BSC 310 Lecture Notes - Lecture 34: Gram-Negative Bacteria, Syncytium, Lysosome
Contributing Factors
Infectious disease results from a competition for supremacy between the parasite and
the host. If the parasite overcomes the host, there is a change in the general state of
good health and disease develops.
Several contributing factors are involved in the establishment of infectious disease.
These factors determine whether the infecting organism will survive in the body.
Portals of entry. In order for a pathogen to gain access to the host, the pathogen must
pass through a portal of entry. One of the most common portals of entry is
the mucous membranes, especially those of the respiratory, gastrointestinal, and
urogenital tracts. Another important portal of entry is the skin. Penetration of the skin
occurs during a wound or by a hair follicle. When microorganisms penetrate below the
skin, the portal of entry is said to be the parenteral route.
Dose. The dose of an organism refers to the number of microorganisms required to
establish an infection. For some diseases, such as typhoid fever, the dose is a few
hundred bacteria. For other diseases, such as cholera, the dose may be several million
bacteria. The dose may be expressed as the LD 50 , which refers to the dose of
microorganisms that will kill 50 percent of the hosts it enters.
Invasiveness. Invasiveness is a property that encourages disease because it refers to
the ability of pathogens to penetrate into the tissues. Those organisms that cause
intestinal ulcers, such as Entamoeba histolytica, penetrate the tissue effectively. Tissue
invasion often begins with adherence, the ability of pathogens to attach to the tissue by
using structures such as pili. The presence of a capsule or glycocalyx encourages
adherence because they are composed of sticky materials.
Capsules. Microorganisms that possess capsulesare able to resist host defenses by
interfering with phagocytosis. Normally, the body uses white blood cells to engulf and
destroy pathogens. However, toxic substances in the capsule are able to destroy the
white blood cells before the white blood cells perform phagocytosis. The organism of
pneumonia Streptococcus pneumoniae is well known for the toxic materials in its
capsule. Many other pathogens also possess capsules.
Enzymes and Toxins
Enzymes. Many pathogens produce a series of enzymes to help overcome body
defenses and establish themselves in the host. One example is leukocidins, a group of
enzymes that destroy white blood cells. This destruction lessens the body's ability to
perform phagocytosis.
Other bacterial enzymes are hemolysins.These enzymes destroy red blood cells.
Streptococci, staphylococci, and certain Clostridium species produce hemolysins.
Coagulases are bacterial enzymes that clot the blood. These enzymes convert
fibrinogen into fibrin, which forms the threads of a blood clot. The clot helps
staphylococci avoid the body's phagocytes and contributes to its pathogenicity.
Other important enzymes are streptokinase and hyaluronidase. Streptokinase is a
streptococcal enzyme that dissolves blood clots. This activity helps the organism
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Infectious disease results from a competition for supremacy between the parasite and the host. If the parasite overcomes the host, there is a change in the general state of good health and disease develops. Several contributing factors are involved in the establishment of infectious disease. These factors determine whether the infecting organism will survive in the body. In order for a pathogen to gain access to the host, the pathogen must pass through a portal of entry. One of the most common portals of entry is the mucous membranes, especially those of the respiratory, gastrointestinal, and urogenital tracts. Another important portal of entry is the skin. Penetration of the skin occurs during a wound or by a hair follicle. When microorganisms penetrate below the skin, the portal of entry is said to be the parenteral route. The dose of an organism refers to the number of microorganisms required to establish an infection.
