Bacterium in the context of Immune system

Marine viruses are defined by their habitat as viruses that are found in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. Viruses are small infectious agents that can only replicate inside the living cells of a host organism, because they need the replication machinery of the host to do so. They can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.

When not inside a cell or in the process of infecting a cell, viruses exist in the form of independent particles called virions. A virion contains a genome (a long molecule that carries genetic information in the form of either DNA or RNA) surrounded by a capsid (a protein coat protecting the genetic material). The shapes of these virus particles range from simple helical and icosahedral forms for some virus species to more complex structures for others. Most virus species have virions that are too small to be seen with an optical microscope. The average virion is about one one-hundredth the linear size of the average bacterium.

Diarrhea (American English), also spelled diarrhoea or diarrhœa (British English), is the condition of having at least three loose, liquid, or watery bowel movements in a day. It often lasts for a few days and can result in dehydration due to fluid loss. Signs of dehydration often begin with loss of the normal stretchiness of the skin and irritable behaviour. This can progress to decreased urination, loss of skin color, a fast heart rate, and a decrease in responsiveness as it becomes more severe. Loose but non-watery stools in babies who are exclusively breastfed, however, are normal.

The most common cause is an infection of the intestines due to a virus, bacterium, or parasite—a condition also known as gastroenteritis. These infections are often acquired from food or water that has been contaminated by feces, or directly from another person who is infected. The three types of diarrhea are: short duration watery diarrhea, short duration bloody diarrhea, and persistent diarrhea (lasting more than two weeks, which can be either watery or bloody). The short duration watery diarrhea may be due to cholera, although this is rare in the developed world. If blood is present, it is also known as dysentery. A number of non-infectious causes can result in diarrhea. These include lactose intolerance, irritable bowel syndrome, non-celiac gluten sensitivity, celiac disease, inflammatory bowel disease such as ulcerative colitis, hyperthyroidism, bile acid diarrhea, and a number of medications. In most cases, stool cultures to confirm the exact cause are not required.

Pneumonic plague is a severe lung infection caused by the bacterium Yersinia pestis. Symptoms include fever, headache, shortness of breath, chest pain, coughing, and coughing up blood. They typically start about three to seven days after exposure. It is one of three forms of plague, the other two being septicemic plague and bubonic plague.

The pneumonic form may occur following an initial bubonic or septicemic plague infection. It may also result from breathing in airborne droplets from another person or animal infected with pneumonic plague. The difference between the forms of plague is the location of infection; in pneumonic plague the infection is in the lungs, in bubonic plague the lymph nodes, and in septicemic plague within the blood. Diagnosis is by testing the blood, sputum, or fluid from a lymph node.

The germ theory of disease is the currently accepted scientific theory for many diseases. It states that microorganisms known as pathogens or "germs" can cause disease. These small organisms, which are too small to be seen without magnification, invade animals, plants, and even bacteria. Their growth and reproduction within their hosts can cause disease. "Germ" refers not just to bacteria but to any type of microorganism, such as protists or fungi, or other pathogens, including parasites, viruses, prions, or viroids. Diseases caused by pathogens are called infectious diseases. Even when a pathogen is the principal cause of a disease, environmental and hereditary factors often influence the severity of the disease, and whether a potential host individual becomes infected when exposed to the pathogen. Pathogens are disease-causing agents that can pass from one individual to another, across multiple domains of life.

Basic forms of germ theory were proposed by Girolamo Fracastoro in 1546, and expanded upon by Marcus von Plenciz in 1762. However, such views were held in disdain in Europe, where Galen's miasma theory remained dominant among scientists and doctors.

Pesticides are substances that are used to control pests. They include herbicides, insecticides, nematicides, fungicides, and many others (see table). The most common of these are herbicides, which account for approximately 50% of all pesticide use globally. Most pesticides are used as plant protection products (also known as crop protection products), which in general protect plants from weeds, fungi, or insects.

In general, a pesticide is a chemical or biological agent (such as a virus, bacterium, or fungus) that deters, incapacitates, kills, or otherwise discourages pests. Target pests can include insects, plant pathogens, weeds, molluscs, birds, mammals, fish, nematodes (roundworms), and microbes that destroy property, cause nuisance, spread disease, or are disease vectors. Pesticides thus increase agricultural yields. Along with these benefits, pesticides also have drawbacks, such as potential toxicity to humans and other species.

Anthrax is an infection caused by the bacterium Bacillus anthracis or Bacillus cereus biovar anthracis. Infection typically occurs by contact with the skin, inhalation, or intestinal absorption. Symptom onset occurs between one day and two months after the infection is contracted. The skin form presents with a small blister with surrounding swelling that often turns into a painless ulcer with a black center. The inhalation form presents with fever, chest pain, and shortness of breath. The intestinal form presents with diarrhea (which may contain blood), abdominal pains, nausea, and vomiting.

According to the U.S. Centers for Disease Control and Prevention, the first clinical descriptions of cutaneous anthrax were given by Maret in 1752 and Fournier in 1769. Before that, anthrax had been described only in historical accounts. The German scientist Robert Koch was the first to identify Bacillus anthracis as the bacterium that causes anthrax.

Syphilis (/ˈsɪfəlɪs, ˈsɪfɪlɪs/) is a sexually transmitted infection caused by the bacterium Treponema pallidum subspecies pallidum. The signs and symptoms depend on the stage it presents: primary, secondary, latent or tertiary. The primary stage classically presents with a single chancre (a firm, painless, non-itchy skin ulceration usually between 1 cm and 2 cm in diameter), though there may be multiple sores. In secondary syphilis, a diffuse rash occurs, which frequently involves the palms of the hands and soles of the feet. There may also be sores in the mouth or vagina. Latent syphilis has no symptoms and can last years. In tertiary syphilis, there are gummas (soft, non-cancerous growths), neurological problems, or heart symptoms. Syphilis has been known as "the great imitator", because it may cause symptoms similar to many other diseases.

Syphilis is most commonly spread through sexual activity. It may also be transmitted from mother to baby during pregnancy or at birth, resulting in congenital syphilis. Other diseases caused by Treponema bacteria include yaws (T. pallidum subspecies pertenue), pinta (T. carateum), and nonvenereal endemic syphilis (T. pallidum subspecies endemicum). These three diseases are not typically sexually transmitted. Diagnosis is usually made by using blood tests; the bacteria can also be detected using dark field microscopy. The Centers for Disease Control and Prevention (U.S.) recommends for all pregnant women to be tested.

Gas exchange is the physiological process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a biological membrane that forms the boundary between an organism and its extracellular environment.

Gases are constantly consumed and produced by cellular and metabolic reactions in most living things, so an efficient system for gas exchange between, ultimately, the interior of the cell(s) and the external environment is required. Small, particularly unicellular organisms, such as bacteria and protozoa, have a high surface-area to volume ratio. In these creatures the gas exchange membrane is typically the cell membrane. Some small multicellular organisms, such as flatworms, are also able to perform sufficient gas exchange across the skin or cuticle that surrounds their bodies. However, in most larger organisms, which have small surface-area to volume ratios, specialised structures with convoluted surfaces such as gills, pulmonary alveoli and spongy mesophylls provide the large area needed for effective gas exchange. These convoluted surfaces may sometimes be internalised into the body of the organism. This is the case with the alveoli, which form the inner surface of the mammalian lung, the spongy mesophyll, which is found inside the leaves of some kinds of plant, or the gills of those molluscs that have them, which are found in the mantle cavity.

Bacteriochlorophylls (BChl) are photosynthetic pigments that occur in various phototrophic bacteria. They were discovered by C. B. van Niel in 1932. They are related to chlorophylls, which are the primary pigments in plants, algae, and cyanobacteria. Organisms that contain bacteriochlorophyll conduct photosynthesis to sustain their energy requirements, but the process is anoxygenic and does not produce oxygen as a byproduct. They use wavelengths of light not absorbed by plants or cyanobacteria. Replacement of Mg with protons gives bacteriophaeophytin (BPh), the phaeophytin form.

• Structures of major bacteriochlorophylls
• 
  bacteriochlorophyll a
• 
  bacteriochlorophyll b
• 
  bacteriochlorophyll c
• 
  bacteriochlorophyll d
• 
  bacteriochlorophyll e
• 
  bacteriochlorophyll f
• 
  bacteriochlorophyll g

A zoonosis (/zoʊˈɒnəsɪs, ˌzoʊəˈnoʊsɪs/ ; plural zoonoses) or zoonotic disease is an infectious disease of humans caused by a pathogen (an infectious agent, such as a virus, bacterium, parasite, fungi, or prion) that can jump from a non-human vertebrate to a human. When humans infect non-humans, it is called reverse zoonosis or anthroponosis.

Major modern diseases such as Ebola and salmonellosis are zoonoses. HIV was a zoonotic disease transmitted to humans in the early part of the 20th century, though it has now evolved into a separate human-only disease. Human infection with animal influenza viruses is rare, as they do not transmit easily to or among humans. However, avian and swine influenza viruses in particular possess high zoonotic potential, and these occasionally recombine with human strains of the flu and can cause pandemics such as the 2009 swine flu. Zoonoses can be caused by a range of disease pathogens such as emergent viruses, bacteria, fungi and parasites; of 1,415 pathogens known to infect humans, 61% were zoonotic. Most human diseases originated in non-humans; however, only diseases that routinely involve non-human to human transmission, such as rabies, are considered direct zoonoses.

Driftwood is a wood that has been washed onto a shore or beach of a sea, lake, or river by the action of winds, tides or waves. It is part of beach wrack.

In some waterfront areas, driftwood is a major nuisance. However, the driftwood provides shelter and food for birds, fish and other aquatic species as it floats in the ocean. Gribbles, shipworms and bacteria decompose the wood and gradually turn it into nutrients that are reintroduced to the food web. Sometimes, the partially decomposed wood washes ashore, where it also shelters birds, plants, and other species. Driftwood can become the foundation for sand dunes.

A sympagic environment is one where water exists mostly as a solid, ice, such as a polar ice cap or glacier. Solid sea ice is permeated with channels filled with salty brine. These briny channels and the sea ice itself have its ecology, referred to as "sympagic ecology".

Residents of temperate or tropical climates often assume, mistakenly, that ice and snow are devoid of life. In fact, a number of varieties of algae such as diatoms engage in photosynthesis in arctic and alpine regions of Earth. Other energy sources include Aeolian dust and pollen swept in from other regions. These ecosystems also include bacteria and fungi, as well as animals like flatworms and crustaceans. A number of sympagic worm species are commonly called ice worms.

Septicemic plague is one of the three forms of plague, and is caused by Yersinia pestis, a gram-negative species of bacterium. Septicemic plague is a systemic disease involving infection of the blood and is most commonly spread by bites from infected fleas. Septicemic plague can cause disseminated intravascular coagulation and is always fatal when untreated. The other varieties of the plague are bubonic plague and pneumonic plague.

Yersinia enterocolitica is a Gram-negative, rod-shaped bacterium, belonging to the family Yersiniaceae. It is motile at temperatures of 22–29 °C (72–84 °F), but it becomes nonmotile at normal human body temperature. Y. enterocolitica infection causes the disease yersiniosis, which is an animal-borne disease occurring in humans, as well as in a wide array of animals such as cattle, deer, pigs, and birds. Many of these animals recover from the disease and become carriers; these are potential sources of contagion despite showing no signs of disease. The bacterium infects the host by sticking to its cells using trimeric autotransporter adhesins.

Y. enterocolitica is widespread in nature, occurring in reservoirs ranging from the intestinal tracts of numerous mammals, avian species, cold-blooded species, and even from terrestrial and aquatic niches. Most environmental isolates are avirulent; however, isolates recovered from porcine sources contain human pathogenic serogroups. In addition, dogs, sheep, wild rodents, and environmental water may also be a reservoir of pathogenic Y. enterocolitica strains. Human pathogenicstrains are usually confined to the intestinal tract and lead to enteritis/diarrhea.

Yersinia pseudotuberculosis is a Gram-negative bacterium that causes Far East scarlet-like fever in humans, who occasionally get infected zoonotically, most often through the food-borne route. Animals are also infected by Y. pseudotuberculosis. The bacterium is urease positive.

Skin flora, also called skin microbiota, refers to microbiota (communities of microorganisms) that reside on the skin, typically human skin.Many of them are bacteria of which there are around 1,000 species upon human skin from nineteen phyla. Most are found in the superficial layers of the epidermis and the upper parts of hair follicles.

Skin flora is usually non-pathogenic, and either commensal (are not harmful to their host) or mutualistic (offer a benefit). The benefits bacteria can offer include preventing transient pathogenic organisms from colonizing the skin surface, either by competing for nutrients, secreting chemicals against them, or stimulating the skin's immune system. However, resident microbes can cause skin diseases and enter the blood system, creating life-threatening diseases, particularly in immunosuppressed people.

Transduction is the process by which foreign DNA is introduced into a cell by a virus or viral vector. An example is the viral transfer of DNA from one bacterium to another and hence an example of horizontal gene transfer. Transduction does not require physical contact between the cell donating the DNA and the cell receiving the DNA (which occurs in conjugation), and it is DNase resistant (transformation is susceptible to DNase). Transduction is a common tool used by molecular biologists to stably introduce a foreign gene into a host cell's genome (both bacterial and mammalian cells).

A human pathogen is a pathogen (microbe or microorganism such as a virus, bacterium, prion, or fungus) that causes disease in humans.

The human physiological defense against common pathogens (such as Pneumocystis) is mainly the responsibility of the immune system with help by some of the body's normal microbiota. However, if the immune system or "good" microbiota are damaged in any way (such as by chemotherapy, human immunodeficiency virus (HIV), or antibiotics being taken to kill other pathogens), pathogenic bacteria that were being held at bay can proliferate and cause harm to the host. Such cases are called opportunistic infections.

An antitoxin is an antibody with the ability to neutralize a specific toxin. Antitoxins are produced by certain animals, plants, and bacteria in response to toxin exposure. Although they are most effective in neutralizing toxins, they can also kill bacteria and other microorganisms. Antitoxins are made within organisms, and can be injected into other organisms, including humans, to treat an infectious disease. This procedure involves injecting an animal with a safe amount of a particular toxin. The animal's body then makes the antitoxin needed to neutralize the toxin. Later, blood is withdrawn from the animal. When the antitoxin is obtained from the blood, it is purified and injected into a human or other animal, inducing temporary passive immunity. To prevent serum sickness, it is often best to use an antitoxin obtained from the same species (e.g. use human antitoxin to treat humans).

Most antitoxin preparations are prepared from donors with high titers of antibody against the toxin, making them hyperimmune globulins.