Two Australian isolates of Treponema hyodysenteriae which did not fit within the current serological grouping system for these bacteria wrere examined by agarose gel double immunodiffusion tests (AGDP). Isolate Vic1 was serologically unique, and we propose that it becomes the type organism for a new sixth serological group of T. hyodysenteriae (Group F). Isolate Q1 was unusual in that lipopolysaccharide (LPS) extracted from it reacted strongly in AGDP with serum raised against the type organism for serogroup D (A1), and also weakly with serum raised against the type organism for serogroup B (WA1). The nature of this cross-reactivity was examined by using cross-absorbed antisera in AGDP, and by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot analysis.
The pattern of serological cross-reactivity between Q1, A1 and WA1 was complex and was not fully defined, but the isolate Q1 apparently shared low molecular weight ‘serogroup’ LPS antigens with A1, and shared higher molecular weight LPS antigens with WA1. On this basis Q1 was designated as belonging to serogroup D, although it was recommended that this be qualified as D (B) to indicate the presence of weak cross-reactivity with serogroup B. Such serological cross-reactivity may have significance in relation to the development of immunity to T. hyodysenteriae. Isolate Q1 may be a potentially useful organism for vaccine development because of its ability to induce a good serological response to LPS of treponemes from both serogroups D and B.