Mutator clones of Neisseria meningitidis in epidemic serogroup A disease

Abstract

Serogroup A Neisseria meningitidis has repeatedly caused widespread epidemics of meningitis and septicemia throughout the 20th century. Recently, in a limited collection of strains, epidemic serogroup A isolates were found to have elevated mutation rates that was caused by defects in mismatch repair pathways. To ascertain the role of these mutators in the epidemic spread of this serogroup, the prevalence of hypermutability in a collection of 95 serogroup A N. meningitidis invasive isolates was determined. Overall mutability in Neisseriae can be described by measuring both missense mutation rates as well as phase variation frequencies of "contingency loci." Fifty-seven percent of serogroup A isolates possessed elevated mutability, which could be divided into two classes: intermediate and high level. Eleven of 20 high-level mutators, with phase variation rates >100-fold higher than wild-type isolates, were defective in mismatch repair. Ten of the 34 intermediate mutators possessing >10-fold increases in phase variation rates could be partially complemented by a wild-type mutL allele. A high prevalence of mutators in epidemic isolates indicates that hypermutability may play a major role in the transmission of this pathogen. The added diversity derived from increased phase variation rates may allow fixation of mutator alleles more frequently during epidemic spread.

Figure 1

The linear relationship between the length of the hmbR poly(G) tract and phase variation frequency. Alleles with varying lengths of repeat tracts were used to replace the hmbR locus in wild-type strain IR4048 (○) and a mutator isolate IR4027 (■). The least squares fit lines describing the two isolates are: y_WT(x) = 2.5x + 9.3 × 10⁻⁹ (P < 0.01), and y_Mutator(x) = 2.1x + 7.7 × 10⁻⁶ (P < 0.01). Each data point represents the medians of at least 10 independent measurements with error bars depicting plus/minus quartiles.

Figure 2

Distribution of corrected Hb receptor phase variation frequencies in serogroup A N. meningitidis. The phase variation rates of hmbR (diamonds) and hpuAB (circles) were linearly regressed by using the data from Fig. 1 (see supporting information). The data are divided into subgroups: epidemic subgroups I, III, and IV, as well as geographically isolated subgroups II, V, VI, VII, and VIII (OTHERS). Nearly all of the isolates with elevated Hb receptor phase variation frequencies also possess HIGH rifampicin resistance rates (red symbols), whereas strains with LOW rifampicin resistance rates (blue symbols) generally have SLOW phase variation rates. Mutator isolates that could have wild-type mutability restored by complementation with either MutS or MutL are indicated by vertical dark arrows. MEDIUM switching strains that could be partially complemented by wild-type MutL (see text) are indicated with horizontal white arrows. Each data point represents the median of at least three independent measurements.

Figure 3

Distribution of the rates of spontaneous resistance to rifampicin in serogroup A N. meningitidis. Spontaneous rifampicin resistance rates were determined for each of the 95 menA isolates and sorted from the lowest to the highest. The division between LOW and HIGH rifampicin rates was determined to be 8.8 × 10⁻⁹ per cfu (see text). Each division is normally distributed and significantly different from the other (P < 0.01). The 95% confidence intervals about the mean for each group are shown at the left (red diamond for HIGH and blue diamond for LOW). Data points are coded for Hb receptor phase variation rate phenotypes: SLOW (blue), MEDIUM (green), and FAST (red) (see supporting information for definitions). Mutator isolates that could have wild-type mutability restored by complementation with either MutS or MutL are indicated (dark arrows). MEDIUM switching strains that could be partially complemented by wild-type MutL (see text) are indicated with white arrows. Each data point represents the median of at least three independent measurements.