Molecular interactions between Mycoplasma hyopneumoniae and host cells
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The Mycoplasmas are a group of wall-less bacteria belonging to the Mollicutes that are believed to have diverged from the Gram-positive Firmicutes. Mollicutes have undergone reductive evolution, losing genes for the biosynthesis of essential biomolecules, subsequently having to form parasite relationships with their hosts in order to acquire these nutrients. They form these relationships as both commensals and pathogens, and a number of Mycoplasma species cause significant clinical and agricultural diseases. Mycoplasma hyopneumoniae is the causative agent of porcine enzootic pneumonia, a chronic respiratory disease that affects swine populations worldwide. M. hyopneumoniae colonises the upper respiratory tract by adhering to the rapidly beating cilia where it causes ciliostasis and eventual cilial death . M. hyopneumoniae possesses a family of surface adhesins referred to as the P97 and P102 paralog family that it utilises to adhere to the cilia [2-10]. A hallmark of M. hyopneumoniae infection is a potent inflammatory response which is believed to be one of the contributing factors to the gross lung lesions observed in infected swine [11-13]. M. hyopneumoniae is described as a strict extracellular pathogen that only adheres to cilia and knowledge is lacking on additional receptors that M. hyopneumoniae binds to. Recent studies have however, shown that viable M. hyopneumoniae cells can be cultured from the liver, spleen, kidneys and lymph nodes of infected swine [14-16]. These observations suggest that M. hyopneumoniae has the capability to invade through the epithelial barrier and disseminate to distal tissue sites. In addition to this, large microcolonies have been observed in the respiratory tract of swine infected with M. hyopneumoniae . These microcolonies are reminiscent of biofilms, and although biofilm formation has never been investigated in M. hyopneumoniae it is likely that they play a role in the chronicity of disease. Notably, even when lung lesions in M. hyopneumoniae-infected swine are cleared, bronchial swabs can still test positive for M. hyopneumoniae up to 185 days post-infection (P.I.)  and pigs can act as convalescent carriers for up to 200 days P.I. . This suggests that M. hyopneumoniae possesses mechanisms in which it can remain dormant within its host whilst remaining infectious. Vaccines against M. hyopneumoniae can successfully reduce lung lesions but they are unable to prevent transmission in swine herds . In order to create vaccines that inhibit the transmission of M. hyopneumoniae, a better understanding of the disease process is required. This PhD project has thus been devised in order to address the problems outlined above. This work has investigated the ability of adhesins to undergo extensive endoproteolytic processing; demonstrating that proteolytic processing in the P97 and P102 adhesins occurs much more extensively than what has previously been shown. I also show that these adhesins can bind to a myriad of host components such as heparin, fibronectin (Fn) and plasminogen (Plg) and investigate the domains responsible. Additionally, this work presents a number of novel receptors that M. hyopneumoniae targets within its host as well as a comprehensive list of putative adhesins that it utilises to do so. This work has also investigated the ability of M. hyopneumoniae to form biofilms on abiotic surfaces, host cells and within the swine respiratory tract and further demonstrate that surface adhesins play a role in biofilm formation. A number of putative biofilm-associated genes have been identified by screening a transposon mutant library, these genes being potential vaccine candidates. Finally, this work has investigated the ability of M. hyopneumoniae to become internalised by host cells and reside within the cytoplasm. M. hyopneumoniae becomes internalised by vacuole-like structures, and that internalised cells appear to escape from lysosomes to reside free within the cytoplasm. Overall, this PhD project has contributed significantly to understanding how M. hyopneumoniae causes disease. Future work on the novel mechanisms described in this thesis will aid in future vaccine development programs and potentially aid in the control of this important veterinary disease.
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