs) of all bacteria are modified with an array of methyl groups, which serve to fine-tune the process of protein synthesis. These modifications are added to specific rRNA nucleotides by methyltransferase enzymes, and one such enzyme, TlyA, is found in several pathogenic bacteria. The TlyA ortholog in Campylobacter jejuni (termed TlyA^I) adds a methyl group to a single specific rRNA nucleotide in the large subunit of the ribosome, while the ortholog of Mycobacterium spp. (termed TlyA^II) methylates this same site plus an additional rRNA nucleotide in the small ribosomal subunit. Loss of the respective TlyA enzymes in Mycobacterium tuberculosis and C. jejuni alters their sensitivity to the antibiotic capreomycin and reduces an array of their virulence properties. The change in antibiotic sensitivity fits well with our understanding of ribosome function, because the methylations are situated on nucleotides within the capreomycin binding site. However, the mechanism by which the rRNA methylations affect virulence in the absence of antibiotics has remained unclear.

Dr. Agnieszka Sałamaszyńska-Guz and Dr. Izabela Serafińska from Warsaw University of Life Sciences together with Dr. Paweł Bącal at University of Warsaw and Professor Stephen Douthwaite at University of Southern Denmark have investigated these effects in C. jejuni. They showed that inactivation of the tlyA^I gene in C. jejuni reduces several of the pathogen’s virulence traits including its ability to form biofilms and to adhere to and invade human intestinal epithelial cells. These traits can be to a large extent recovered by engineering an active copy of the  tlyA^I gene back into the C. jejuni null-mutant at a different site in its chromosome.

In their most recent work published in Cellular Microbiology, the research team extended the investigation, this time using the mycobacterial tlyA^II gene for complementation of the C. jejuni null-mutant. When expressed in C. jejuni, the mycobacterial TlyA^II enzyme methylates the same two rRNA nucleotides as it would in its original Mycobacterium host. Addition of the two methyl groups not only restores the ability of C. jejuni to act as a pathogen, but surprisingly enhances some (but not all) of its virulence traits. For instance, the biofilms formed by the C. jejuni tlyA^II-recombinant were more uniform and significantly denser, surpassing those of the wild-type. Furthermore, it was shown that the adhesion and invasion capacity of C. jejuni on Caco‐2 human colon epithelial cells, which was reduced to less than half that of the wild‐type following the loss of TlyA^I‐directed methylation, was markedly increased upon complementation with the mycobacterial tlyA^II gene. In contrast, other traits such as motility, survival time in epithelial cells, and sensitivity to capreomycin-type antibiotics were not significantly changed compared to C. jejuni cells expressing the original tlyA^I gene.

In a nutshell, the authors demonstrate that the different methylation patterns on the C. jejuni rRNA affect its ability to be an effective pathogen. The authors propose that the methylations subtly modulate the rRNA’s function during the translation of certain mRNAs and thereby change the relative proportions of proteins in the bacterial cell. Such changes in the bacterial proteome would consequently alter the phenotype and virulence properties. These researchers are currently mapping the proteomes of the various C. jejuni recombinants using mass spectrometric approaches to identify key proteins involved in pathogenicity. It is envisaged that this approach will lead to a better understanding of virulence mechanisms and form a basis for finding novel means of treating Campylobacter jejuni infections and, ultimately, infections with other bacterial pathogens.