Abstract
Salmonella enterica causes a range of important diseases in humans and a in a variety of animal species. The ability of bacteria to adhere to, invade and survive within host cells plays an important role in the pathogenesis of Salmonella infections. In systemic salmonellosis, macrophages constitute a niche for the proliferation of bacteria within the host organism. Salmonella enterica serovar Typhimurium is flagellated and the frequency with which this bacterium collides with a cell is important for infection efficiency. We investigated how bacterial motility affects infection efficiency, using a combination of population-level macrophage infection experiments and direct imaging of single-cell infection events, comparing wild-type and motility mutants. Non-motile and aflagellate bacterial strains, in contrast to wild-type bacteria, collide less frequently with macrophages, are in contact with the cell for less time and infect less frequently. Run-biased Salmonella also collide less frequently with macrophages but maintain contact with macrophages for a longer period of time than wild-type strains and infect the cells more readily. Our results suggest that uptake of S. Typhimurium by macrophages is dependent upon the duration of contact time of the bacterium with the cell, in addition to the frequency with which the bacteria collide with the cell.
Highlights
Salmonella enterica infection is a major cause of disease in humans and animals
The contact potential (N ) of each bacterial stain was calculated as follows: N 1⁄4 n/(local multiplicity of infection (MOI) Â nwt), where n is the average number of contact event per macrophage in 5 min, and nwt is the number of contacts for the appropriate wild-type strain (SJW1103 for DmotAB and DfliOPQR, and fliMWT for fliMR60C and fliMP220L) in 5 min, itself normalized by its local MOI
Our results show that non-motile bacteria, once in contact with the macrophage, had similar average periods of time associated with the cell as wild-type strains, while the run-biased strain maintained the longest bacterium–cell contact time
Summary
Salmonella enterica infection is a major cause of disease in humans and animals. Human typhoid fever is responsible for around 27 million cases each year [1,2] and non-typhoidal Salmonella (NTS) causes around 93.8 million cases annually [3]. Non-motile mutants of Salmonella enterica serovar Typhimurium exhibit reduced uptake into host cells. We characterize the adhesion and infection dynamics, using live imaging to directly visualize individual cell –bacterial interactions to determine how motility affects the contact between bacteria and macrophages, and their subsequent uptake. The number of macrophages and infection events in each movie was determined manually, assisted by a Matlab script for counting all bacterium – cell contacts and tracking the duration of each contact event This analysis was performed on 400 frames (5 min) of each movie (see electronic supplementary material, table S3 for contact counts). The contact potential (N ) of each bacterial stain was calculated as follows: N 1⁄4 n/(local MOI Â nwt), where n is the average number of contact event per macrophage in 5 min, and nwt is the number of contacts for the appropriate wild-type strain (SJW1103 for DmotAB and DfliOPQR, and fliMWT for fliMR60C and fliMP220L) in 5 min, itself normalized by its local MOI
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