Microbiology

Microbiology Study Guide

For the transmission to happen, there should be a portal of exit from which the infections are shed and transmitted across other organisms. These portals are mostly the infected part of the organism and can include the surface of the skin, intestine, mouth, etc. The infection is transmitted when the pathogen transfers from the portal of exit (of the infected organism) to the portal of entry of the other organism. There can be various portals of entry like mucous membranes in the respiratory, gastrointestinal, and urogenital tracts. Wounds, hair follicles, and skin follicles are also portals of entry. When the organism penetrates deep under the skin, the route for the portal of entry is known as the parenteral route . Dose The number or count of the microorganisms required in order to establish the infection is termed as the dose of the organism. The dose is written as follows: LD 50 , which can be interpreted as the number of microorganisms needed to kill 50 percent of host. The dose of the organisms varies with the disease. For instance, a few hundred bacteria are enough to cause typhoid fever, while millions of bacteria are needed to infect humans with cholera. Virulence Factors Virulence factors cause infections and diseases in the host and constitute the molecules secreted by various species of pathogens. These virulence factors are responsible for forming colonies of the niche in host cells, capable of evading the immune response of the host cells, and invade the immune response of the host cells, enter and exit the cells of intracellular host cells, and extract nutrients from the host cells. By doing this, the pathogens are able to inhibit certain host functions and hence, they are capable of causing infections and diseases among the host cells. One of the most common virulence factors is bacterial toxins, which are further categorized as endotoxins and exotoxins. 4.2 Nonspecific Resistance to Disease Pathogens often observe defense from the non-specific immune system, known as the innate immune system. The non-specific immune system includes various molecules and cells that provide immediate defense against infections and diseases. These cells and molecules recognize the entry of the pathogens and respond to them in a generic manner. However, the non-specific immune system is not as efficient as the adaptive immune system, and thus, provides only short-lived immunity to the host. Mechanical and Chemical Factors Skin The skin acts as a successful defense against many infection-causing pathogens. It constitutes the outer, pigmented, protective, and innervated layer of keratinized stratified squamous epithelium that covers the dermis and contains the derivatives like hair, sweat, nails, and sebaceous glands, which

©2018

Achieve

Page 55

of 132