Pathogenesis: what is the process of pathogenesis and how do microorganisms cause disease?

Pathogenesis is defined as the manner of development of a disease and the biological mechanisms that leads to a diseased state. Pathogenesis can be caused by a variety of things however; microbial infection is the most common. Damage or destruction of the host cells is the most common form of pathogenesis for example in the case of Salmonella. The organism enters the body by one of the entry routes (in this case oral) and is taken up by the epithelial cells of the stomach. Here a host specific receptor-ligand reaction occurs, whereby the ligand on the pathogen attaches to the receptor proteins on the host. Certain hosts are particularly more susceptible than others due to the genetic coding and consequent structure of the proteins present. This binding results in the destruction of the microvilli in the epithelium, creating a ruffled surface where invaded cells detach from the stomach wall. This results in inflamed areas which secret large amounts of fluid into the lumen of the gut, therefore causing the symptom of diarrhoea.Secondly, pathogens may produce toxins within the body, i.e. Vibrio cholerae. The toxins produced by VC are released from the cell and result in a loss of chloride and bicarbonate ions from the cells of the intestine. This results in a move of concentration gradient and an osmotic shift and impairs the absorption of waters and salt from the gut. Instead up to 10litres of water can be lost per day – resulting in the disease state of extreme water diarrhoea and potential death from dehydration.Thirdly, the body’s own immune response to pathogens being present can be responsible for causing disease state. Immune cells aim to kill the invading bacteria by the initiation of the inflammation cascade around the infected cells. Damage cells result in lesions, which in the case of Mycobacterium tuberculosis which attacks lung cells, will leave spongy holes that may damage blood vessels. Some pathogens may produce all three responses to infection. The ability of pathogens to cause disease relies on the location, i.e. what tissue in the body is infected with the pathogen. The infectivity level of the pathogen; how easily it can enter the host cell. The invasiveness of the pathogen; how easily the pathogen or its toxin can spread around the body and finally the pathogenicity, how easily it can cause disease.