Loss of meningococcal PilU delays microcolony formation and attenuates virulence in vivo.

Neisseria meningitidis is a major cause of sepsis and bacterial meningitis worldwide. This bacterium expresses type IV pili (Tfp), which mediate important virulence traits such as the formation of bacterial aggregates, host cell adhesion, twitching motility, and DNA uptake. The meningococcal PilT protein is a hexameric ATPase that mediates pilus retraction. The PilU protein is produced from the pilT-pilU operon and shares a high degree of homology with PilT. The function of PilT in Tfp biology has been studied extensively, whereas the role of PilU remains poorly understood. Here we show that pilU mutants have delayed microcolony formation on host epithelial cells compared to the wild type, indicating that bacterium-bacterium interactions are affected. In normal human serum, the pilU mutant survived at a higher rate than that for wild-type bacteria. However, in a murine model of disease, mice infected with the pilT mutant demonstrated significantly reduced bacterial blood counts and survived at a higher rate than that for mice infected with the wild type. Infection of mice with the pilU mutant resulted in a trend of lower bacteremia, and still a significant increase in survival, than that of the wild type. In conclusion, these data suggest that PilU promotes timely microcolony formation and that both PilU and PilT are required for full bacterial virulence.