Population dynamics of Neisseria gonorrhoeae in Shanghai, China: a comparative study

Abstract

Background: Gonorrhea is a major sexually transmitted disease (STD) in many countries worldwide. The emergence of fluoroquinolone resistance has complicated efforts to control and treat this disease. We report the first study of the evolutionary processes acting on transmission dynamics of a resistant gonococcal population from Shanghai, China. We compare these findings with our previous study of the evolution of a fluoroquinolone sensitive gonococcal population from Baltimore, MD.

Methods: Ninety six gonococcal samples were collected from male patients in Shanghai, China. All samples were fluoroquinolone resistant. Seven MLST housekeeping genes, two fluoroquinolone resistance genes (gyrA and parC) and the porB gene were sequenced and subjected to population genetic and evolutionary analyses. We estimated genetic diversity, recombination, growth, and selective pressure. The evolutionary history and population dynamics of the Shanghai population were also inferred and compared with that observed in a fluoroquinolone sensitive gonococcal population from Baltimore.

Results: For both populations, mutation plays a larger role than recombination in the evolution of the porB gene, whereas the latter seems to be the main force driving the evolution of housekeeping and fluoroquinolone resistance genes. In both populations there was evidence for positively selected sites in all genes analyzed. The phylogenetic analyses showed no temporal clustering in the Shanghai gonococcal population, nor did we detect shared allelic profiles between the Shanghai and the Baltimore populations. Past population dynamics of gonococcal strains from Shanghai showed a rising relative effective population size (Ne) in MLST genes with a declining relative Ne for gyrA and parC, whereas among sensitive strains from Baltimore we previously observed concordance among these genes. In both Shanghai and Baltimore, the past population dynamics of gonococcal strains tracked changes in the prevalence of gonorrhea.

Conclusions: Our study illustrates both similarities and differences in the evolutionary processes acting on gonococcal populations in different geographic areas. An explanation of this pattern that may apply in China is the continued use of quinolone antibiotics despite widespread resistance. Population genetic analysis of gonococcal strains in conjunction with epidemiological surveillance may provide insights into the epidemic behavior of antibiotic resistant strains and help to design control measures.

Figure 1

Statistical parsimony network of concatenated housekeeping genes (fumC, gdh, glnA, gnd, pilA, pyrD, and serC). The solid squares (putative outgroups) and the solid circles represent actual sequences derived from the strains analyzed. The size of squares and circles is proportional to the number of sequences displaying the same genotype. The open circles represent putative sequences in the evolutionary pathway. The solid lines on a network represent mutational connections among unique genotypes with at least a 95% degree of confidence, whereas the dashed lines represent a more tenuous connection.

Figure 2

N. gonorrhoeae population dynamics in Shanghai over the past 40 years. Relative population size was estimated from seven housekeeping genes (black lines), two fluoroquinolone resistance genes (green lines), and porB gene (blue lines) with the solid lines showing the mean estimates and the dashed lines showing the 95% highest posterior density (HPD) limits.

Figure 3

N. gonorrhoeae population dynamics in Shanghai and Baltimore over the past 35 years. Comparison of reported cases and relative population size of N. gonorrhoeae in Shanghai and Baltimore over the past 35 years (from 1970 to 2005). The light blue line shows the number of reported cases of N. gonorrhoeae in Baltimore (1970 - 2004), and the dark blue line shows the reported cases of N. gonorrhoeae in Shanghai (1994 - 2005). Relative population size was estimated from the housekeeping genes (brown line: Shanghai; green line: Baltimore), and the fluoroquinolone resistance genes (purple line: Shanghai; pink line: Baltimore). Only the mean estimates are shown.

Figure 4

eBURST diagram displaying the recent evolutionary relationships among MLST sequences in the gonococcal population. (A) eBURST diagram displaying the relatedness of 96 Shanghai gonococcal isolates. The size of each circle is proportional to the number of isolates with the same ST. Clusters of linked isolates correspond to clonal complexes. Primary founders in the cluster are shown in blue, and the subgroup founders are shown in yellow. (B) eBURST diagram displaying the comparison between the Baltimore and the Shanghai populations. The size of each circle is proportional to the number of isolates with the same ST. Clusters of linked isolates correspond to clonal complexes. The STs found only in the Shanghai population are shown in green, and the STs found only in the Baltimore population are shown in black. If STs are found in both populations, they are shown in cyan.