Resources Find an Expert. Start Here. Diagnosis and Tests. Prevention and Risk Factors. Treatments and Therapies. Related Issues. Statistics and Research. Bleach vs. 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. Upon learning that Pankaj became sick the day after the party, the physician orders a blood test to check for pathogens associated with foodborne diseases. There he is to receive additional intravenous antibiotic therapy and fluids. Following the initial exposure, the pathogen adheres at the portal of entry. The term adhesion refers to the capability of pathogenic microbes to attach to the cells of the body using adhesion factors , and different pathogens use various mechanisms to adhere to the cells of host tissues.
Figure 4. Glycocalyx produced by bacteria in a biofilm allows the cells to adhere to host tissues and to medical devices such as the catheter surface shown here. Molecules either proteins or carbohydrates called adhesins are found on the surface of certain pathogens and bind to specific receptors glycoproteins on host cells.
Adhesins are present on the fimbriae and flagella of bacteria, the cilia of protozoa, and the capsids or membranes of viruses. Protozoans can also use hooks and barbs for adhesion; spike proteins on viruses also enhance viral adhesion. The production of glycocalyces slime layers and capsules Figure 4 , with their high sugar and protein content, can also allow certain bacterial pathogens to attach to cells.
Biofilm growth can also act as an adhesion factor. A biofilm is a community of bacteria that produce a glycocalyx, known as extrapolymeric substance EPS , that allows the biofilm to attach to a surface. Persistent Pseudomonas aeruginosa infections are common in patients suffering from cystic fibrosis, burn wounds, and middle-ear infections otitis media because P. The EPS allows the bacteria to adhere to the host cells and makes it harder for the host to physically remove the pathogen.
The EPS not only allows for attachment but provides protection against the immune system and antibiotic treatments, preventing antibiotics from reaching the bacterial cells within the biofilm.
In addition, not all bacteria in a biofilm are rapidly growing; some are in stationary phase. Since antibiotics are most effective against rapidly growing bacteria, portions of bacteria in a biofilm are protected against antibiotics. Once adhesion is successful, invasion can proceed. Invasion involves the dissemination of a pathogen throughout local tissues or the body. Pathogens may produce exoenzymes or toxins, which serve as virulence factors that allow them to colonize and damage host tissues as they spread deeper into the body.
Pathogens may also produce virulence factors that protect them against immune system defenses. Figure 5 shows the invasion of H. Figure 5. Some are obligate intracellular pathogens meaning they can only reproduce inside of host cells and others are facultative intracellular pathogens meaning they can reproduce either inside or outside of host cells.
By entering the host cells, intracellular pathogens are able to evade some mechanisms of the immune system while also exploiting the nutrients in the host cell. Entry to a cell can occur by endocytosis.
For most kinds of host cells, pathogens use one of two different mechanisms for endocytosis and entry. One mechanism relies on effector proteins secreted by the pathogen; these effector proteins trigger entry into the host cell.
This is the method that Salmonella and Shigella use when invading intestinal epithelial cells. When these pathogens come in contact with epithelial cells in the intestine, they secrete effector molecules that cause protrusions of membrane ruffles that bring the bacterial cell in.
This process is called membrane ruffling. The second mechanism relies on surface proteins expressed on the pathogen that bind to receptors on the host cell, resulting in entry. For example, Yersinia pseudotuberculosis produces a surface protein known as invasin that binds to beta-1 integrins expressed on the surface of host cells. Some host cells, such as white blood cells and other phagocytes of the immune system, actively endocytose pathogens in a process called phagocytosis.
Although phagocytosis allows the pathogen to gain entry to the host cell, in most cases, the host cell kills and degrades the pathogen by using digestive enzymes.
Normally, when a pathogen is ingested by a phagocyte, it is enclosed within a phagosome in the cytoplasm; the phagosome fuses with a lysosome to form a phagolysosome, where digestive enzymes kill the pathogen see Pathogen Recognition and Phagocytosis. However, some intracellular pathogens have the ability to survive and multiply within phagocytes.
Bacteria such as Mycobacterium tuberculosis , Legionella pneumophila , and Salmonella species use a slightly different mechanism to evade being digested by the phagocyte. These bacteria prevent the fusion of the phagosome with the lysosome, thus remaining alive and dividing within the phagosome.
Following invasion, successful multiplication of the pathogen leads to infection. Infections can be described as local, focal, or systemic, depending on the extent of the infection. A local infection is confined to a small area of the body, typically near the portal of entry. White blood cells , antibodies , and other mechanisms go to work to rid your body of the foreign invader.
Indeed, many of the symptoms that make a person suffer during an infection—fever, malaise, headache, rash—result from the activities of the immune system trying to eliminate the infection from the body. Pathogenic microbes challenge the immune system in many ways. Viruses make us sick by killing cells or disrupting cell function. Many bacteria make us sick in the same way that viruses do, but they also have other strategies at their disposal.
Sometimes bacteria multiply so rapidly they crowd out host tissues and disrupt normal function. An aggressive worldwide vaccination campaign, headed by the World Health Organization WHO , managed to wipe out smallpox. However, some viruses — such as those that cause the common cold — are capable of mutating from one person to the next. This is how an infection with essentially the same virus can keep dodging the immune system.
Vaccination for these kinds of viruses is difficult, because the viruses have already changed their format by the time vaccines are developed. This page has been produced in consultation with and approved by:. Anthrax is a rare but potentially fatal bacterial disease that occasionally infects humans. The Western obsession with cleanliness may be partly responsible for the increase in allergic asthma and conditions such as rhinitis. Careful prescribing of antibiotics will minimise the emergence of antibiotic resistant strains of bacteria.
Aspergillus is a fungus that commonly grows on rotting vegetation. It can cause asthma symptoms. The simplest form of prevention for lyssavirus is to avoid close contact with bats. Content on this website is provided for information purposes only. Information about a therapy, service, product or treatment does not in any way endorse or support such therapy, service, product or treatment and is not intended to replace advice from your doctor or other registered health professional.
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