Salmonella, a type of bacteria commonly associated with foodborne illness, is motile. This means the bacterium can move independently through its environment. Motility allows for active navigation rather than passive displacement by currents or other external forces.
Mechanism of Movement
Salmonella achieves its movement primarily through specialized structures called flagella. Flagella are thin, whip-like appendages that extend from the bacterial cell surface. They rotate rapidly, much like miniature propellers, to generate thrust.
The flagellar motor, a complex molecular machine, powers this rotation. It is composed of a rotor and multiple stator units, with each stator unit acting as a channel for ions like protons or sodium ions. The flow of these ions across the bacterial membrane provides the energy for the flagellum to spin.
Salmonella typically has multiple flagella distributed around its cell body, a characteristic known as peritrichous flagellation. When all flagella rotate in a counterclockwise direction, they bundle together behind the cell, propelling the bacterium in a straight line, a movement referred to as a “run.” If one or more flagellar motors switch to clockwise rotation, the bundle disrupts, causing the bacterium to “tumble” and reorient itself, allowing for changes in swimming direction. This run-and-tumble pattern enables Salmonella to navigate and explore its surroundings.
Role in Infection
Salmonella’s motility plays a role in establishing and spreading infection within a host. Motility helps the bacterium navigate the complex, often viscous environment of the intestinal tract. Flagella allow Salmonella to penetrate the mucus layer, which acts as a protective barrier in the gut.
Motility also contributes to Salmonella’s ability to reach and attach to host cells. Studies indicate that flagellum-associated motility is linked to the invasiveness of Salmonella enterica serovar Typhimurium. This movement facilitates initial contact between bacteria and host cells, a step before attachment and subsequent invasion.
Once inside the host, flagella contribute to the bacterium’s spread within tissues. Motility through host tissues can help Salmonella disseminate beyond the initial site of infection. Flagella can increase the invasiveness of Salmonella.
Motility enhances the bacterium’s ability to colonize and invade. Impaired flagellar motility in Salmonella enterica serovar Enteritidis has been shown to reduce its ability to colonize the gut in chickens during the early stages of infection. The flagellar structure and its motility are relevant to the initial phases of intestinal colonization.