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Role of Microorganisms in Disease Microorganisms can enter the body and make a person ill. These types of microbes are called pathogens. They cause infectious diseases such as the common cold and measles. Different pathogens cause different illnesses.
A pathogen can enter the body in one of four ways—through the respiratory tract, the gastrointestinal tract, the urogenital tract, or through a break in the skin’s surface. To cause an infection, the microorganism must attach to its target site in the body. Then, it must multiply rapidly, obtain its nutrients from its host, and avoid any harm from the host’s immune system. Once the pathogen starts causing damage to a person’s vital functions and systems, it progresses from an infection to a disease.
Diseases have specific signs and symptoms that are a result of the pathogen invading a specific location within the body.
Germ Theory The germ theory of disease is a belief that microorganisms are the cause of many diseases. Once they invade a living organism, they grow and multiply within their host, causing localized damage to tissues and therefore, disease. Louis Pasteur was one of the first scientists to prove the germ theory of disease in the 1860s. Before his work, it was believed that microorganisms spontaneously generated. He isolated nutrient broth and proved that without outside access, no living organisms appeared in the broth. Following Pasteur, Robert Koch demonstrated that diseases are caused by specific microorganisms. He helped build the framework for isolating and identifying specific pathogens to see how they were related to different infectious diseases. Vaccine Strategies A vaccine is an agent that helps the body develop an immunity to a disease as a preventative measure. It looks like a disease-causing pathogen, but it is usually a weakened or dead form of it, and stimulates the body’s immune system to react against it. The body then gets programmed to remember that microorganism and destroy it during any future encounters. There are several different types of vaccines: - Live: A living microbe that has been weakened in a lab so that it can cause an immune response but will not cause the disease associated with it; examples include chicken pox, measles, and mumps vaccines. - Inactivated: Previously-threatening microorganisms treated to become inactive by chemicals, heat, radiation, or antibiotics; examples include influenza, hepatitis A, and rabies vaccines. - Toxoid: Made from inactivated toxic compounds (not microorganisms) that cause diseases; examples include tetanus and diphtheria vaccines. - Subunit: A fragment of the whole microorganism, which can still cause an immune response; examples include hepatitis B and HPV vaccines. - Conjugate: A polysaccharide that resembles the outer coat of certain bacteria and is linked to toxic proteins that cause an immune response; an example includes the Haemophilus influenza type B vaccine - DNA: A genetically-engineered strand of DNA that causes cells to produce an antigen to a microbe directly; these are still in the experimental stages for human use against diseases such as influenza and herpes. - Recombinant vector: Microbial DNA is inserted into a harmless virus, which stimulates an immune response; these are still in the experimental stages for diseases such as HIV, rabies, and measles. Laboratory Diagnostic Methods There are several laboratory methods that can help to identify a microbial infection. Direct examination with a microscope can identify specific microorganisms. The microbes can be stained to microscopically illuminate distinct features. Microorganisms can also be cultured in vitro for identification, but the results often take days or weeks. Phenotypic identification in culture can be based on colony size, color, and shape of the organisms. Once in culture, the susceptibility of the microorganism to different antimicrobial agents can also be determined. Genetic identification can also be performed using immunoassays. Immunization Protocols There are many immunizations recommended by the Centers for Disease Control and Prevention (CDC). While some vaccines can be administered with one dose, others require several boosters over time to ensure protection against the disease. The table below gives further detail about the immunization protocols recommended from birth through 18 years of age.
Cancer and Microbiology While most microorganisms only cause infections or diseases, some cause more serious health problems like cancer. The bacteria Helicobacter pylori (H pylori) can cause ulcers in the stomach, as well as inflammation and damage to the inner stomach lining. Long-term infection with H pylori can lead to stomach cancer. The bacteria Chlamydia trachomatis can infect the female reproductive tract. When these bacteria encounter HPV, they can act together to promote growth of cancerous cells and cause cervical cancer. Disease Transmission Diseases can be transmitted from person to person either directly or indirectly. Direct transmission happens when an infected person exchanges bodily fluids with another person, either through direct contact or through droplet, including sneezing and coughing. Indirect transmission can occur through airborne transmission or touching contaminated objects, such as a doorknob, contaminated food or water, or insect bites. The most effective way to prevent disease transmission is through frequent hand-washing. Host Defense Mechanisms There are several ways a host can protect itself against infection. Natural barriers, such as skin and mucous membranes, are a physical barrier to the invasion of microorganisms. The respiratory tract has built-in filters against microorganisms. The acidic pH of the stomach does not allow for the growth of microorganisms. The immune system can send both nonspecific and specific immune responses to fight off pathogens. While nonspecific responses include cytokines that fight general microorganisms, the specific response includes antigens produced by vaccinations and other antibodies that target a specific pathogen. Nosocomial Infections A nosocomial infection is an infection contracted from a healthcare facility or hospital. The microorganisms that cause these infections are generally specific to these locations. Nosocomial infections occur up to 48 hours after a hospital admission, up to 3 days after hospital discharge, up to 30 days after an operation, or in a healthcare facility where the person was admitted for a different reason. These infections often occur because the person already has a compromised immune system, and therefore has increased susceptibility to acquire a new infection.
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