The surface structure seen on gonococci after treatment with CMP-NANA is due to sialylation of surface lipopolysaccharide previously described as a 'capsule'.

Phenotypic serum resistance of gonococci in urethral exudates is due to sialylation of lipopolysaccharide (LPS) by host cytidine 5'-monophospho-N-acetyl neuraminic acid (CMP-NANA). A surface structure was visible on gonococci [strain BS4 (agar)] that had been stained with ruthenium red after incubation with CMP-NANA. This structure was not visible after neuraminidase treatment, which released sialic acid but not LPS. The LPS profiles of strain BS4 (agar) had another in vivo-selected strain Gc40 (variant D1), were similar. A monoclonal antibody (mAb) which recognises epitope C on the LPS of 'capsulated' gonococci was shown by immunoblotting to react with several LPS components, including one of about 4.5 kDa which contains the probable site of sialylation by CMP-NANA. The reactions with the mAb were not affected by growing the strains with CMP-NANA nor by neuraminidase treatment of the sialylated LPS. The mAb also gold-labelled the surface of both strains before and after treatment with CMP-NANA. These data indicate that sialylation by CMP-NANA and staining with ruthenium red renders more visible the surface LPS which, sometimes in the past, has been seen as a 'capsule'.