Cell wall arabinogalactan is responsible for Fungitell cross reactivity in nocardiosis.

Nocardiosis is a serious infection in immunosuppressed patients, especially transplant recipients. The slow-growing phenotype of the bacterium and the variety of symptoms complicate diagnosis and delay antimicrobial therapy, resulting in high mortality rates despite effective treatments. A further complication is that some nocardiosis patients test positive in fungal diagnostics that detect (1,3)-β-D-glucan (the Fungitell assay), but the basis for this cross-reactivity remains unknown. We demonstrate that nocardial cell wall arabinogalactan is a cryptic antigen responsible for cross-reactivity in the Fungitell assay and that this antigen is revealed in vivo following bacterial cell lysis. We further show that the reactivity results from a β-glucose substitution of the galactan domain, a modification specific to nocardia, and identify the optimal antigen as a tetramer of the trisaccharide repeating unit. By providing structural evidence for Fungitell cross-reactivity during nocardiosis, this work paves the way for developing specific diagnostic tools that are currently lacking.