A distinct phylum of helminths (parasitic worms), commonly known as the spiny- or thorny-headed worms. The phylum Acanthocephala comprises three classes: Archiacanthocephala, Palaeacanthocephala, and Eoacanthocephala (a suggested fourth class, Polyacanthocephala, has been determined to be part of Eoacanthocephala). The adult members (Fig. 1) of the phylum—termed acanthocephalans, spiny-headed worms, or thorny-headed worms—are parasitic in the alimentary canal of vertebrates. All acanthocephalans are obligatory parasites throughout their entire life cycle; no known member exists as a free-living organism. Approximately 1100 species have been described from all classes of vertebrates, although more species occur in fish (but not elasmobranch fish) than in birds and mammals and only a relatively few species are found in amphibians and reptiles. The geographical distribution of acanthocephalans is worldwide, but genera and species do not have a uniform distribution because some species are confined to limited geographic areas. Host specificity is well established in some species, whereas others exhibit a wide range of host tolerance. The same species never occurs normally, as an adult, in cold-blooded and warm-blooded definitive hosts. The fact that larval development occurs in arthropods gives support to the postulation that the ancestors of acanthocephalans were parasites of primitive arthropods during or before the Cambrian Period and became parasites of vertebrates as this group arose and utilized arthropods for food. Molecular analyses have indicated a close relationship with rotifers (microscopic, mainly free-living aquatic animals), and some researchers have suggested that acanthocephalans and rotifers should be combined into a single taxonomic group (Syndermata). See also: Archiacanthocephala; Arthropoda; Eoacanthocephala; Invertebrate phylogeny; Medical parasitology; Palaeacanthocephala; Parasitology; Rotifera; Vertebrata

A genus of Gram-negative, immotile and nonspore-forming, oval to rod-shaped, often pleomorphic bacteria that occur as parasites or pathogens in mammals (including humans), birds, and reptiles. Actinobacillus bacteria are facultatively aerobic, capable of fermenting carbohydrates (without production of gas) and of reducing nitrates. Most species are oxidase- and catalase-positive. Some cultures tend to stick on the surface of agar media, particularly on primary isolation. The genome deoxyribonucleic acid contains between 40 and 47 mol % guanine plus cytosine. The actinobacillus group shares many biological properties with the genus Pasteurella. At least two of the following features or combinations of features differentiate members of the Actinobacillus group from Pasteurella: hemolysis, delayed or lacking fermentation of d-galactose or d-mannose, fermentation of inositol, positive reactions for both urease and β-galactosidase, hydrolysis of salicin or esculin, and fermentation of maltose together with negative reactions for trehalose fermentation and ornithine decarboxylase. See also: Bacteria; Bacteriology; Medical bacteriology; Medical parasitology; Parasitology; Pasteurella; Pathogen

A heterogeneous collection of bacteria with diverse ecologies and physiologies. The initial definition was based on their branching filamentous cellular morphology during a stage of the growth cycle. The name actinomycete means “ray fungus,” and for many years the actinomycetes were erroneously considered to be fungi, or at least closely related to the fungi. True fungi, such as the common bread mold, also grow as branching filaments (hyphae) that are several micrometers in diameter, whereas those of the actinomycetes are about 1 μm in diameter. The genetic material of the eukaryotic true fungi consists of DNA associated with proteins, and is enclosed in a membrane (the nucleus), whereas the genetic material of the prokaryotic actinomycetes consists of free DNA in direct contact with the cell sap (cytoplasm). In general, actinomycetes do not have membrane-bound organelles, whereas the fungi have an array of organelles such as mitochondria and vacuoles. The cell walls of fungi are made of chitin or cellulose, whereas the cell walls of the actinomycetes are made of a cross-linked polymer containing short chains of amino acids and long chains of amino sugars. Actinomycetes are susceptible to a wide range of antibiotics used to treat bacterial diseases, such as penicillin and tetracycline. See also: Amino acids; Antibiotic; Deoxyribonucleic acid (DNA); Fungi

A bacterial genus in the family Vibrionaceae comprising oxidase-positive, facultatively anaerobic, monotrichously flagellated Gram-negative rods. The mesophilic species of Aeromonas are A. hydrophila, A. caviae, and A. sobria; the psychrophilic one is A. salmonicida. Aeromonads are of aquatic origin and are found in surface and waste water, but not in seawater. They infect chiefly cold-blooded animals, including fishes, reptiles, and amphibians, and only occasionally warm-blooded animals and humans. See also: Bacteria

Any of a group of secondary metabolites produced by the common molds Aspergillus flavus and A. parasiticus that cause a toxic response in vertebrates when introduced in low concentration by a natural route. The group constitutes a type of mycotoxin. Discovered in 1960 after a massive poisoning of turkey poults fed moldy peanut meal, aflatoxin has become the focus of a highly interdisciplinary research field involving chemists, mycologists, veterinarians, agriculturalists, toxicologists, and other basic and applied scientists. See also: Toxin

An infection caused by anaerobic bacteria (organisms that are intolerant of oxygen). Most anaerobic infections are mixed, involving more than one anaerobe and often aerobic or facultative bacteria as well. See also: Bacteria; Infection

An acute, infectious worldwide zoonotic disease (transmissible to humans from other animals) caused by the spore-forming bacterium Bacillus anthracis to which most animals, especially grazing herbivores, are susceptible. Anthrax is a serious, though rare, infectious disease caused by a rod-shaped bacterium, Bacillus anthracis (Fig. 1). In natural conditions, anthrax infections in humans are predominantly cutaneous and usually result from contact with infected animals or contaminated animal products, such as hides or wool. The awareness of the use of Bacillus anthracis as a bioterrorist weapon has considerably increased. Anthrax is endemic as a zoonosis in many areas of Africa, Asia, and the Americas, where spores can lie dormant in the soil for many years and commonly affect grazing animals, including sheep, cattle, and goats. See also: Agricultural science (animal); Anthrax bacillus and the immune response; Bioterrorism; Infectious disease; Medical bacteriology; Zoonoses

A chemical substance, produced by microorganisms, plants, marine organisms, and synthetically, that has the capacity in dilute solutions to inhibit the growth of bacteria or destroy bacteria. Antibiotics are important antibacterial agents. Most are produced by the fermentation of microorganisms or by semisynthetic approaches that involve chemical derivatizations of the naturally occurring antibiotics. In addition, some antibiotics have been isolated from terrestrial plants and marine organisms. Since the discovery of penicillin by Alexander Fleming in 1928, thousands of antibiotics have been isolated and identified. However, relatively few of the natural or semisynthetic compounds have reached commercial status because most of these compounds are not active in an animal model, are too toxic, or have very limited efficacy. Still, some have particular value in the treatment of infectious diseases (Fig. 1). In general, antibiotics differ markedly in their physicochemical and pharmacological properties, antimicrobial spectra, and mechanisms of action. See also: Antimicrobial agents; Infectious disease; Public health

The capability that a bacterium acquires to counteract an inhibitory chemical molecule or compound that was formerly effective in killing it or preventing its growth. Antibiotic resistance in bacteria (Fig. 1) is a hot topic in the media and in scientific and health care settings worldwide. Antibiotic-resistant bacteria pose a catastrophic threat to people in every country in the world. It is estimated that more than 2.8 million people in the United States become infected by resistant bacteria each year and that, worldwide, around five million people might die as a consequence of an infection by antimicrobial resistant (AMR) bacteria per year. The CDC indicates that, in the United States, at least 23,000 people die per year as a direct result of AMR infections, with more individuals dying because of complications that occur as a result of these infections. The CDC keeps an pdated list of infections considered as urgent threats to public health and, depending on the year, at the top of the list there have been organisms like carbapenem-resistant Enterobacteriaceae (CRE), drug-resistant gonorrhea, carbapenem-resistant Acinetobacter (CRAB), Clostridioides difficile (formerly, Clostridium difficile), which is linked to serious diarrheal illnesses associated with antibiotic use, and methicillin-resistant Staphylococcus aureus (MRSA). The current status of antibiotic resistance worldwide needs to be investigated meticulously, and ways must be found to halt its progression. See also: Antibiotic; Antimicrobial agents; Methicillin-resistant Staphylococcus aureus (MRSA); Bacteria; Bacteriology; Drug resistance; Infection; Medical bacteriology; Microbiology; Public health

A sesquiterpene lactone (C₁₅H₂₂O₅), derived from the leaves of Artemisia annua (sweet wormwood or qinghao), which is known for its antimalarial activity. Artemisinin (C₁₅H₂₂O₅; see illustration) is an important antimalarial agent. Around 2000 years ago, artemisinin was used as an herbal remedy by Chinese herbal medicine practitioners. The discovery of artemisinin in the early 1970s as a therapy for malaria was a remarkable medical achievement. Chinese pharmaceutical chemist Youyou Tu was awarded the 2015 Nobel Prize in Physiology or Medicine for discovering artemisinin as a most efficacious and safe antimalarial drug. Presently, artemisinin-based combination therapies (ACTs) are recommended by the World Health Organization (WHO) as the first-line treatment for malaria caused by the parasite Plasmodium falciparum. See also: Malaria; Medical parasitology; Medicine; Parasitology; Pharmaceutical chemistry; Pharmacology; Pharmacy