Mycoplasma hominis variable adherence-associated antigen: A major adhesin and highly variable surface membrane protein

Mycoplasma hominis is a member of the genus mycoplasma and has only been isolated from humans. It is most frequently isolated from the urogenital tract in the absence of symptoms, but has been isolated from wounds, brain abscess, inflamed joints, blood and placenta from pregnancy with adverse outcomes (especially preterm birth and occasionally term stillbirth). Controversy surrounds whether this organism is a commensal or a pathogen; however, Mycoplasma hominis has been shown to induce preterm birth and foetal lung injury in an experimental primate model as a sole pathogen. These bacteria are known to exist as a parasitic infection, due to a number of missing synthetic and metabolism pathway enzymes from their minimal genome; therefore, the ability to adhere to host cells is important. Here we provide a review that clarifies the different nomenclature (variable adherence-associated antigen and P50) that has been used to investigate the major surface adhesin for this organism, as well as reported mechanisms responsible for turning off its expression. Variation in the structure of this protein can be used to separate strains into six categories, a method that we were able to use to distinguish and characterise 12 UK strains isolated from between 1983 and 2012. We propose that the Vaa should be used in further investigations to determine if commensal populations and those that are associated with disease utilise different forms of this adhesin, as this is under-studied and identification of pathogenic determinants is overdue for this organism