Moneran toxins

The prefixes “exo-” and “endo-” are retained in classifying the bacterial toxins mainly for historical reasons rather than because they are found either outside or inside the bacterial cell. The main differences in these toxins lie in their chemical structure.

Poisonous proteins from bacteria are sometimes referred to as bacterial exotoxins. The exotoxins are generally produced by gram-positive organisms (i.e., bacteria that react in certain ways to the staining procedure known as Gram staining); at least two bacteria, Shigella dysenteriae and Vibrio cholerae, that produce exotoxins are gram-negative, however. The exotoxins usually do not contain any nonprotein substances, and most are antigenic; i.e., they stimulate the formation of antibodies. The exotoxins may appear in the culture medium in which the bacteria are growing during the declining phases of growth; in some cases they are released at the time of normal destruction of the cells after death (autolysis). The exotoxins are less stable to heat than are the endotoxins, and they may be detoxified by agents that do not affect endotoxins. They are more toxic than endotoxins, and each exotoxin exerts specific effects which are collectively known as pharmacological properties. Exotoxins are neutralized by homologous antibodies—i.e., the active agents in blood serum produced by a process involving the bacteria against which the serum is to be used.

Endotoxins are antigens composed of complexes of proteins, polysaccharides (large molecules built up of numerous sugars), and lipids (fats). The protein part determines the antigenicity, or quality of being reacted against as a foreign substance in a living organism. The polysaccharide part determines the immunological specificity, or limitations on the types of antibodies that can react with the endotoxin molecule and neutralize it (the immunological reaction). Some of the lipids possibly determine the toxicity. Endotoxins are derived from the bacterial cell wall and, when cells are grown in culture, are released only on autolysis. Endotoxins are not neutralized by homologous antibodies and are relatively stable to heat; all of them have the same pharmacological properties.

The Cyanobacteria, or blue-green algae, are among the most primitive and widely distributed of all organisms. They have extreme temperature tolerances. Some strains of a species are toxic; other strains of the same species are not. Water blooms of blue-green algae have been responsible for the death of fishes, waterfowl, cattle, horses, swine, and other animals. Blue-green algae have also been implicated as causes of human intoxications.

Mycotoxins

Fungi are plantlike members of the kingdom Fungi (Mycota) that do not contain chlorophyll. A significant number are known to produce poisons of various types. Toxic fungi can be roughly divided into two main categories on the basis of their size: the smaller microfungi and the larger mushrooms. The toxic microfungi are members of one of two classes: Ascomycetes, or the sac fungi, and the Deuteromycetes, or the imperfect fungi (i.e., fungi in which no sexual reproductive stages are known). The large toxic mushrooms, or toadstools, are mostly members of the class Basidiomycetes, although some Ascomycetes, such as the poisonous false morel (Gyromitra esculenta), may attain a size as large as some of the mushrooms.

The ability of certain fungi, such as ergot (Claviceps purpurea) and some mushrooms, to produce intoxication has long been known. During the 19th century it was recognized that molds are responsible for such diseases as yellow-rice toxicoses in Japan and alimentary toxic aleukia in Russia. The eruption of so-called turkey X disease in England in 1960 and the resulting discovery of the substance known as aflatoxin (see Table 4) stimulated study of the subject of mycotoxicology. Because mycotoxins have now been recognized as potential cancer-producing agents (carcinogens) that can become involved in man’s food supply, they have become important in the study of environmental carcinogenesis.

Poisonous mushrooms, or toadstools as they are commonly called, are the widely distributed members of the class Basidiomycetes, although only a few are known to be poisonous when eaten (see Table 5); some of the poisons, however, are deadly. Most deaths attributed to mushroom poisoning result from eating members of the genus Amanita. Wild mushrooms should be eaten only if they have been accurately identified by an experienced person; the safest procedure is to eat only cultivated species. The problem of toxicity in mushrooms is complex; no single rule or test method exists by which the toxicity of a mushroom can be determined. The most poisonous species closely resemble some of the most prized edible species; in addition, toxicity within a given wild species may vary from one set of ecological conditions or from one geographical locality to the next. Moreover, although some mushrooms that are poisonous when fresh are edible when cooked, dried, salted, or preserved in some other way, others remain poisonous in spite of all preparation procedures. It has also been observed that some people may become poisoned by eating mushrooms that apparently do not affect others. As with microfungi, the mushroom poisons vary in their chemical and biological properties from species to species.

Protistan poisons

The dinoflagellates, important producers of the primary food supply of the sea, are microscopic one-celled organisms that are dependent upon various inorganic nutrients in the water and upon radiant energy for photosynthesis, the process by which they produce their own food supplies. Although dinoflagellates inhabit both marine waters and freshwaters, most species are marine. Dinoflagellates are most often found in cool or temperate waters. During periods of planktonic blooms (times of high concentrations of microscopic organisms in the water) dinoflagellates multiply in large numbers. These planktonic blooms, sometimes referred to as red tide because they discolour the water, are often associated with weather disturbances that may bring about changes in water masses or upwellings. During periods of bloom large numbers of toxic dinoflagellates may be ingested by shellfish; the poisons accumulate in their digestive glands. Animals and humans may in turn be poisoned by eating poisoned shellfish. Certain species of dinoflagellates are capable of producing some of the most toxic substances known. The two species of dinoflagellates most commonly involved in human intoxications have been Gonyaulax catenella along the Pacific coast of North America and G. tamarensis along the eastern coast of North America. Intoxications from these organisms are known as paralytic shellfish poisoning. The symptoms, which begin with a tingling or burning sensation, then numbness of the lips, gums, tongue, and face, gradually spread. Gastrointestinal upset may be present. Other symptoms include weakness, joint aches, and muscular paralysis; death may result. There is no specific treatment or antidote. The poison, variously called paralytic shellfish poison, mussel poison, and saxitoxin, is a complex nonprotein nitrogen-containing compound. Paralytic shellfish poisoning is best avoided by following local public-health quarantine regulations.

Respiratory irritation may result from the inhalation of toxic products in the windblown spray from red-tide areas containing the toxic dinoflagellate Gymnodinium breve, which is found in the Gulf of Mexico and Florida; the nature of the poison is unknown. Deaths of large numbers of brackish-water pond fishes because of Prymnesium parvum have been reported in Israel; the poison is known as prymnesin.