Genetic variation in the MacAB-TolC efflux pump influences pathogenesis of invasive Salmonella isolates from Africa.

The various sub-species ofSalmonella entericacause a range of disease in human hosts. The human-adaptedSalmonellaenterica serovar Typhi enters the gastrointestinal tract and invades systemic sites to cause enteric (typhoid) fever. In contrast, most non-typhoidal serovars ofSalmonellaare primarily restricted to gut tissues. Across Africa, invasive non-typhoidalSalmonella(iNTS) have emerged with an ability to spread beyond the gastrointestinal tract and cause systemic bloodstream infections with increased morbidity and mortality. To investigate this evolution in pathogenesis, we compared the genomes of African iNTS isolates with otherSalmonella entericaserovar Typhimurium and identified severalmacAandmacBgene variants unique to African iNTS. MacAB forms a tripartite efflux pump with TolC and is implicated inSalmonellapathogenesis. We show thatmacABtranscription is upregulated during macrophage infection and after antimicrobial peptide exposure, withmacABtranscription being supported by the PhoP/Q two-component system. Constitutive expression ofmacABimproves survival ofSalmonellain the presence of the antimicrobial peptide C18G. Furthermore, thesemacABvariants affect replication in macrophages and influence fitness during colonization of the murine gastrointestinal tract. Importantly, the infection outcome resulting from thesemacABvariants depends upon both theSalmonellaTyphimurium genetic background and the host geneNramp1, an important determinant of innate resistance to intracellular bacterial infection. The variations we have identified in the MacAB-TolC efflux pump in African iNTS may reflect evolution within human host populations that are compromised in their ability to clear intracellularSalmonellainfections. Author summary SalmonellaTyphimurium will generally cause acute gut infections in humans. However,S. Typhimurium strains causing severe, systemic infections have emerged in sub-Saharan Africa and are phylogenetically distinct from otherS. Typhimurium strains. Our comparative genomic analysis revealedS. Typhimurium sequence-type 313 (ST313) from Africa have notable sequence variations within themacAandmacBgenes. These genes are already known to play a role inSalmonellapathogenesis and are otherwise conserved inSalmonellaand many other Gram-negative bacteria. We show that regulation ofmacABtranscription depends, in part, on the keySalmonellavirulence system PhoP/Q and that expression of MacAB improvesSalmonellaresistance to an antimicrobial peptide. AfricanmacABvariants interfere with this antimicrobial peptide resistance function and can alterSalmonellareplication within macrophages. Using competitive infection experiments in mice, we see that thesemacABvariants influence fitness in the mammalian gut and systemic sites, with AfricanS. Typhimurium reliant upon itsmacABgenotype for systemic infection of susceptible hosts. These results suggest that the evolution of AfricanS. Typhimurium has been shaped by human populations with impaired ability to control intracellularSalmonellainfections.