A new multi drug resistant strain of Salmonella Typhimurium that is causing life-threatening disease in Africa has been described by scientists.
Such type of Salmonella bug normally causes diarrhoea and is rarely fatal.
The new strain, called ST313, infects vulnerable children and adults in many regions of sub-Saharan Africa leading to death in up to one in four cases.
The new genome work- a collaboration between the Wellcome Trust Sanger Institute, the Malawi-Liverpool-Wellcome Trust Clinical Research Programme and the Kemri-Wellcome Trust Programme- shows how genetic changes transformed this infection into a new antibiotic-resistant form of the disease.
ST313 appears to be better adapted to affect humans, is resistant to several commonly used antibiotics and may spread from person to person.
The severe effects of ST313 - invasive disease and high mortality rates - are seen predominantly in individuals with the immune compromising conditions, HIV, malaria, malnutrition or anaemia.
Until today, it had been widely assumed that this wasn't a new deadly form of the organism but, rather, that the existing diarrhoea-causing strain of the pathogen was taking advantage of the weakened immune system of patients with underlying health problems to cause a more severe disease.
However, the new study has painted a new picture about the strain.
"Multi-drug resistant forms of this deadly type of S. Typhimurium emerged only in the last decade. It seems to have evolved to acquire a unique armoury that allows it to efficiently infect vulnerable children and adults in some African regions. The results highlight the power of in-depth genome sequence analysis to challenge basic medical assumptions and distinguish closely related pathogens. This is a new form of pathogen," explained Dr Robert Kingsley from the Sanger Institute and lead author on the paper.
For the study, researchers analysed approximately 50 samples of the bacterial DNA, extracted from blood samples of African patients with severe symptoms of infection and also suffering HIV, malaria, malnutrition or anaemia.
They used these samples to produce a high-quality reference genome sequence.
Based on this sequence, the team could look in fine detail at the genetic differences between ST313 and strains associated with milder disease symptoms across the globe.
The team's findings suggest that ST313 may be spreading by a new method, not seen before in S. Typhimurium.
This pathogen normally circulates among animals and is introduced to humans through food poisoning. It spends much of its time in the animal hosts.
However, ST313 may be passing predominantly from person to person and so can adapt more rapidly to its human hosts: it doesn't spend time acquiring mutations to help it thrive in the animal host, but on the contrary may be losing them.
This is likely to have helped seal its success and helped it to become the dominant strain among humans.