Glyceraldehyde-3-phosphate dehydrogenase facilitates Mycoplasma synoviae colonisation in the lower respiratory system

Mycoplasma synoviae is a significant pathogen in commercial poultry, causing respiratory disease, synovitis, and reduced egg production. The live attenuated vaccine MS-H, derived from the virulent strain 86079/7NS (7NS) via random mutagenesis, is widely used for disease control. Comparative genomics of MS-H and 7NS revealed mutations in the genes encoding glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a key glycolysis pathway enzyme, as well as the essential GTPase ObgE and the peptide transporter OppF. To examine the role of GAPDH in pathogenicity and immunogenicity, a MS-H transformant complemented with a plasmid expressing the wild-type GAPDH (MS-H + gapdh) was compared in vivo to 7NS containing the empty plasmid (7NS + EP), MS-H with or without the empty plasmid (MS-H + EP), and MS-H complemented with obgE (MS-H + obgE) or oppF (MS-H + oppF). Six groups of 20 specific-pathogen-free chickens were inoculated with each transformant and MS-H, via aerosol and eye drop routes. Postmortem examinations at 2- and 3-week post-challenge (WPI) evaluated colonisation, serological responses, and respiratory lesions in the birds. While all strains colonised the upper respiratory tract, only 7NS + EP and MS-H + gapdh colonised the lower respiratory tract. The 7NS + EP group induced significantly higher airsac lesions and tracheal mucosal thickness when compared to the remaining groups. Early systemic antibody responses were also higher in birds inoculated with MS-H + gapdh and 7NS + EP at 2-WPI compared to other groups. Thus, GAPDH enhances the colonisation and immunogenicity of MS-H in the absence of pathological lesions.