Salmonellosis

Salmonella are rod-shaped bacteria, which is why they are called bacilli; they move on their own, and on Gram staining they do not hold the purple dye: that is what gram-negative means. They can cause gastroenteritis, enteric fever, bacteremia, which is the presence of the bacterium in the blood, and also a state of chronic carriage, in which someone carries it without being sick.

They have tools for getting in. The flagella are the whips they swim with, and the type III secretion system is the apparatus they use to grab onto cells and push their way inside. With that they cross the lining of the intestine through the M cells, a special type of cell in the wall, and reach the Peyer's patches, clumps of immune tissue in the intestine. They slip in through a service door of the gut, not the front door. They then spread inside macrophages, the cells that were supposed to eat them.

Nontyphoidal strains, the ones that do not cause enteric fever, most commonly produce gastroenteritis: diarrhea, fever and cramps. Infection is linked to undercooked eggs, meat and dairy.