SecReT6 has classified T6SSs into four types, type i, ii, iii and iv, by employing the schemes proposed by Barret et al. (Microbiology, 2007; Environ Microbiol Rep, 2013) and Russell et al. (Cell Host Microbe, 2014). Type i system in turn contains six subtypes (i1, i2, i3, i4a, i4b and i5), while the type ii class is uniquely populated by the Francisella pathogenicity island-encoded system, leading to this group being also known as type ii. The type iii class is identified in Bacteroidetes. Indeed, our phylogenetic analysis of 825 proteins assigned as the TssB (VipA or IglA) component matches the above described classification scheme, suggesting that TssB alone may be a suitable classification marker once appropriate quantitative phylogenetic criteria are established.
Only experimentally validated T6SSs are counted.