Neisseria meningitidis (Nme) is the causative agent of Invasive Meningococcal Disease (IMD). The ability of Nme to survive complement-mediated killing is crucial in the pathogenesis of IMD. In addition to capsule and surface proteins, partial sialylation of LOS by the LOS sialyltransferase Lst inhibits all three complement pathways and contributes to complete serum resistance in Nme even in the presence of capsule.
Our study sought to assess the LOS sialylation and serum resistance profiles among closely related isolates from IMD outbreak caused by clonal complex 11 meningococci in Western Australia between 2016-2020. We compared isolates from two phylogenetic subclusters of the outbreak: Cluster A, which is closely related to global cc11 isolates; and Cluster B, which is genetically derived and likely represents a high-fitness variant. LOS sialylation was determined by densitometry analysis using Tricine-SDS-PAGE. Resistance to 75% normal human sera was assessed for up to 30 minutes at 37°C. Transcript levels of lst were measured using RT-qPCR.
Among 6 isolates tested, a consistent pattern of regulation was seen during growth: both LOS sialylation and lst transcription peaking during early log-phase. Compared to Cluster A isolates (33.7-64.2% sialylated by densitometry), Cluster B isolates were hypersialylated, expressing constitutively higher sialylation (50.3-82.5% sialylated) and lst transcript (1.54-11.77-fold higher than Cluster A) regardless of growth phase. All wild-type strains were resistant to 75% human sera. However, Lst knockout mutants of Cluster B isolates exhibited a greater survival defect (39.0-43.8% decrease) compared to wild-type than Cluster A isolates (16.1-31.2% decrease), suggesting that LOS sialylation makes a greater contribution to serum resistance in Cluster B isolates.
Our study demonstrates that lst is regulated according to growth phase in meningococci. In Cluster B isolates the baseline expression of lst was higher and resulted in hypersialylation of LOS. Given the importance of LOS sialylation as a virulence factor in Nme, further study to elucidate the nature of this regulatory mechanism is warranted.