Novel Pili of Mycobacterium tuberculosis

by Alteri, Christopher.

Mycobacterium tuberculosis is responsible for nearly three million human deaths every year. Understanding the mechanisms and bacterial factors responsible for M. tuberculosis’ ability to cause disease in humans is critical for the development of improved treatment strategies. Using negative staining and transmission electron microscopy it was discovered that mycobacteria, including the human pathogen M. tuberculosis, produce fine surface structures known as pili. Mass spectroscopy analysis demonstrated that purified pili from M. tuberculosis are comprised of protein subunits encoded by the predicted M. tuberculosis H37Rv ORF designated Rv3312A. These pili termed M. tuberculosis pili, Mtp, are highly aggregative 2-5 nm diameter fibers and are recognized by IgG antibodies contained within TB patient sera. These results indicate that Mtp are produced during human infection. Mtp bind to the extracellualr matrix protein laminin in vitro suggesting that Mtp are a newly identified adherence factor for M. tuberculosis. A second pili morphotype that appeared as rope-like bundles were observed for M. tuberculosis and it was found that the M. tuberculosis chromosome contains a type IVB pili gene cluster. The M. tuberculosis type IV pili belong to the Flp sub-family of type IVB pili. RT-PCR analysis reveals that flp is expressed by M. tuberculosis and IF microscopy with Flp-specific antibodies shows the Flp protein is secreted from the bacteria. Evidence presented herein also demonstrates that an Flp-derived peptide is capable of polymerizing into pili-like fibers in vitro over a pH range of 4.5-7.5. Further studies show that the M. tuberculosis type IV pili are encoded by a novel 5-kb genomic 14 island that contains the flp prepilin and putative biogenesis genes. The flp genomic island is characterized by an increased G+C content of 70% (the mean G+C content of the M. tuberculosis chromosome is 65%) and is flanked by multiple direct repeats. The identification of type IV pili in M. tuberculosis is the first report of any classical virulence factor for the bacillus and the genetic characteristics of the locus strongly suggest this chromosomal region was horizontally acquired. 15