How Bacteria Cause Disease
The body has a complex bacterial population that is an essential part of normal physiology. These microbiota are associated with the skin, upper respiratory tract, throat, gastrointestinal system and vagina. Intact epithelial surfaces as well as anatomical and physiological defences separate these bacteria from the sterile tissues. While small numbers of bacteria can ascend the relatively short urethra and reach the bladder of the premenopausal female, defences on the bladder epithelium and regular micturition with complete emptying usually remove them. It is reasonable to assume that a few bacteria can occasionally cross the bowel epithelium and enter the blood, to be transported by the portal vein to the liver. In most circumstances they are taken up and destroyed in the sinusoids by macrophages, the Kupffer cells.