To cause disease, many bacteria must first enter the body and invade host tissues. The ability to invade and cause disease is part of a pathogen's virulence.Entry. Bacteria enter through breaks in the skin (cuts, bites, wounds), or through the mucous membranes of the respiratory, digestive and reproductive tracts.Attachment. Bacteria use hair-like projections called pili (fimbriae) and surface proteins to bind to receptors on host cells. This stops them being washed away by mucus, urine or gut contents.Spreading through tissues. Some bacteria secrete enzymes that break down the substances holding host cells and tissues together:
• hyaluronidase breaks down hyaluronic acid, the 'cement' between cells, so bacteria can pass between them;
• collagenase breaks down collagen in connective tissue;
• coagulase, made by Staphylococcus aureus, makes blood plasma clot around the bacteria, shielding them from phagocytes.Getting inside cells. Some bacteria, such as Salmonella, cause epithelial cells to take them in, so they multiply inside the cells, hidden from antibodies. Mycobacterium tuberculosis survives inside the macrophages that engulf it.Avoiding the immune system. A slimy capsule makes it hard for phagocytes to bind to and engulf the bacteria. Some bacteria also release toxins that kill phagocytes.Once established, bacteria may spread in the blood or lymph to other organs, and the damage caused by invasion adds to the damage done by their toxins.
• hyaluronidase breaks down hyaluronic acid, the 'cement' between cells, so bacteria can pass between them;
• collagenase breaks down collagen in connective tissue;
• coagulase, made by Staphylococcus aureus, makes blood plasma clot around the bacteria, shielding them from phagocytes.Getting inside cells. Some bacteria, such as Salmonella, cause epithelial cells to take them in, so they multiply inside the cells, hidden from antibodies. Mycobacterium tuberculosis survives inside the macrophages that engulf it.Avoiding the immune system. A slimy capsule makes it hard for phagocytes to bind to and engulf the bacteria. Some bacteria also release toxins that kill phagocytes.Once established, bacteria may spread in the blood or lymph to other organs, and the damage caused by invasion adds to the damage done by their toxins.