Evolution of Salmonella within Hosts

Abstract

Within-host evolution has resulted in thousands of variants of Salmonella that exhibit remarkable diversity in host range and disease outcome, from broad host range to exquisite host restriction, causing gastroenteritis to disseminated disease such as typhoid fever. Within-host evolution is a continuing process driven by genomic variation that occurs during each infection, potentiating adaptation to a new niche resulting from changes in animal husbandry, the use of antimicrobials, and emergence of immune compromised populations. We discuss key advances in our understanding of the evolution of Salmonella within the host, inferred from (i) the process of host adaptation of Salmonella pathovars in the past, and (ii) direct observation of the generation of variation and selection of beneficial traits during single infections.

Keywords: Salmonella; adaptation; evolution; pathogenesis.

Figure 1

Within-Host Evolution of Salmonella and Its Consequences. Illustration of Salmonella (white circle) entering a host animal (blue or green outline) resulting in the emergence of variation during replication (colour-filled circles). Variants with beneficial traits increase in frequency due to diversifying selection (red and purple circles), while those with detrimental or neutral traits are lost from the population by genetic drift or purifying selection (marked ‘X’). However, within-host evolution, resulting in the increased frequency of beneficial mutations, may not be beneficial outside of the host and counter-selected due to its loss in fitness for transmission (red circle with ‘X’) or result in increased fitness (purple circle) in one host (blue outline) but decreased fitness (purple circle marked with ‘X’) in a second host species (green outline), in which case host restriction may occur.

Figure 2

Within-Host Evolution Inferred from Salmonella Typhi Host Adaptation. Broad host range serovar S. Typhimurium and host-restricted serovar S. Typhi initiate infection by invasion of the intestinal mucosa mediated by T3SS-1-dependent invasion of the epithelium. The expression of flagella and lipopolysaccharide (LPS) with very long O-antigen chains by S. Typhimurium and the concomitant transfer of effector proteins by the T3SS-1, induces an acute host inflammatory response that prevents dissemination of S. Typhimurium to extraintestinal sites. Key to limiting S. Typhimurium dissemination is the recruitment of neutrophils by both complement-dependent and -independent mechanisms – where complement activation occurs through the complement component 3 (C3) cleavage product, C3b, covalently interacting with the exposed O-antigen chains , . S. Typhi, on the other hand, evades detection by the host immune response to facilitate dissemination. Through the action of the acquired TviA regulator (encoded by tviA), S. Typhi downregulates the expression of flagella and T3SS-1 while activating the expression of genes (black arrows) encoding Vi exopolysaccharide (viaB locus) , , . To potentially accommodate the Vi capsule, S. Typhi no longer produces LPS with very long O-antigen chains, due to disruption (orange ‘X’) of fepE encoding a regulator of O-antigen chain lengths. These changes in gene expression and coding capacity work together to reduce the host immune response by inhibiting complement activation (C3) and reducing proinflammatory cytokine production. For pathogenesis in the human host, S. Typhi produces the typhoid toxin (red stars) upon internalization via expression of the acquired and subsequently adapted cdtB, pltA, and pltB genes.

Figure I

Within-Host Evolution of S. Enteritidis during Clinical Infection. Summary of disease progression and within-host evolution in a patient with interleukin-12/23 (IL 12/23) β1 receptor deficiency. A maximum likelihood tree based on core genome sequence variation of 11 sequential strains isolated from the blood of the patient over a 10-year period, and a typical S. Enteritidis PT4 strain (centre), as reported previously . Numerals and colored edges in the phylogenetic tree indicate the number of pseudogenes that were introduced .