Comparison of serologic and genetic porB-based typing of Neisseria gonorrhoeae: consequences for future characterization

Abstract

Due to temporal changes in the epidemiology of gonorrhea, a precise characterization of Neisseria gonorrhoeae is essential. In the present study genetic heterogeneity in the porB genes of N. gonorrhoeae was examined, and serovar determination was compared to porB gene sequencing. Among 108 N. gonorrhoeae isolates, phylogenetic analysis of the entire porB alleles (924 to 993 bp) identified 87 unique sequences. By analyzing only the four to six most heterogeneous porB gene regions (174 to 363 bp), 86 out of these 87 genetic variants were identified. Consequently, analysis of shorter highly variable regions of the porB gene generates high-level discriminatory ability as well as fast, objective, reproducible, and portable data for epidemiological characterization of N. gonorrhoeae. Regarding putative antigenic epitopes of PorB for Genetic Systems monoclonal antibodies (MAbs), some of the previous findings were confirmed, but new findings were also observed. For several of the MAbs, however, the precise amino acid residues of PorB critical for single-MAb reactivity were difficult to identify. In addition, repeated serovar determination of 108 N. gonorrhoeae isolates revealed discrepancies for 34 isolates, mostly due to nonreproducible reactivity with single MAbs. Thus, the prospects of a genetic typing system with congruent translation of the serovar determination seem to be limited. In conclusion, analysis of short highly variable regions of the porB gene could form the basis for a fast molecular epidemiological tool for the examination of emergence and transmission of N. gonorrhoeae strains within the community.

FIG. 1.

(A) Neighbor-joining phylogenetic tree of the entire porB1a alleles (924 unambiguously aligned nucleotides) coding for the mature PorB1a proteins of N. gonorrhoeae isolates (n = 35), comprising nine different serovars and two nonserotypeable (NT) serovars according to the initial serovar determination. The serovars of the isolates are included; discrepancies between initial and repeated (one to three times) serovar determination are indicated by boldface italics. *, the isolates were reactive with MAbs 2F12, 5G9, 5D1, and assigned serovar IA-25 in the present study. (B) Neighbor-joining phylogenetic tree of the loop sequences 1, 2, 4, and 8 (174 unambiguously aligned nucleotides) of the porB1a alleles in the isolates described above. The entire and partial porB1a sequences, respectively, of the isolates 34/98 and 114/98, which represent an outgroup, were used to root the trees. The lengths of the branches leading to the outgroups have been reduced by a factor of three. All bootstrap values (as a percentage of 1,000 resamplings) are shown.

FIG. 2.

(A) Neighbor-joining phylogenetic tree of the entire porB1b alleles (999 unambiguously aligned nucleotides) coding for the mature PorB1b proteins of N. gonorrhoeae isolates (n = 73), comprising 19 different serovars according to the initial serovar determination. The serovars of the isolates are included; discrepancies between initial and repeated (one or two times) serovar determination are indicated by boldface italics. (B) Neighbor-joining phylogenetic tree of the loop sequences 1, 3, 5, 6, 7, and 8 (369 unambiguously aligned nucleotides) of the porB1b alleles in the isolates described above. The entire and partial porB1b sequence, respectively, of the strain CCUG 42287, which represents an outgroup, was used to root the trees. All bootstrap values (as a percentage of 1,000 resamplings) are shown. The isolates that were indistinguishable by using only the loop sequences are shaded.