Intracontinental spread of human invasive Salmonella Typhimurium pathovariants in sub-Saharan Africa

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Abstract
Gordon Dougan and colleagues report whole-genome sequencing of a global collection of 179 Salmonella Typhimurium isolates, including 129 diverse sub-Saharan African isolates associated with invasive disease. They determine the phylogenetic structure of invasive Salmonella Typhimurium in sub-Saharan Africa and find that the majority are from two closely related highly conserved lineages, which emerged in the last 60 years in close temporal association with the current HIV epidemic. A highly invasive form of non-typhoidal Salmonella (iNTS) disease has recently been documented in many countries in sub-Saharan Africa. The most common Salmonella enterica serovar causing this disease is Typhimurium (Salmonella Typhimurium). We applied whole-genome sequence–based phylogenetic methods to define the population structure of sub-Saharan African invasive Salmonella Typhimurium isolates and compared these to global Salmonella Typhimurium populations. Notably, the vast majority of sub-Saharan invasive Salmonella Typhimurium isolates fell within two closely related, highly clustered phylogenetic lineages that we estimate emerged independently ∼52 and ∼35 years ago in close temporal association with the current HIV pandemic. Clonal replacement of isolates from lineage I by those from lineage II was potentially influenced by the use of chloramphenicol for the treatment of iNTS disease. Our analysis suggests that iNTS disease is in part an epidemic in sub-Saharan Africa caused by highly related Salmonella Typhimurium lineages that may have occupied new niches associated with a compromised human population and antibiotic treatment.