e Neisseria meningitidis serogroup B (MnB) is a leading cause of bacterial meningitis; however, MnB is most commonly associated with asymptomatic carriage in the nasopharyngeal cavity, as opposed to the disease state. Two vaccines are now licensed for the prevention of MnB disease; a possible additional benefit of these vaccines could be to protect against disease indirectly by disrupting nasopharyngeal carriage (e.g., herd protection). To investigate this possibility, accurate diagnostic approaches to characterize MnB carriage isolates are required. In contrast to invasive meningococcal disease (IMD) isolates, which can be readily serogrouped, carriage isolates often lack capsule expression, making standard phenotypic assays unsuitable for strain characterization. Several antibody-based methods were evaluated for their abilities to serogroup isolates and were compared with two genotyping methods (real-time PCR [rt-PCR] and whole-genome sequencing [WGS]) to identify which approach would most accurately ascertain the polysaccharide groups associated with carriage isolates. WGS and rt-PCR were in agreement for 99% of IMD isolates, including those with coding sequences for MnB, MnC, MnW, and MnY, and the phenotypic methods correctly identified serogroups for 69 to 98% of IMD isolates. In contrast, only 47% of carriage isolates were groupable by genotypic methods, due to mutations within the capsule operon; of the isolates identified by genotypic methods, <43% were serogroupable with any of the phenotypic methods tested. These observations highlight the difficulties in the serogrouping and capsular genogrouping of meningococcal carriage isolates. Based on our findings, WGS is the most suitable approach for the characterization of meningococcal carriage isolates. N eisseria meningitidis, a Gram-negative bacterium that causes both epidemic and endemic life-threatening disease, is also an obligate human commensal organism that colonizes the nasopharyngeal mucosa with no or minimal harm to the host, a phenomenon known as carriage (1). Asymptomatic pharyngeal colonization with N. meningitidis in young adults is relatively common, and these asymptomatic carriers represent a potential reservoir for the transmission of pathogenic isolates in the community (2, 3). The rates of asymptomatic carriage in the United States and Europe are highest among adolescents and young adults, peaking at the age of 19 years, and are estimated to be 10% to 35% (4-6). Rates of transmission and carriage are higher in closed and semiclosed populations, such as university students living in dormitories and military recruits housed in barracks (7). Higher rates of carriage are also found among people in close contact with patients with active meningococcal infections (8). For the majority of people, carriage is an immunizing process that results in systemic, serogroup-specific, protective antibody responses (9, 10). Invasive meningococcal disease (IMD) usually occurs shortly after the onset of colonization of a susceptible host, when the bacteria ...
