A lethal form of Salmonella prevalent in sub-Saharan Africa may have evolved to take advantage of weaknesses in the immune system created by the emergence of HIV, according to a report in Nature Genetics released this week (September 30).
A relative of the Salmonella commonly linked to cases of food poisoning, this strain—known as invasive nontyphoidal Salmonella (iNTS)—is spread from person to person and kills up to half the adults it infects. But little is known about how it became so widespread throughout sub-Saharan Africa in the past 50 years.
So researchers at the Wellcome Trust Sanger Institute in Cambridge, UK, sequenced the genomes of 129 isolates of Salmonella typhimurium—the most common cause of the disease—collected from seven African countries between 1988 and 2010. Organizing them into family trees, they found two distinct lineages that emerged and spread in ways that overlapped the spread of HIV. And when compared to strains that cause gastroenteritis or typhoid, the iNTSs were less genetically diverse, which suggests they were introduced only a few times and spread through human populations. “It's remarkable that these are such tightly grouped sets of organisms, and it's quite clear that it's an epidemic,” molecular microbiologist and leader of the research team Gordon Dougan told ScienceNOW.
Many scientists had thought that iNTSs jumped from animals to humans and then died off, as did other nontyphoidal strains. But this study suggests that these two lineages underwent genetic changes that allowed them to jump between people. It also indicates that their spread followed a similar geographic and temporal pathway as that of HIV.
The authors suggest that these deadly strains became an epidemic because HIV opened up a niche, though critics argue that such a thesis relies on several assumptions. “It's great to suggest the possibility that there's a causal relationship, but at the end of the day this is pretty classic correlation," Michael Worobey, an evolutionary biologist at the University of Arizona in Tucson, who was not involved in the study, told Science. "It's really quite possible that the dating [of when these lineages first surfaced] is off by enough so that [causal argument] doesn't make sense.”