Classification of Disease
CLASSIFICATION OF DISEASE
The word "classification" can refer to either a "thing" or an "activity." As a thing, a classification is a set of categories (pigeonholes) into which may be placed all the objects in the universe for which it has been designed. As an activity, classification is the process of placing the objects into the categories. This article shall deal only with the first meaning: a framework for organizing information.
Two terms are used in describing a classification: "universe" and "axis." The universe is the totality of the objects that are to be classified—all diseases, all automobiles, all causes of death, all reasons why people encounter the health system, all persons in a given population, and so on. An axis is an attribute or property shared by members of the universe. In health matters, there are many axes—ages of patients, causes of illness, disorders produced, physiological systems disrupted, reasons for encounters, and so on.
Every classification has basic attributes:
- It deals with a defined universe.
- It is designed for a specific purpose, which determines its scheme of organization.
- It groups the objects, using as few groups as consistent with its purpose. In public health and epidemiology, classifications are designed primarily for compilation of statistics.
- It uses a schema that depends on the logic of its author (which often is a committee).
- It must accommodate all the objects in its universe and as a result always has one or more categories termed other, which are often called wastebasket categories.
In public health, diseases are only one of the kinds of "objects" that cause death or disability, so it is rare to develop classifications for them alone. The earliest use of classifications in public health was for presenting "causes of death," which of course included injuries. Later, as classifications were expanded to include morbidity as well as mortality, their titles were expanded to "diseases, injuries, and causes of death." In the latest version of the standard classification used in public health, the International Statistical Classification of Diseases (ICD), the universe has become even broader, and the title of the tenth revision (1992) includes Related Health Problems.
In the early twenty-first century, only ICD is in widespread use. Its universe is all individuals who have (or should have) any contact with health services—for prevention, rehabilitation, acute care, long-term care, behavioral problems, investigation of abnormal findings, or for any other reasons. It is not surprising that, to handle this diversity, a number of different axes are found in the classification.
An early need for multiple axes involved trauma. Early mortality statistics showed deaths by external causes. But it was equally valid to tabulate the same deaths according to the injuries sustained. These are two different axes that are used by ICD.
Certain diseases are caused by infectious agents, and one chapter in the classification uses infectious agents as the organizing axis. Other chapters use physiological systems as their organizing axes— respiratory and circulatory, for example. Conflict arises, of course, because a disease such as bacterial pneumonia is both infectious and respiratory. If it is classified both ways, it will almost certainly be counted twice in the statistics.
Largely because of the multiple axis nature of ICD, an extensive set of rules called conventions has been developed to instruct the classifier how to handle these and other conflicting demands. One convention (which the United States has resisted) is the use of dagger and asterisk coding in which a code marked with a dagger (†) indicates the underlying disease and that with an asterisk (*) indicates the manifestation.
It should be clear by this point that classifications in health care are not really classifications of diseases or injuries or causes of disability but are actually classifications of individuals who are of interest to the public health community. As a result, almost never can the classifying be done from a single factor, such as the diagnosis. Rather, the person's other attributes, such as age and other diagnoses, must at least be considered and taken into the classification decision when called for by the conventions.
Retrieval of information for making statistical tabulations or finding individual case records is done by referring to the codes that have been substituted for the labels of the categories; retrieval is code-dependent. It is essential, then, to know the limitations presented by this fact. Retrieval can never produce any more detailed information than the category level—the code is equivalent to the category label. This is a serious limitation when, for example, an epidemic appears and it is a condition that is hidden in a wastebasket category. For example, in the 1970s, Guillain-Barré syndrome was lost in "Polyneuritis and polyradiculitis," where it could not be separated from the other miscellany.
Also, in twenty-first-century information systems, neither the category label nor the category content can be known with certainty, because there is no method for determining the source of the code, that is, the classification from which the code was taken and the version of that classification. For example, code 395 was for Meniere's disease in ICD-6 and ICD-7. With ICD-8 and ICD-9 it was used for diseases of the aortic valve. Especially in ICD 's derivatives, such as the International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM) in North America, changes are made annually to reflect new diseases and new knowledge. The result may be to add a new disease to an existing category or to move a disease from one category to another. In the mid-1980s, code 279.1 (deficiencies of cell-mediated immunity) was the category to which AIDS (acquired immunodeficiency syndrome) was assigned; after 1986, AIDS was supposed to go to an infectious disease category. Whether it did or not is moot—279.1 looks just the same. This problem, the ambiguity of category labels and codes, will persist until the information systems are modified and standardized to tag each code with an unique source identifier.
In view of the requirement that a classification be designed with a purpose, it is no surprise that ICD is increasingly unsatisfactory. Beginning with the desire simply to tabulate mortality statistics, it has taken on the burden of trying to serve multiple purposes, to accommodate morbidity, health care reimbursement, quality review, epidemiological surveillance, evidence-based medicine, facility planning, public policy, and others. Developers of electronic medical records are expecting it to serve the needs for clinical care for individual patients as well. One classification cannot serve all masters equally well.
It will not be possible to have optimal classifications for public health and epidemiology, as well as for the other legitimate uses of health and health care information, until a simple but major modification of the information system is adopted. That modification is to capture and uniquely and permanently code the specific diagnoses (clinical entities) which go into the classification categories. When that is done, the entities can be distributed into a variety of classifications, each designed optimally for its intended purpose.
(see also: International Classification of Diseases )
Israel, R. A. (1978). "The International Classification of Diseases: Two Hundred Years of Development." Public Health Reports 93:150–152.
L'Hours, A. G. P. (1990). An Overview of the Tenth Revision of the International Statistical Classification of Diseases and Related Health Problems (JCD-10). Geneva: World Health Organization.
Slee, V. N.; Slee, D. A.; and Schmidt, H. J. (2000). The Endangered Medical Record: Ensuring Its Integrity in the Age of Informatics. St. Paul, MN: Tringa Press.
White, K. L. (1985). "Restructuring the International Classification of Diseases: Need for a New Paradigm." The Journal of Family Practice 21:17–20.
World Health Organization (1992). International Statistical Classification of Diseases and Related Health Problems, 10th revision (ICD-10), 3 vols. Geneva: Author.
Classification is a method of organizing plants and animals into categories based on their appearance and the natural relationships between them. Also called scientific classification, it is science's way of identifying and grouping living things. The classification of organisms is a science called taxonomy, or systematics.
The first person to attempt any type of systematic grouping of organisms was the Greek philosopher and scientist, Aristotle (384–322 b.c.) Until his work, most people simply divided all plants and animals into two basic categories: useful and harmful. As a careful observer of the natural world, Aristotle began arranging organisms according to their physical similarities. Since there were only about one thousand organisms known in his time, he classified animals according to those with red blood (vertebrates or having a backbone) and those with no red blood (invertebrates or no backbone). He also classified plants by size and by whether they were herbs, shrubs, or trees. Despite many mistakes and an over-simplified idea, Aristotle's impulse to classify and to categorize organisms was a necessary attempt to make sense of the diversity of life in order to study it better.
THE BENEFITS OF CLASSIFICATION
In the life sciences, the need to organize is very important and extremely useful. Classification helps biologists keep track of living things and to study their differences and similarities. It also shows biologists how living things are related to one another through evolution (the process by which living things change over generations). Classifying also saves time and effort. There are many possible ways to classify life: appearance, behavior, evolutionary history, or life development from fertilization to adulthood. The modern classification system is considered a natural system since it represents genuine relationships between organisms. In this natural system, the more closely organisms are related to each other, the more features they have in common. This system is also hierarchical, meaning that its categories are grouped according to size in a series of successively larger ranks.
CAROLUS LINNAEUS DEVELOPS BINOMIAL SYSTEM OF NOMENCLATURE
The system used today is based on the work of one individual, the Swedish physician and naturalist, Carolus Linnaeus (1707–1778). In his day, it sometimes took as many as ten words to name a particular organism and no standard system existed upon which everyone agreed. Linnaeus traveled throughout Europe compiling lists of the animals and plants he encountered, and in 1735 published a book which tried to make some sense of this great diversity. By 1758 he had completed his huge encyclopedia called System of Nature which described and classified all known organisms by their structure and placed them in one of the seven levels of his hierarchical system. Linnaeus also developed the binomial system of nomenclature, which gave a distinctive two-word name to each species. This system is still followed with the first part being the genus name, and the second part serving as its species name. For example, both the bobcat and the house cat belong to the genus Felis, but the bobcat's species is rufa (Felis rufa) while that of the domestic cat is Felis domestica. The second name is usually descriptive of the particular animal. Among the rules for this system, the two-part name is always used. The species part
Swedish botanist (a person specializing in the study of plants) Carolus Linnaeus (1707–1778) devised the first orderly system of classifying living things. He also introduced the binomial system of nomenclature (a two-part naming system) that is still in use today. He is called the father of taxonomy (the science of classifying living things) because his system was able to impose a much-needed order on the study of life itself.
Being able to identify a plant or animal, to tell how it is different from others, and to know how it fits into the entire natural world is something that people simply take for granted in today's world. However, there was a time in the history of the life sciences when naturalists (people specializing in the study of plants and animals in their natural surroundings) used as many as ten words to give something a specific, descriptive name, and even with all that effort, there was no guarantee that it would be used by others or that someone who spoke a different language would know what the name meant. This describes what the life sciences were like before the great classifier, Carolus Linnaeus, gave science a practical way of naming organisms that was based on clear and simple standards upon which everyone could agree.
Linnaeus was born Carl von Linne in South Rashult, Sweden. (Linnaeus is the Latin version of his name.) His father was a clergyman, and the very young Linnaeus was so interested in gardens and growing things that the locals called him "the little botanist." When his father sent him to medical school, Linnaeus was able to combine school with botanical exploring trips that only made him more interested in plants. When he became lecturer in botany at Uppsala University at the age of twenty-three, he was able to go on longer, more extensive excursions to Lapland in 1732. After traveling 4,600 miles (7,407 kilometers) throughout northern Scandinavia discovering new plant species and observing animal life, he began to formulate the details of an idea that he had first expressed in a paper some years before. By 1735, Linnaeus had published his System of Nature in which he proposed the idea of classifying plants in the simplest and most clear way that one could. To Linnaeus, that meant a system based on the specimen's external characteristics that were most obvious to the eye. His system would therefore be based on observable characteristics such as structure, or anatomy, or on the details of the way a thing reproduced itself. In his landmark book, he showed how this could be done. First, he created a hierarchical system (in a hierarchy, things are arranged in a certain order) in which the categories above included all of the ones below it. Thus, he created a system in which living things were grouped according to their similarities, with each succeeding level possessing a larger number of shared traits. He named these levels class, order, genus, and species. He also popularized what is called binomial nomenclature, which gave every living thing a Latin name consisting of its genus and species. For example, this would distinguish two very different species, like a lion and a cougar, simply by their Latin names. The lion belongs to Panthera leo and the cougar belongs to Felis concolor. Thus each organism has a generic name, telling which group it belongs to, and a specific name for itself.
Although classification might not seem to be as important a subject to science as some others, it proved absolutely essential to such a broad and diverse field as the life sciences. In fact, only after Linnaeus's system was accepted and regularly used did biology and botany begin to make real progress. The advantages of his system are numerous. For example, his use of Latin allows scientists to communicate worldwide about organisms without having to understand different languages. Since each type of organism can fit into his system in a logical and orderly way, it can be expanded indefinitely. It is also a great advantage that the levels of his hierarchical system provide a framework for seeing and understanding the relationships among different organisms or groups of organisms. His system is also flexible and adaptable. Since it was first introduced, the number of levels have grown, with phylum being inserted above class and kingdom being placed at the very top. Finally, although his system was introduced before the theory of evolution (the process by which gradual genetic change occurs over time to a group of living things), it always has been able to accommodate any new discoveries or modifications which that theory has made. Linnaeus was said to have been an almost obsessive classifier, yet he was a person who turned his passion for an idea into a truly great scientific contribution.
is never used alone. The generic name always begins with a capital letter but the species name is always lowercase. Both names are written in italics or are underlined. Latin is used to avoid any confusion in translating different languages. Altogether, this system allows everyone in the world to use the same name for the same organism and to immediately understand each other.
Seven major groups or categories make up the scientific classification system. The groups or categories themselves are called taxons (from taxonomy, which is the science of naming and classifying organisms). These groups range in order of size, so from the largest or most general to the smallest and most specific, they are: kingdom, phylum, class, order family, genus, and species. Each kingdom is divided into smaller and smaller groups until each type of organism is placed in a unique category. One way of remembering this general-to-specific scheme is the rhyme or formula, "King Philip Came Over From Great Spain."
Kingdom is the largest unit and is composed of five separate kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. Beginning with Linnaeus and for a long time afterward, there were only two kingdoms, Plantae and Animalia. But with the improvement of the microscope and the discovery of microorganisms, the number was expanded to five. From kingdom on down to species, organisms are grouped together with increasing similarity. Besides these seven major groups, biologists are able to use various subgroups to deal with minor differences among organisms when those differences are not large enough to form a new group. For example, species may be divided up into subspecies.
Classifying a dog and a wolf offers a good example of how two animals would fit into these seven categories. Both are in the kingdom Animalia since they cannot make their own food. Next, they would both be in the phylum Chordata since they have a notochord (like a vertebrae or backbone). Both also belong to the class Mammalia since they have fur and feed milk to their young. Both are members of the order carnivora since they are meat eaters. They also both belong to the family canidae because they cannot retract their claws and they hunt and stalk their prey. However, while both are similar enough to be in the same genus, canis, they are different enough to be in separate species. Therefore, the wolf's scientific name is Canis lupus and the dog's is Canis familiaris.
A classification system provides a method that best represents genuine relationships between organisms. It is a natural system that is based on overall resemblances and which reflects how each organism is related from an evolutionary standpoint.
classification, in biology, the systematic categorization of organisms into a coherent scheme. The original purpose of biological classification, or systematics, was to organize the vast number of known plants and animals into categories that could be named, remembered, and discussed. Modern classification also attempts to show the evolutionary relationships among organisms (see the table entitled Examples of Systematic Classification). A system based on categories that show such relationships is called a natural system of classification; one based on categories assigned only for convenience (e.g., a classification of flowers by color) is an artificial system.
Modern classification is part of the broader science of taxonomy, the study of the relationships of organisms, which includes collection, preservation, and study of specimens, and analysis of data provided by various areas of biological research. Nomenclature is the assigning of names to organisms and to the categories in which they are classified.
A modern branch of taxonomy, called numerical taxonomy, uses computers to compare very large numbers of traits without weighting any type of trait—in contrast to the traditional view that certain characteristics are more significant than others in showing relationships. For example, the structure of flower parts is considered more significant than the shape of the leaves in flowering plants because leaf shape appears to evolve much more quickly. Much of the science of taxonomy has been concerned with judging which traits are most significant. If new evidence reveals a better basis for subdividing a taxon than that previously used, the classification of the group in question may be revised. A considerable number of classification changes as well as insights in recent years have been the result of comparisons of nucleic acid (genetic material) sequences of organisms.
See also cladistics.
The broadest division of organisms has been into kingdoms. Traditionally there were two kingdoms, Animalia and Plantae, but many unicellular and simple multicellular organisms are not easily classified as either plants or animals. In 1866 the zoologist Ernst Heinrich Haeckel proposed a third kingdom, the Protista, to include all protozoans, algae, fungi, and bacteria. In the 20th cent. his proposal was refined, and a grouping became widely accepted that was made up of five kingdoms: animals; plants; Protista, including protozoans and some algae; Monera, comprising the prokaryotic bacteria and cyanobacteria (blue-green algae); and Fungi. Other groupings have been proposed from time to time.
Analysis of genetic sequences in various organisms has recently suggested placement of the Archaebacteria into a separate major group called the archaea. In this system, the second and third major groups are the other bacteria and the eukarya (or eukaryotes), organisms that have cell nuclei and include the fungi, plants, and animals.
The Lower Taxa
Kingdoms are divided into a hierarchical system of categories called taxa (sing. taxon). The taxa are, from most to least inclusive: phylum (usually called division in botany), class, order, family, genus, and species. Intermediate divisions, such as suborder and superfamily, are sometimes added to make needed distinctions. The lower a taxon is in the hierarchy, the more closely related are its members.
The species, the fundamental unit of classification, consists of populations of genetically similar interbreeding or potentially interbreeding individuals. If two populations of a species are completely isolated geographically and therefore evolve separately, they will be considered two species once they are no longer capable of mixing genetically if brought together. In a few cases interbreeding is possible between members of closely related species—for example, horses, asses, and zebras can all interbreed. The offspring of such crosses, however, are usually sterile, so the two groups are nonetheless kept separate by their genetic incompatibility. Populations within a species that show recognizable, inherited differences but are capable of interbreeding freely are called subspecies, races, or varieties.
The genus (pl. genera) is a grouping of similar, closely related species. For example, the domestic cat and the jungle cat are species of the genus Felis; dogs, wolves, and jackals belong to the genus Canis. Often the genus is an easily recognized grouping with a popular name; for example, the various oak species, such as black oak and live oak, form the oak genus (Quercus). Similarly, genera are grouped into families, families into orders, orders into classes, and classes into phyla or divisions.
The present system of binomial nomenclature identifies each species by a scientific name of two words, Latin in form and usually derived from Greek or Latin roots. The first name (capitalized) is the genus of the organism, the second (not capitalized) is its species. The scientific name of the white oak is Quercus alba, while red oak is Quercus rubra. The first name applies to all species of the genus—Quercus is the name of all oaks—but the entire binomial applies only to a single species. Many scientific names describe some characteristic of the organism (alba=white; rubra=red); many are derived from the name of the discoverer or the geographic location of the organism. Genus and species names are always italicized when printed; the names of other taxa (families, etc.) are not. When a species (or several species of the same genus) is mentioned repeatedly, the genus may be abbreviated after its first mention, as in Q. alba. Subspecies are indicated by a trinomial; for example, the southern bald eagle is Haliaeetus leucocephalus leucocephalus, as distinguished from the northern bald eagle, H. leucocephalus washingtoniensis.
The advantages of scientific over common names are that they are accepted by speakers of all languages, that each name applies only to one species, and that each species has only one name. This avoids the confusion that often arises from the use of a common name to designate different things in different places (for example, see elk), or from the existence of several common names for a single species. There are two international organizations for the determination of the rules of nomenclature and the recording of specific names, one for zoology and one for botany. According to the rules they have established, the first name to be published (from the work of Linnaeus on) is the correct name of any organism unless it is reclassified in such a way as to affect that name (for example, if it is moved from one genus to another). In such a case definite rules of priority also apply.
The earliest known system of classification is that of Aristotle, who attempted in the 4th cent. BC to group animals according to such criteria as mode of reproduction and possession or lack of red blood. Aristotle's pupil Theophrastus classified plants according to their uses and methods of cultivation. Little interest was shown in classification until the 17th and 18th cent., when botanists and zoologists began to devise the modern scheme of categories. The designation of groups was based almost entirely on superficial anatomical resemblances.
Before the idea of evolution there was no impetus to show more meaningful relationships among species; the species was thought to be uniquely created and fixed in character, the only real, or natural, taxon, while the higher taxa were regarded as artificial means of organizing information. However, since anatomical resemblance is an important indication of relationship, early classification efforts resulted in a system that often approximated a natural one and that—with much modification—is still used. The most extensive work was done in the mid-18th cent. by Carolus Linnaeus, who devised the presently used system of nomenclature. As biologists came to accept the work of Charles Darwin in the second half of the 19th cent., they began to stress the significance of evolutionary relationships for classification.
Although comparative anatomy remained of foremost importance, other evidence of relationship was sought as well. Paleontology provided fossil evidence of the common ancestry of various groups; embryology provided comparisons of early development in different species, an important clue to their relationships. In the 20th cent., evidence provided by genetics and physiology became increasingly important. Recently there has been much emphasis on the use of molecular genetics in taxonomy, as in the comparison of nucleic acid sequences in the genetic makeup of organisms. Computers are increasingly used to analyze data relevant to taxonomy.
See E. Mayr, Principles of Systematic Zoology (1969); T. Savory, Animal Taxonomy (1972); H. M. Hoenigswald and L. F. Wiener, eds., Biological Metaphor and Cladistic Classification (1987); F. A. Stafleu and R. S. Cown, Taxonomic Literature: A Selective Guide to Botanical Publications and Collections (1988); N. Eldredge, Fossils: The Evolution and Extinction of Species (1991).
See also 288. NAMES ; 301. ORDER and DISORDER .
- a name composed of two terms, a generic and a specific. —binomial , adj.
- biosystematy. —biosystematic, biosystematical , adj.
- the science of the classification of living things. Also called biosystematics . —biosystematic , —biosystematical , adj.
- the area of taxonomy that uses cytological structures, as chromosomes, in classifying organisms.
- division of material into two parts for the purpose of classification. —dichotomist , n.
- 1. the science of method or orderly arrangement and classification.
- 2. any system created to impose order. See also 250. LOGIC . —methodological , adj.
- the investigation and classification of trivial matters. —micrologist , n. —micrologic, micrological , adj.
- the enumeration and description of a museum’s collection. —museographer, museographist , n.
- a person who invents or assigns names, as in nomenclature. See also 53. BOOKS .
- 1. a system of names used in the classification of an art or science or other field or subject.
- 2. a naming system peculiar to a social group. See also 53. BOOKS .
- Biology. a technical name, as one that forms part of a system of nomenclature or classification.
- the application of onyms; classification or systematic nomenclature.
- the nomenclature of organs. —organonymal, organonymic , adj.
- any of the basic divisions of the plant or animal kingdom.
- the systematic classification and description of nature. See also 178. GEOGRAPHY ; 179. GEOLOGY . —physiographer , n. —physiographic, physiographical , adj.
- an advocate of the quinary system of animal classification, which regarded all animal groups as being naturally divisible by five. —quinarian, quinary , adj.
- the condition or quality of being of the same type. —syntypic , adj.
- the study of classification and methods of classification. —systematician, systematist , n.
- the practice or act of systematizing.
- the study or science of systematizing.
- a botanical or zoological name in which the two terms, the generic name and the specific, are the same (a practice no longer approved by the International Code of Botanical Nomenclature). —tautonymic, tautonymous , adj.
- taxonomy, taxology
- 1. the technique or science of classification.
- 2. the scientific identification, naming, and classification of living things. Also called systematics . —taxonomist , n. —taxonomie, taxonomical , adj.
- 1. the terms of any branch of knowledge, field of activity, etc.
- 2. the classification of terms associated with a particular field; nomenclature.
- 3. Rare. the science of classification. —terminologic, terminological , adj.
- division into three parts, especially the theological division of man’s nature into the body, the soul, and the spirit. —trichotomic, trichtomous , adj.
- the use of three terms or names in the classification of a species, genus, variety, etc. —trinomial , adj.
- a trinomial or name composed of three terms.
- Rare. a universal system of nomenclature or classification.
- zoological classification; the scientific classification of animals.
1. Any scheme for structuring data that is used to group individuals. In ecological and taxonomic studies numerical classification schemes have been devised, but various hierarchical or non-hierarchical classificatory strategies have also been used. In taxonomy, the fundamental unit is the species. Among living forms species are groups of individuals that look alike and can interbreed, but cannot interbreed with other species. In palaeontology, where breeding capability cannot be determined, species are defined according to morphological similarities. In formal nomenclature, taxonomists follow the binomial system devised by Linnaeus. In this system each species is defined by two names: the generic (referring to the genus) and the specific (referring to the species). Thus various related species may share a common generic name. Genera (sing. genus) may be combined with others to form families, and related families combined into an order. Orders may be combined into classes, and classes into phyla (sing. phylum) or divisions in the case of Metaphyta. For example, the brachiopods comprise some 11 orders split between two classes and these two classes are the major subdivisions of the phylum Brachiopoda. The basic groupings, the phyla, are combined together into kingdoms, e.g. Plantae (the plants) and Animalia (animals). Some workers have tackled the uncertainties arising from subjectivity in classification by using numerical methods. In their view, if enough characters were measured and represented by cluster statistics, the distances between clusters could be used as a measure of difference. Even so, the worker has to decide (subjectively) how best to analyse the measurements, and so objectivity is lost. Other workers emphasize those features shared by organisms that show a hierarchical pattern (see CLADISTICS).
2. In remote sensing, the computer-assisted recognition of surface materials. The process assigns individual pixels of an image to categories (e.g. vegetation, road) based on spectral characteristics compared to spectral characteristics of known parts of an image (training areas). Assignation of pixels is not always possible when the parameter space of different training areas overlaps. In such cases a principal component analysis prior to classification may be used to allow better separation of training areas by increasing the overall parameter space. See also BOX CLASSIFICATION; MINIMUM-DISTANCE-TO-MEANS CLASSIFICATION; and MAXIMUM-LIKELIHOOD CLASSIFICATION.
The shapes of bacterial cells, often of keen interest to forensic investigators, are classified as spherical (coccus), rodlike (bacillus), spiral (spirochete), helical (spirilla), and comma-shaped (vibrio) cells. Many bacilli and vibrio bacteria have whiplike appendages (called flagella) protruding from the cell surface. Flagella are composed of tight, helical rotors made of chains of globular protein called flagellin, and act as tiny propellers, making the bacteria very mobile. On the surface of some bacteria are short, hairlike, proteinaceous projections that may arise at the ends of the cell or over the entire surface. These projections, called fimbriae, facilitate bacteria adherence to surfaces.
Other proteinaceous projections, called pili, occur singly or in pairs, and join pairs of bacteria together, facilitating transfer of DNA between them.
Oxygen may or may not be a requirement for a particular species of bacteria, depending on the type of metabolism used to extract energy from food (aerobic or anaerobic). Obligate aerobes must have oxygen in order to live. Facultative aerobes can also exist in the absence of oxygen by using fermentation or anaerobic respiration. Anaerobic respiration and fermentation occur in the absence of oxygen, and produce substantially less ATP than aerobic respiration.
During periods of harsh environmental conditions some bacteria can produce within themselves a dehydrated, thick-walled endospore. These endospores can survive extreme temperatures, dryness, and exposure to many toxic chemicals and to radiation. Endospores can remain dormant for long periods (hundreds of years in some cases) before being reactivated by the return of favorable conditions.
Pathogens are disease-causing bacteria that release toxins or poisons that interfere with some function of the host's body.
An understanding of the basic classification of bacteria found at crime scenes and taken from bodies at autopsy is critical to forensic investigators (including forensic epidemiologists) attempting to identify bacteria. The identification schemes of Bergey's Manual are based on morphology (e.g., coccus, bacillus), staining (gram-positive or negative), cell wall composition (e.g., presence or absence of peptidoglycan), oxygen requirements (e.g., aerobic, facultatively anaerobic) and biochemical tests (e.g., which sugars are aerobically metabolized or fermented).
Another important identification technique is based on the principles of antigenicity—the ability to stimulate the formation of antibodies by the immune system . Commercially available solutions of antibodies against specific bacteria (antisera) are used to identify unknown organisms in a procedure called a slide agglutination test. A sample of unknown bacteria in a drop of saline is mixed with antisera that has been raised against a known species of bacteria. If the antisera causes the unknown bacteria to clump (agglutinate), then the test positively identifies the bacteria as being identical to that against which the antisera was raised. The test can also be used to distinguish between strains of slightly different bacteria belonging to the same species.
see also Anthrax; Bacterial biology; Bacteria, growth and reproduction; Bacterial resistance and response to antibacterial agents; Biological weapons, genetic identification; Biosensor technologies; Bubonic plague; Decontamination methods.