Some have been implicated as a cause of acne. Corynebacterium diphtheriae, the agent of diphtheria, was considered a member of the normal flora before the widespread use of the diphtheria toxoid, which is used to immunize against the disease. Looking for the most current news, updates, and articles relating to microbiology, go to The American Society for Microbiology educational website Microbe World.
Print this Page To search the entire book, enter a term or phrase in the form below. However, the surface tissues, i. The mixture of organisms regularly found at any anatomical site is referred to as the normal flora , except by researchers in the field who prefer the term " indigenous microbiota ". The normal flora of humans consists of a few eucaryotic fungi and protists, but bacteria are the most numerous and obvious microbial components of the normal flora.
Figure 1. Gram stain of a species of Micrococcus, commonly isolated from the skin and nasal membranes of humans. The predominant bacterial flora of humans are shown in Table 1. This table lists only a fraction of the total bacterial species that occur as normal flora of humans. A recent experiment that used 16S RNA probes to survey the diversity of bacteria in dental plaque revealed that only one percent of the total species found have ever been cultivated. Similar observations have been made with the intestinal flora.
Also, this table does not indicate the relative number or concentration of bacteria at a particular site. 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. Is normal flora harmful? Members of the normal flora may cause endogenous disease if they reach a site or tissue where they cannot be restricted or tolerated by the host defenses.
Many of the normal flora are potential pathogens, and if they gain access to a compromised tissue from which they can invade, disease may result. How do antibiotics affect normal flora? Antibiotics that are prescribed to treat pathogenic bacteria also have an impact on the normal microbial flora of the human gut.
Antibiotics can alter the composition of microbial populations potentially leading to other illnesses and allow micro-organisms that are naturally resistant to the antibiotic to flourish. What does Flora mean in medical terms? Medical Definition of Flora Flora: The population of microbes inhabiting the outside or inside surfaces of people or other animals. Also, the population of plants including flowers, usually in a particular area.
Where are normal flora found in the body? Normal flora can be found in many sites of the human body including the skin especially the moist areas, such as the groin and between the toes , respiratory tract particularly the nose , urinary tract, and the digestive tract primarily the mouth and the colon.
Are fungi normal flora? Like bacteria, fungi can comprise part of our normal flora. The overall beneficial effects of microbes are summarized below. The normal flora synthesize and excrete vitamins in excess of their own needs, which can be absorbed as nutrients by their host.
For example, in humans, enteric bacteria secrete Vitamin K and Vitamin B12, and lactic acid bacteria produce certain B-vitamins. Germ-free animals may be deficient in Vitamin K to the extent that it is necessary to supplement their diets. The normal flora prevent colonization by pathogens by competing for attachment sites or for essential nutrients. This is thought to be their most important beneficial effect, which has been demonstrated in the oral cavity, the intestine, the skin, and the vaginal epithelium.
In some experiments, germ-free animals can be infected by 10 Salmonella bacteria, while the infectious dose for conventional animals is near 10 6 cells. The normal flora may antagonize other bacteria through the production of substances which inhibit or kill nonindigenous species.
The intestinal bacteria produce a variety of substances ranging from relatively nonspecific fatty acids and peroxides to highly specific bacteriocins, which inhibit or kill other bacteria.
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