Question

Explain the stages needed to establish microbial infections. Classify bacterial toxins. Provide two examples and describe their mechanism of action on mammalian cells.

Answer 1

Stages that lead to the establishment of microbial infection have been discussed below-

Entry inside the host- in order to cause an infection, the host must get exposed to the microbe. This can occur in several ways, for example, through contaminated food and water, air droplets containing the microbe, soil, etc. After the initial exposure, the microbe gains entry inside the host through a specific portal of entry. This can be the mouth, vagina, eye, nose, skin, etc.

The microbe uses several adhesion factors to attach/adhere to the host cells. This is necessary to avoid elimination from the host body (inside the host body several mechanisms are there which function to throw a pathogen out of the body, such as ciliated epithelium, mucus, etc). Different mechanisms which microbes use to successfuly adhere to the host cells include- biofilm formation, production of adhesins, pili and fimbriae, etc.

Invasion- The microbe invades through the host cells, to spread and rapidly multiply in different regions. While doing so, it damages the host tissues, by secreting hydrolytic enzymes. It also escapes the host immune system, which is again necessary to avoid elimination from the body. Many microbes have the ability to escape phagocytosis; they do this by producing capsules. Some can also survive and multiply inside the phagocytes. After successful invasion, the microbe multiplies rapidly, leading to infection.

Bacterial toxins are classified into endotoxins (the outer layer of gram negative bacteria acts as the toxin) and exotoxins (secreted by the bacterial cells in the extracellular environment). Examples of endotoxin includes lipopolysaccharide and O-antigen present in gram negative bacteria such as E.coli , Shigella , etc. Examples of exotoxins are diphtheria toxin and cholera toxin.

Endotoxins are a part of the gram negative cell. Inside the mammalian body, when bacterial cells are lysed, the toxin is released into the body. It binds to macrophages and lymphocytes, which initiates an inflammatory response. This can occur by activation of complement system. Release of cytokines into the bloodstream can raise the body temperature, which leads to fever. In severe cases, it can lead to lowering of blood pressure, septic shock and death.

Exotoxins are released by the bacterial cells in the host body. Most exotoxins are AB type toxins. They are composed of two different subunits. The B subunit helps the toxin attach to the target site, and the A subunit carries out the toxic activity. It can act by modifying the target site, such as by ADP-ribosylation, which stops protein synthesis (for example, in case of diphtheria toxin), can lead to loss of ions and diarrhea (in case of cholera toxin), etc, which ultimately leads to cell death.