PURPOSE OF REVIEW: To summarize recent evidence regarding the presence and 
potential role of the microbiome in systemic vasculitides.
RECENT FINDINGS: Microbiomic descriptions are now available in patients with 
small, medium and large vessel vasculitis. The majority of studies have 
evaluated gastrointestinal inhabitants, with a smaller number of studies 
describing the nasal, pulmonary or vascular microbiomes. Most published studies 
are observational and cross-sectional. Dysbiosis is seen frequently in 
vasculitis patients with reduced microbial diversity observed in nasal, fecal 
and vascular samples compared with disease and/or healthy controls. Predominant 
bacteria vary, but overall, patients with vasculitis tend to have more 
pathogenic and less commensal bacteria in active disease. In the few 
longitudinal studies available, improvement or resolution of dysbiosis has been 
observed following vasculitis treatment and improved disease activity.
SUMMARY: Dysbiosis and reduced microbial diversity has been identified in 
patients with small, medium and large vessel vasculitis. Although limited data 
suggests microbiomes may 'normalize' following immunosuppression, cause or 
effect cannot be determined. It is hypothesized that microbial disruption in a 
genetically susceptible individual may trigger excessive host immune activation 
and vasculitis; however, larger studies with longitudinal and translational 
design are needed to further our current understanding.