Virulence is a continuum. On one end of the spectrum are organisms that are avirulent not harmful and on the other are organisms that are highly virulent. Highly virulent pathogens will almost always lead to a disease state when introduced to the body, and some may even cause multi-organ and body system failure in healthy individuals.
Less virulent pathogens may cause an initial infection, but may not always cause severe illness. Pathogens with low virulence would more likely result in mild signs and symptoms of disease, such as low-grade fever, headache, or muscle aches.
Some individuals might even be asymptomatic. An example of a highly virulent microorganism is Bacillus anthracis , the pathogen responsible for anthrax. The most serious form of anthrax is inhalation anthrax. After B.
An active infection develops and the bacteria release potent toxins that cause edema fluid buildup in tissues , hypoxia a condition preventing oxygen from reaching tissues , and necrosis cell death and inflammation.
Signs and symptoms of inhalation anthrax include high fever, difficulty breathing, vomiting and coughing up blood, and severe chest pains suggestive of a heart attack. With inhalation anthrax, the toxins and bacteria enter the bloodstream, which can lead to multi-organ failure and death of the patient. If a gene or genes involved in pathogenesis is inactivated, the bacteria become less virulent or nonpathogenic. Figure 2. A graph like this is used to determine LD 50 by plotting pathogen concentration against the percent of infected test animals that have died.
Virulence of a pathogen can be quantified using controlled experiments with laboratory animals. Two important indicators of virulence are the median infectious dose ID 50 and the median lethal dose LD 50 , both of which are typically determined experimentally using animal models.
To calculate these values, each group of animals is inoculated with one of a range of known numbers of pathogen cells or virions. In graphs like the one shown in Figure 2, the percentage of animals that have been infected for ID 50 or killed for LD 50 is plotted against the concentration of pathogen inoculated. Figure 2 represents data graphed from a hypothetical experiment measuring the LD 50 of a pathogen.
Interpretation of the data from this graph indicates that the LD 50 of the pathogen for the test animals is 10 4 pathogen cells or virions depending upon the pathogen studied. Table 2 lists selected foodborne pathogens and their ID 50 values in humans as determined from epidemiologic data and studies on human volunteers. Keep in mind that these are median values.
The actual infective dose for an individual can vary widely, depending on factors such as route of entry; the age, health, and immune status of the host; and environmental and pathogen-specific factors such as susceptibility to the acidic pH of the stomach. For example, just a single cell of Salmonella enterica serotype Typhimurium can result in an active infection. In contrast, S.
Pathogens can be classified as either primary pathogens or opportunistic pathogens. Individuals susceptible to opportunistic infections include the very young, the elderly, women who are pregnant, patients undergoing chemotherapy, people with immunodeficiencies such as acquired immunodeficiency syndrome [AIDS] , patients who are recovering from surgery, and those who have had a breach of protective barriers such as a severe wound or burn. An example of a primary pathogen is enterohemorrhagic E.
This toxin inhibits protein synthesis, leading to severe and bloody diarrhea, inflammation, and renal failure, even in patients with healthy immune systems. Staphylococcus epidermidis , on the other hand, is an opportunistic pathogen that is among the most frequent causes of nosocomial disease.
However, in hospitals, it can also grow in biofilms that form on catheters, implants, or other devices that are inserted into the body during surgical procedures. Once inside the body, S. Other members of the normal microbiota can also cause opportunistic infections under certain conditions.
This often occurs when microbes that reside harmlessly in one body location end up in a different body system, where they cause disease. For example, E. This is the leading cause of urinary tract infections among women. Members of the normal microbiota may also cause disease when a shift in the environment of the body leads to overgrowth of a particular microorganism.
For example, the yeast Candida is part of the normal microbiota of the skin, mouth, intestine, and vagina, but its population is kept in check by other organisms of the microbiota. If an individual is taking antibacterial medications, however, bacteria that would normally inhibit the growth of Candida can be killed off, leading to a sudden growth in the population of Candida , which is not affected by antibacterial medications because it is a fungus.
An overgrowth of Candida can manifest as oral thrush growth on mouth, throat, and tongue , a vaginal yeast infection , or cutaneous candidiasis.
Other scenarios can also provide opportunities for Candida infections. Untreated diabetes can result in a high concentration of glucose in the saliva, which provides an optimal environment for the growth of Candida, resulting in thrush. Vaginal yeast infections can result from decreases in estrogen levels during the menstruation or menopause. The amount of glycogen available to lactobacilli in the vagina is controlled by levels of estrogen; when estrogen levels are low, lactobacilli produce less lactic acid.
The resultant increase in vaginal pH allows overgrowth of Candida in the vagina. To cause disease, a pathogen must successfully achieve four steps or stages of pathogenesis : exposure contact , adhesion colonization , invasion, and infection.
In many cases, the cycle is completed when the pathogen exits the host and is transmitted to a new host. An encounter with a potential pathogen is known as exposure or contact. The food we eat and the objects we handle are all ways that we can come into contact with potential pathogens. Yet, not all contacts result in infection and disease. For a pathogen to cause disease, it needs to be able to gain access into host tissue.
An anatomic site through which pathogens can pass into host tissue is called a portal of entry. These are locations where the host cells are in direct contact with the external environment. Major portals of entry are identified in Figure 3 and include the skin, mucous membranes, and parenteral routes.
Figure 3. Shown are different portals of entry where pathogens can gain access into the body. With the exception of the placenta, many of these locations are directly exposed to the external environment. Mucosal surfaces are the most important portals of entry for microbes; these include the mucous membranes of the respiratory tract, the gastrointestinal tract, and the genitourinary tract.
Although most mucosal surfaces are in the interior of the body, some are contiguous with the external skin at various body openings, including the eyes, nose, mouth, urethra, and anus. Most pathogens are suited to a particular portal of entry. The respiratory and gastrointestinal tracts are particularly vulnerable portals of entry because particles that include microorganisms are constantly inhaled or ingested, respectively.
Pathogens can also enter through a breach in the protective barriers of the skin and mucous membranes. Pathogens that enter the body in this way are said to enter by the parenteral route. For example, the skin is a good natural barrier to pathogens, but breaks in the skin e. In pregnant women, the placenta normally prevents microorganisms from passing from the mother to the fetus. However, a few pathogens are capable of crossing the blood-placental barrier. The gram-positive bacterium Listeria monocytogenes , which causes the foodborne disease listeriosis, is one example that poses a serious risk to the fetus and can sometimes lead to spontaneous abortion.
Other pathogens that can pass the placental barrier to infect the fetus are known collectively by the acronym TORCH Table 3. Transmission of infectious diseases from mother to baby is also a concern at the time of birth when the baby passes through the birth canal. Babies whose mothers have active chlamydia or gonorrhea infections may be exposed to the causative pathogens in the vagina, which can result in eye infections that lead to blindness.
Fifth disease erythema infectiosum Treponema pallidum bacterium. Later, you could get or spread an infection that those antibiotics cannot cure. The information on this site should not be used as a substitute for professional medical care or advice.
Contact a health care provider if you have questions about your health. Bacterial Infections. Learn More Related Issues Specifics. See, Play and Learn Images.
Resources Find an Expert. Start Here. Diagnosis and Tests. Prevention and Risk Factors. Treatments and Therapies. In response to infection, your immune system springs into action.
An army of white blood cells, antibodies and other mechanisms goes to work to rid your body of whatever is causing the infection. For instance, in fighting off the common cold, your body might react with fever, coughing and sneezing.
What's the best way to stay disease-free? Prevent infections. You can prevent infections through simple tactics, such as washing your hands regularly, avoiding close contact with people who are sick, cleaning surfaces that are touched often, avoiding contaminated food and water, getting vaccinations, and taking appropriate medications.
Your doctor can perform diagnostic tests to find out if you're infected, the seriousness of the infection and how best to treat that infection.
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