Characterisation of an acapsular mutant of Burkholderia pseudomallei identified by signature tagged mutagenesis

Researchers used signature tagged mutagenesis to identify virulence factors in Burkholderia pseudomallei, the bacterium causing melioidosis. They identified a mutant (1E10) with a transposon insertion in a gene encoding a mannosyltransferase within the capsular biosynthetic operon. This acapsular mutant was severely attenuated in mice infected intraperitoneally, showing a 10^5-fold increase in lethal dose compared to wild-type bacteria and failing to react with antibodies against the bacterial capsular polysaccharide. The mutant could not proliferate in the peritoneal cavity and was rapidly cleared from infected tissues. Notably, while highly attenuated by intraperitoneal challenge, the mutant remained virulent when administered intravenously, though with delayed onset of death. Mice surviving the mutant challenge were not protected against subsequent wild-type infection, suggesting the capsule is a major protective antigen. The study demonstrates that the polysaccharide capsule is a critical virulence determinant in B. pseudomallei, but that virulence is multifactorial, with other factors contributing to pathogenesis via different infection routes.

Key findings

• Signature tagged mutagenesis identified an acapsular mutant of B. pseudomallei with 10^5-fold increased lethal dose in mice via intraperitoneal challenge
• The transposon inserted into a mannosyltransferase gene within the capsular polysaccharide biosynthetic operon, eliminating capsule production
• The acapsular mutant could not proliferate in the peritoneal cavity and was rapidly cleared, unlike wild-type bacteria which proliferated
• The mutant remained virulent via intravenous infection with delayed mortality, indicating virulence is multifactorial and route-dependent
• The acapsular mutant failed to induce protective immunity against wild-type challenge, suggesting capsular polysaccharide is a major protective antigen