The pathogen bypasses barrier defenses and starts multiplying in the host’s body. During the first 4 to 5 days, the innate immune response will partially control, but not stop, pathogen growth.
In order to survive and multiply in a host, a successful pathogenmust be able to: (1) colonize the host; (2) find a nutritionally compatible niche in the host body; (3) avoid, subvert, or circumvent the host innate and adaptive immune responses; (4) replicate, using host resources; and (5) exit and spread to a new host.
Infection with a pathogen does not necessarily lead to disease. Infection occurs when viruses , bacteria, or other microbes enter your body and begin to multiply. Disease occurs when the cells in your body are damaged as a result of infection and signs and symptoms of an illness appear.
Introduction to Pathogens. Disable Glossary Links In this Page Pathogens Have Evolved Specific Mechanisms for Interacting with Their Hosts The Signs and Symptoms of Infection May Be Caused by the Pathogen or by the Host's Responses Pathogens Are Phylogenetically Diverse
Pathogens can be transmitted a few ways depending on the type. They can be spread through skin contact, bodily fluids, airborne particles, contact with feces, and touching a surface touched by an infected person.
In order to survive and multiply in a host, a successful pathogen must be able to: (1) colonize the host; (2) find a nutritionally compatible niche in the host body; (3) avoid, subvert, or circumvent the host innate and adaptive immune responses; (4) replicate, using host resources; and (5) exit and spread to a new ...
Infectious diseases are caused by microorganisms such as viruses, bacteria, fungi or parasites and can spread between individuals.
They employ tactics such as modulating their cell surfaces, releasing proteins to inhibit or degrade host immune factors, or even mimicking host molecules.
Obligate pathogens tend to be highly adapted to their hosts, with sophisticated mechanisms to synchronise their life cycles with that of the host, and the ability to manipulate the host's immune system, metabolism and sometimes even behaviour.
Why are pathogens that are transmitted person-to-person often less virulent than those transmitted by arthropod vectors? Person-to-person transmission requires the infected host to remain alive long enough to transmit the organism to another susceptible host.
Pathogen. A microorganism that causes disease.
Infection occurs when viruses, bacteria, or other microbes enter your body and begin to multiply. Disease, which typically happens in a small proportion of infected people, occurs when the cells in your body are damaged as a result of infection, and signs and symptoms of an illness appear.
The number, route, mode of transmission, and stability of an infectious agent outside the host determines its infectivity.
Tears, mucus and saliva Your nose, mouth and eyes are obvious entry points for pathogens. However, tears, mucus and saliva contain an enzyme that breaks down the cell wall of many bacteria.
Two different strategies for survival are assumed by viruses. One is hit and run infection whereby there is successive propagation in a series of hosts. The other is hit and stay with viral persistence in the same host. Hit and run viruses are mainly cytolytic and destroy the cells of the host in which they multiply.
pathogens. The Immune System If pathogens get through the body's barriers: ATTACKED BY THE IMMUNE SYSTEM Page 20 Immune system uses two major strategies: • Inflammatory response. Specific defenses.
The body fights bacterial pathogens with a wide variety of immunological mechanisms, essentially trying to find one that is effective. Bacteria such as Mycobacterium leprae, the cause of leprosy, are resistant to lysosomal enzymes and can persist in macrophage organelles or escape into the cytosol. In such situations, infected macrophages receiving cytokine signals from Th1 cells turn on special metabolic pathways. Macrophage oxidative metabolism is hostile to intracellular bacteria, often relying on the production of nitric oxide to kill the bacteria inside the macrophage.
Mucosal tissues are major barriers to the entry of pathogens into the body. The IgA (and sometimes IgM) antibodies in mucus and other secretions can bind to the pathogen, and in the cases of many viruses and bacteria, neutralize them.
Immune responses in some mucosal tissues such as the Peyer’s patches (see Chapter 21.1 Figure 21.1.10) in the small intestine take up particulate antigens by specialized cells known as microfold or M cells ( Figure 21.5.2 ). These cells allow the body to sample potential pathogens from the intestinal lumen. Dendritic cells then take the antigen to the regional lymph nodes, where an immune response is mounted.
The hygiene theory is the idea that the immune system is geared to respond to antigens, and if pathogens are not present, it will respond instead to inappropriate antigens such as allergens and self-antigens.
Think of a primary infection as a race between the pathogen and the immune system. The pathogen bypasses barrier defenses and starts multiplying in the host’s body.
During the first 4 to 5 days, the innate immune response will partially control, but not stop, pathogen growth. As the adaptive immune response gears up, however, it will begin to clear the pathogen from the body, while at the same time becoming stronger and stronger.
Neutralization is the process of coating a pathogen with antibodies, making it physically impossible for the pathogen to bind to receptors. Neutralization, which occurs in the blood, lymph, and other body fluids and secretions, protects the body constantly.
In response to infection, your immune system springs into action. 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.
Each of us has a unique set of microbial communities, which are believed to play an important role in digestion and in protection from disease.
Microbes occupy all of our body surfaces, including the skin, gut, and mucous membranes. In fact, our bodies contain at least 10 times more bacterial cells than human ones, blurring the line between where microbes end and humans begin. Microbes in the human gastrointestinal tract alone comprise at least 10 trillion organisms, representing more than 1,000 species, which are thought to prevent the gut from being colonized by disease-causing organisms. Among their other beneficial roles, microbes synthesize vitamins, break down food into absorbable nutrients, and stimulate our immune systems.
Types of Microbes. There are five major categories of infectious agents: Viruses, bacteria, fungi, protozoa, and helminths. Viruses. Viruses are tiny, ranging in size from about 20 to 400 nanometers in diameter (see page 9). Billions can fit on the head of a pin.
Bacteria are 10 to 100 times larger than viruses and are more self-sufficient. These single-celled organisms, generally visible under a low-powered microscope, come in three shapes: spherical (coccus), rodlike (bacillus), and curved (vibrio, spirillum, or spirochete).
The most common vector for human infection is the mosquito, which transmits malaria, West Nile virus, and yellow fever. Airborne transmission:Pathogens can also spread when residue from evaporated droplets or dust particles containing microorganisms are suspended in air for long periods of time.
E. colibacteria directly transferring genetic material via a pilus (the thin strand connecting the two).
Pathogenic microbes challenge the immune system in many ways. Viruses make us sick by killing cells or disrupting cell function. Our bodies often respond with fever (heat inactivates many viruses), with the secretion of a chemical called interferon (which blocks viruses from reproducing), or by marshaling the immune system’s antibodies ...
Infection occurs when viruses , bacteria, or other microbes enter your body and begin to multiply. Disease occurs when the cells in your body are damaged as a result of infection and signs and symptoms of an illness appear.
Cells infected with sporozoites eventually burst, releasing another cell form, merozoites, into the bloodstream. These cells infect red blood cells and then rapidly reproduce, destroying the red blood cell hosts and releasing many new merozoites to do further damage.
Those that reach skeletal muscle cells can survive and form new cysts, thus completing their life cycle. Histoplasma capsulatum, a fungus that causes histoplasmosis, grows in soil contaminated with bird or bat droppings.
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. In response to infection, your immune system springs into action. White blood cells , antibodies, and other mechanisms go to work to rid your body ...
The protozoa that cause malaria, which are members of the genus Plasmodium, have complex life cycles. Sporozoites, the stage of the parasite that infects new hosts, develop in the salivary glands of Anopheles mosquitos. They leave the mosquito during a blood meal from a human, enter the host’s liver, and multiply.
Sometimes bacteria multiply so rapidly they crowd out host tissues and disrupt normal function. Sometimes they kill cells and tissues outright.
In order to survive and multiply in a host, a successful pathogenmust be able to: (1) colonize the host; (2) find a nutritionally compatible niche in the host body; (3) avoid, subvert, or circumvent the host innate and adaptive immune responses; (4) replicate, using host resources; and (5) exit and spread to a new host. Under severe selective pressure to induce only the correct host cell responses to accomplish this complexset of tasks, pathogens have evolved mechanisms that maximally exploit the biology of their host organisms. Many of the pathogens we discuss in this chapter are skillful and practical cell biologists. We can learn a great deal of cell biology by observing them.
Some pathogenic bacteria use several independent mechanisms to cause toxicity to the cells of their host. Anthrax,for example, is an acute infectious disease of sheep, cattle, other herbivores, and occasionally humans. It is usually caused by contact with spores of the Gram-positive bacterium, Bacillus anthracis. Unlike cholera, anthrax has never been observed to spread directly from one infected person to another. Dormant spores can survive in soil for long periods of time and are highly resistant to adverse environmental conditions, including heat, ultraviolet and ionizing radiation, pressure, and chemical agents. After the spores are inhaled, ingested, or rubbed into breaks in the skin, the spores germinate, and the bacteria begin to replicate. Growing bacteria secrete two toxins, called lethal toxinand edema toxin. Either toxin alone is sufficient to cause signs of infection. Like the A and B subunits of cholera toxin, both toxins are made of two subunits. The B subunitis identical between lethal toxin and edema toxin, and it binds to a host cell-surface receptorto transfer the two different A subunits into host cells. The A subunit of edema toxin is an adenylyl cyclase that directly converts host cell ATP into cyclic AMP. This causes an ionimbalance that can lead to accumulation of extracellular fluid (edema)in the infected skin or lung. The A subunit of lethal toxin is a zinc protease that cleaves several members of the MAPkinase kinase family (discussed in Chapter 15). Injection of lethal toxin into the bloodstream of an animal causes shock and death. The molecular mechanisms and the sequence of events leading to death in anthrax remain uncertain.
Because bacteria form a kingdom distinct from the eucaryotes they infect (see Figure 25-3), much of their basicmachinery for DNAreplication, transcription, translation, and fundamental metabolismis quite different from that of their host. These differences enable us to find antibacterial drugs that specifically inhibit these processes in bacteria, without disrupting them in the host. Most of the antibioticsthat we use to treat bacterial infections are small molecules that inhibit macromolecular synthesis in bacteria by targeting bacterial enzymes that are either distinct from their eucaryotic counterparts or that are involved in pathways, such as cell wallbiosynthesis, that are absent in humans (Figure 25-8and Table 6-3).
Genetic organization of Vibrio cholerae. (A) Vibrio choleraeis unusual in having two circular chromosomes rather than one. The two chromosomes have distinct origins of replication (oriC1and oriC2). Three loci in pathogenic strains of V. choleraeare (more...)
Most can be classified broadly by their shape as rods, spheres, or spirals and by their cell-surface properties. Although they lack the elaborate morphological variety of eucaryotic cells, they display a surprising array of surface appendages that enable them to swim or to adhere to desirable surfaces (Figure 25-4). Their genomes are correspondingly simple, typically on the order of 1,000,000–5,000,000 nucleotidepairs in size (compared to 12,000,000 for yeastand more than 3,000,000,000 for humans).
But a pathogenor a parasite, like any other organism, is simply trying to live and procreate.
Some hallmarks of bacterial infection, including the swelling and redness at the site of infection and the production of pus (mainly dead white blood cells), are the direct result of immune systemcells attempting to destroy the invading microorganisms. Fever, too, is a defensive response, as the increase in body temperature can inhibit the growth of some microorganisms. Thus, understanding the biology of an infectious disease requires an appreciation of the contributions of both pathogenand host.