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Typhoid Fever Bacterium Has Evolved Slowly

Carriers are asymptomatic and drug-resistant strain circulating in Asia is now in Africa

FRIDAY, Nov. 24 (HealthDay News) -- A genetic analysis of over 100 global strains of Salmonella enterica Typhi, which causes typhoid fever, has shown that the bacterium evolves slowly except during outbreaks, carriers are asymptomatic and drug resistance is spreading, according to research published in the Nov. 24 issue of Science.

Mark Achtman, Ph.D., of the Max-Planck-Institut fur Infektionsbiologie in Berlin, Germany, and colleagues investigated the genetic diversity of 105 globally representative strains of S. enterica serovar Typhi by sequencing fragments of 200 genes.

The researchers found a "neutral" population structure that has not been under strong selection pressure and has evolved slowly, with haplotypes that have existed for millennia. This structure appears to reflect an asymptomatic carrier state, they suggest, although acute epidemics lead to rapid adaptive evolution by natural selection. The use of fluoroquinolone antibiotics has led to drug resistance, although antibiotic-sensitive strains still persist. One drug-resistant strain found predominantly in southern Asia has recently appeared in Africa, they note.

"Our overview of the current global population diversity in Typhi will allow comparisons of genomic sequences from representative strains without the risk of phylogenetic discovery bias," Achtman and colleagues conclude. "Finally, we suggest that the human carrier state may be of much greater importance for neutral evolution and genetic buffering than had been previously appreciated, an interpretation that would demand major changes in public health campaigns to reduce the incidence of typhoid."

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