Neisseria meningitidis X (NmX) is a minority meningococcus serogroup for which no vaccine is available. Huge meningococcus X outbreaks occurred in West Africa but developed countries have reported only sporadic cases. Two invasive fatal cases are presented.

Isolates were characterized by serogrouping (latex agglutination and genogroup X-specific polymerase-chain-reaction), PorA and FetA typing, multilocus sequence typing and pulsed-field gel electrophoresis. Immunocompetent female balm/C mice inoculated intraperitoneally were use to test virulence of invasive and carrier isolates.

Until April 2010, NmX was absent among 868 invasive meningococci characterized in the Basque Country in a 20-year period. In April 2010, two fatal NmX: P1.21,16: F5-5/ST750 episodes were detected in unrelated immunodeficient patients. After analysis of 803 meningococcal isolates from asymptomatic carriers obtained between 1988 and 2010, eight NmX isolates were detected. The genotype of the two invasive isolates bore no relation to any of the NmX isolates detected in healthy individuals from the Basque Country or to isolates from outbreaks in Africa.

NmX isolates in the north of Spain can cause severe disease in humans, despite their low prevalence. The in vivo animal study showed that virulence of isolates was more closely associated with the genotype than with the serogroup.