Bacteria are minute living organisms that can neither be classified as plants nor animals, but occupy a class of their own. They are less than one-cell thick and due to their rapidly multiplying nature, they are everywhere around us. Some are harmful while others can be of invaluable economic importance. They have evolved quite a number of evasive mechanisms to evade immunity.
A bacterium has motility and attachment factors such as adhesions. These are essential for attaching to the cell surfaces. They can withstand mechanical removal in addition to being able to secrete toxins. They impair the protective functions, which facilitate colonization of their hosts. Bordettella pertussis is an agent that causes whooping cough by paralyzing the ciliary clearing role of the respiratory tract (Klaus, 2009).
Klaus (2009) says that the immune recognition evasion by mucosal surfaces is another mechanism. Mucosal surfaces have a variety of opsonizing factors like antibodies that stop and get rid of forthcoming bacteria. A bacterium opposes this by proteolytic degradation of secretory immunoglobulin. This is a method, used by bacteria that colonize the upper respiratory tract. A good example is Haemophilus influenza, an agent that causes respiratory tract infections. This is a bacterium using such a mechanism to avoid opsonization and Fc receptor-mediated engulfment of the phagocyte.
In addition, they can also resist the effectors on the epithelial surfaces. Epithelial body surface offers innate antimicrobial defense. This adaptive mechanism used by bacteria lowers the vulnerability to antimicrobial peptides expressed by the host (Klaus, 2009). Considered to be resistant to enzymatic digestion, some linear peptide degradation is thought to exist. Some bacteria are able to degrade the extracellular matrix. The resulting fragments in turn bind to the antimicrobial peptides eliminating their efficiency. An example of bacteria using this mechanism is the Staphylococcus aureus which is a causative agent of wound infections.
Moreover, they have strategies for invading and crossing the epithelium. For instance, S.enterica Typhimurium invades a system using which it causes its own absorption. They make use of transfer organ in the shape of syringe termed as type III secretion system. It transfers two bacterial products, SopE and SopE2, from the cytoplasm of bacteria into the cell of the eukaryotic host Klaus, (2009).
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In conclusion, bacteria also have the ability to escape from phagocytic responses. They are able to interfere with cytokine secretion, they can resist humoral defense mechanism, interfere with antigen presentation, and inhibit T and B cell effector functions.
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