The aim of this study was to ascertain and compare genotypic data of NmY isolates causing meningococcal disease in the United States, South Africa, and Israel, where the proportion of NmY disease is higher than the global average. Genotypically, there were 70 different clones among the 300 isolates, as determined by the unique combinations of multilocus ST, 16S type, and PorA type. Overall, the genotypic profiles of NmY isolates in the United States and Israel were similar to each other (ST-23 complex, 16S type 19, and P1.5-1,2-2 or P1.5-2,10-1). This genotypic profile was found in only a minor percentage of the South African isolates. A newly defined clonal complex comprising ST-175 and three closely related STs predominated in the South African NmY population.
NmY isolates in the ST-23 complex have previously been found as carriage isolates in the nasopharynx but were thought to cause invasive disease infrequently (20
). In fact, a study of disease-associated and carriage isolates of N. meningitidis
collected in The Czech Republic, Greece, and Norway from 1991 to 2000 found a negative association of ST-23 with disease (22
). More recent studies, however, have documented the isolation of NmY isolates of the ST-23 complex from patients with invasive meningococcal disease in Taiwan, Canada, and Italy (4
). The proportion of meningococcal disease caused by NmY in Taiwan and Italy, however, remains below 10%. In contrast, the proportion of NmY disease increased in Canada from 8.4% in 1999 to 21.5% in 2003. And 65.7% of the NmY isolates were from the ST-23 clonal complex. In the present study, the high proportion of serogroup Y meningococcal disease in the United States and Israel was due to ST-23, whereas ST-23 accounted for only 15% of the NmY isolates collected in South Africa.
In a recent report, Coulson et al. described the clonal population structure of NmY in South Africa by means of pulsed-field gel electrophoresis typing (5
). The same South African NmY strains were further characterized in the present study using MLST, 16S typing, and PorA VR typing. The majority of the isolates identified by pulsed-field gel electrophoresis as belonging to the clonal cluster Y-1 in the previous study were genotyped as ST-175/16S-21/P1.5-1,2-2. Similarly, the strains identified as belonging to cluster Y-2 in the previous study were ST-23/16S-19/P1.5-2,10-1. These recent data strongly support the previous finding of a clonal population structure of NmY disease in South Africa, and determined that strains belonging to the ST-175 clonal complex were responsible for three-fourths of NmY disease during this period.
The newly designated ST-175 clonal complex includes 32 STs and 110 isolates in the MLST database. Using the strictest criteria for a clonal complex (six of seven alleles identical), whereby all STs are thought to descend from a common founder, there are 17 STs in the MLST database that group with ST-175, including ST-2881, and two STs that were observed in the present study, ST-4367 and ST-4669. All of the 13 STs are single-locus variants of ST-175, indicating that ST-175 may be the founder ST of the group. There are 38 isolates, mainly serogroups W135 and NmY, listed as ST-2881 in the MLST database from the West African countries of Niger, Benin, Burkina Faso, Chad, Togo, and Cameroon. The NmY isolates were from carriers, while many of the W135 isolates were invasive. W135 ST-2881 has caused both sporadic and a cluster of meningitis cases in Niger (2
). Based on genotypic data of carriage and invasive isolates from Niger in 2003, investigators proposed that a capsule switch most likely occurred in the ST-2881 isolates of serogroups W135 and Y (2
). Evidence of capsule switching among isolates of ST-175 also exists as both serogroup W135 and Y isolates from Gambia and Rwanda have been deposited in the Neisseria PubMLST database. An isolate of ST-4942 that differs from ST-175 at only one locus was isolated in The Netherlands in 1970. Therefore, ST-175 and some very close relatives have been circulating at least for the past 30 years and have recently caused disease in several countries in Africa.
Previously, congruence between particular outer membrane protein sequences (PorA, PorB, and FetA) and ST complexes in the major hypervirulent lineages of N. meningitidis
was observed (19
). Urwin et al. concluded that simple clonal and epidemic clonal models of population structure were inadequate to explain the observed strain structure (19
). In our study, we observed a similar congruence between PorA types and clonal complexes. Usually, a PorA type was found among members of a single clonal complex. The exceptions were P1.5,2 that was found among ST-11 and ST-23 clonal complexes, P1.5-1,2-2 that was found among ST-23 and ST-175 clonal complexes, and P1.5-1,10-4 that was found among ST-167 and ST-175 clonal complexes. The clonal complexes that shared PorA types were not found in the same country. Urwin's proposal of a host immunity model of strain structuring whereby shared immunological variants are disadvantaged in the same transmission system (e.g., country) may apply here (19
Antigenic shift in PorA from P1.5-1,2-2 to P1.5-2,10-1 among NmY isolates from Maryland in the 1990s, as noted by Harrison et al. (10
), was confirmed for NmY isolates in the present study collected through the ABCs surveillance program from other states in the United States. Of the 127 isolates (64%) from the United States characterized here from 1999 to 2002, including 9 isolates from Maryland, 81 had PorA type P1.5-2,10-1, and all but one were in the ST-23 complex. The ratio of P1.5-1,2-2 to P1.5-2,10-1 in the NmY ST-23 complex population studied previously by our laboratory decreased from 2.75:1 in 1992 to 0.4:1 in 1998 (21
). The ratio remained below 0.5:1 through 2002 (the present study) (Table ). A similar change in PorA type has been observed over the same time period in Israel. In our previous study, 47% of the isolates from Israel were P1.5-1,2-2, 33% were P1.5-1,2-13, and 10% were P1.5-2,10-1 (21
). The Israeli isolates from 1990 to 2002 showed a decrease to 34% in PorA type P1.5-1, 2-2, a decrease to 17% of P1.5-1,2-13, and an increase in P1.5-2,10-1 to 43%. Historic data were insufficient for a similar analysis of NmY isolates from South Africa. These trends are indicative of cyclical fluctuations of antigenic variants and are predicted by the strain structure model of nonoverlapping antigenic combinations dictated by immune-mediated competition (8
In summary, 70 different clones were found in a population of NmY isolates collected in the United States, Israel, and South Africa, the high proportion of serogroup Y meningococcal disease in the United States and Israel was caused by genotypically distinct clones of N. meningitidis (ST-23 clonal complex) from those causing disease in South Africa (ST-175 clonal complex). ST-175 appears to be the founder of a clonal complex that has recently caused disease in other countries in Africa. Continued active surveillance and genetic characterization of invasive and carriage NmY isolates in the United States, Israel, and South Africa will provide valuable data for local and global epidemiology and allow monitoring for the expansion of existing clonal complexes and detection of the emergence of new virulent clones in the population.