The team pinpointed that S. sonnei
was first established in Europe just a few centuries ago, but in the last few decades has spread to the rest of the world. They also found that a key factor in the spread of this pathogen was a rise in multidrug resistance - the ability to survive exposure to a wide array of antibiotics. Because S. sonnei
is easily transmitted and has high levels of drug resistance, the researchers suggest that drug treatment and better sanitation alone will not be sufficient for controlling the disease. Vaccine development will be crucial.
Dysentery is a disease primarily associated with developing countries and more than one million people, mostly young children, are estimated to die from dysentery caused by Shigella each year. Whilst most people have heard about dysentery, few know about the bacteria that causes it, Shigella. This is because it is relatively understudied and little is known about their population structure or its origins. Traditionally, S. flexneri
has been the most common form of Shigella bacterium to cause dysentery in developing countries with S. sonnei
more prevalent in industrialized countries. Yet, this is beginning to change with S. sonnei
becoming increasingly common as developing countries rapidly industrialize.
"Although S. sonnei
is a relatively new species of bacterium, during its spread it has diversified into an array of different distinguishable clones or strains found right across the world," says Dr Kathryn Holt, first author from the University of Melbourne. "This is hard to see using traditional methods, but by sequencing the genomes of over 100 different forms of the bacteria, we were able to get a glimpse into its past and really start to understand how it is evolving and moving around the world."
"We compared the S. sonnei
family tree and geographical locations of the different strains to determine when and where this bacterium first emerged and why it has become such a problem in industrialized countries with increasing access to clean water. Traditionally we associate dysentery with contaminated water and lack of industrialization."
To investigate why the bacterium was spreading so effectively, the team looked at the S. sonnei
's genetic evolution and found that only a few types of genes were selectively evolving over time, particularly those involved with drug resistance. This suggests that a major driver in the spread of this bacterium was its apparent ability to become resistant to drug treatment.
"Since S. sonnei
originated, we found there have been three, independent, yet closely related lineages that have spread. The two most recent lineages have been continually evolving to become increasingly resistant to antimicrobials," says Dr Stephen Baker, a senior author from the Oxford University Clinical Research Unit in Vietnam. "Our data is consistent with antibiotic resistance as being a main driver of the spread and persistence of S.sonnei
around the world, stressing that antibiotics are not a long-term solution for the elimination of this global health problem."
Despite the fact S. sonnei
and S. flexneri
are closely related they have very different surface antigens or coats that interact with the human immune system. S. sonnei has only one type of outer coat, while S. flexneri
has many, all of which look very different from that of S. sonnei
. It has been speculated for some time that S. sonnei
acquired its outer coat from another bacterium that is commonly found in contaminated water, Plesiomonas shigelloides
Both S. sonnei
and P. shigelloides
have an identical outer coat. It is believed that when a person is exposed to contaminated water containing P. shigelloides
, there is an immune cross reaction and the body builds a natural immunity against S. sonnei
. This theory may explain why the incidence of S. sonnei
increases following economic development and improvements to water quality, and is consistent with the patterns of global spread described in the current report.
"One of the Millennium Development Goals is to improve drinking water and reduce water borne diseases, an undeniably important aim," says Professor Nicholas Thomson, lead author from the Wellcome Trust Sanger Institute. "This may have the unforeseen result of increasing the incidence of S. sonnei
dysentery in transitional countries.
"Our research emphasises the importance of a vaccine against Shigella
. The combination of increased incidence and antibiotic resistance of S. sonnei
, means that a vaccine will be increasingly important for the long-term control and prevention of dysentery."