University of Guyana MLS 3100 Lecture 2B - Taxonomy PDF

UNIVERSITY OF GUYANA MLS 3100 General & Environmental Microbiology Lecture 2B Approaches to Taxonomy Prepared by: Narita Singh Introduction to Taxonomy Taxonomy is the classification, description, identification, and naming of living organisms. Classificatio...

UNIVERSITY OF GUYANA MLS 3100 General & Environmental Microbiology Lecture 2B Approaches to Taxonomy Prepared by: Narita Singh Introduction to Taxonomy Taxonomy is the classification, description, identification, and naming of living organisms. Classification is the practice of organizing organisms into different groups based on their shared characteristics. The most famous early taxonomist was a Swedish botanist, zoologist, and physician named Carolus Linnaeus (1701–1778). In 1735, Linnaeus published Systema Naturae, an 11-page booklet in which he proposed the Linnaean taxonomy, a system of categorizing and naming organisms using a standard format so scientists could discuss organisms using consistent terminology. He continued to revise and add to the book, which grew into multiple volumes. In his taxonomy, Linnaeus divided the natural world into three kingdoms: animal, plant, and mineral (the mineral kingdom was later abandoned). Within the animal and plant kingdoms, he grouped organisms using a hierarchy of increasingly specific levels and sublevels based on their similarities. The names of the levels in Linnaeus’s original taxonomy were kingdom, class, order, family, genus (plural: genera), and species. Swedish botanist, zoologist, and Species was, and continues to be, the most specific and basic physician Carolus Linnaeus taxonomic unit. Introduction to Taxonomy Taxonomy is an area of biological science which comprises three distinct, but highly interrelated disciplines that include classification, nomenclature and identification. Applied to all-living entities taxonomy provides a consistent means to classify name and identify organisms. This consistency allows biologists worldwide to use a common label for every organism they study within their particular disciplines. The common language that taxonomy provides minimizes the confusion about names and allows attention to center on more important scientific issues and phenomena. In diagnostic microbiology, classification, nomenclature and identification of microbes play a central role in providing accurate and timely diagnosis of infection. Classification Classification is the organization of organisms that share similar morphologic, physiologic and genetic traits into specific groups or taxa. Nomenclature, the naming of microorganisms according to established rules and guidelines provide the accepted labels by which organisms are universally recognized. The classification of microbes is based on how they look and what they can do. The correct identification of microorganisms is of fundamental importance to microbial systematists as well as to scientists involved in many other areas of applied research and industry (e.g. agriculture, clinical microbiology and food production). Classification may be defined as the arrangement of organisms into taxonomic groups (taxa) on the basis of their phenotypic (observable) and geno-typic (genetic) similarities and differences. It allows for proper and systematic grouping of microorganisms. A phenetic classification system relies upon the phenotypes or physical appearances of organisms. Phylogenetic classifcation uses evolutionary relationships of organisms. A genotypic classification compares genes or genomes between organisms. Classification Organisms are classified into three main kingdoms: animals, plants and Protista. The Protista contain unicellular microorganisms including eukaryotes and prokaryotes. Carl Woese in 1990 put forward the three domains and six kingdom classifications to group all existing living organisms. This is based primarily on variations in the 16S ribo-somal RNA structure. Domain It is the first and the highest group encompassing all living beings. The three domains are as follows: 1. Archaea: These are prokaryotes and members of this group are the most ancient organisms. Though similar to bacteria, these lack nuclear membranes, the cell wall is devoid of peptidoglycan and these can survive in extremes of temperatures or salinity 2. Bacteria: These are “eubacteria” or true bacteria, having lipid-containing cell membranes and their cell wall contains peptidoglycan. 3. Eukarya: These have a eukaryote cell structure, with membrane-bound nuclei, and peptidoglycan is absent from the cell wall. Classification – Medically Important Kingdoms Kingdom It is the next higher group for organisms and in the older classification, there were five kingdoms: Monera, Protista, Fungi, Plantae and Animalia. With the introduction of the domain system, the Monera kingdom has been split into kingdoms Archaebacteria and Eubacteria (Table 1). Medically important disease-causing organisms belong to four kingdoms only, with archaebacteria and plantae being non-pathogenic (except for some poisonous substances produced by certain plants). Classification – Medically Important Kingdoms Kingdom Eubacteria: These are prokaryotic unicellular organisms with a circular DNA and characteristic rRNA. Peptidoglycan is present in the cell wall. There are five main groups of Eubacteria—Proteobacteria, Cyanobacteria, Firmicutes, Chlamydiae and Spirochetes—of which proteobacteria contain the highest number of pathogens. Kingdom Fungi: For a long time, fungi were classified under the plant kingdom but later a separate kingdom was assigned to these organisms. The morphology of the fungal structures had been used for a long time in classification. The recent application of geno-mic studies and sequencing has led to the description of nine phyla, in which members of a few groups are human pathogens. Kingdom Protista: Protozoan parasites are uni-cellular organisms which have been classed under the Kingdom of Protista. At present, there are 11 phyla of protozoan parasites of which the following phyla comprise medically important protozoan parasites: Amoebozoa, Trichozoa, Percolozoa, Euglenozoa, Miozoa and Ciliophora. Classification – Medically Important Kingdoms Kingdom Animalia: In contrast to protozoa, the animalia kingdom contains multicellular organisms, and the helminth parasites and ectoparasites of medical importance are members of this group. Trematodes, cestodes and nematodes are the three major helminth groups studied in medical parasitology, while ectoparasites are largely insects. In addition to the above mentioned organisms, a distinctly separate group of disease-causing agents are the viruses. By their nature, viruses are considered non-living outside a living body, but can replicate inside a live cell or host. The nature of the nucleic acid content of viruses forms the basis of their classification into DNA and RNA viruses. The two groups are further divided into families based on the special features of the nucleic acids, their mode of replication and basic structural features.  Classification Broad classification of living organisms Classification means the act of arranging a number of objects (of any sort) into groups (or taxa) in relation to attributes possessed by those objects In biological classifications, the primary objects (microorganisms, plants, animals) are usually arranged in groups which are themselves members of larger groups (and so on) in such a way that any item or any group appears as a member of only one larger grouping, i.e., the groups are non-overlapping. This method of classification is the familiar hierarchical system which can be conveniently represented by a 'family tree' or dendrogram. Nomenclature The units at each level (taxonomic rank) of a hierarchical system are given distinctive names or label a branch of taxonomy known as nomenclature. In biology, the system of nomenclature is normally used for living organisms, which is derived from that used by the great eighteenth-century taxonomist Linnaeus (Carl von Linne). In this system, the basic unit (the species) is given two names one denoting its membership of a taxon at the rank that we label genus (generic name) followed by a second denoting the particular species (specific name). These names are written in a latinized form and constitute a so called latinized binomial (e.g., Aspergillus niger, Bacillus subtilis, Clostridium tetani). Taxa of higher rank (families, orders, etc.) are given single latinized names with characteristic endings (e.g., Pseudomonadaceae, family; Pseudomonadales, order). The naming of newly discovered organisms or of newly proposed taxa of higher ranks is governed by rigid rules standardized by international agreement. Nomenclature Linnaeus's text was in Latin because it was the language used in universities at the time; however, since Latin is no longer a spoken language, terms and their meanings remain stable and provide the basis for universally accepted scientific communication. Currently, the binomial names of organisms are italicized when in print and underlined when written by hand, a convention allowing for easy recognition. The two-part name applied to each type of organism indicates where that organism fits into a larger taxonomic schema Taxonomic Rank Taxonomic Ranks – Taxonomic ranks are the categories used in the classification of living organisms. These are nested ranks, with each successively lower level being contained within the one above. A group of organisms occupying a specific rank is called a taxon (pleural = taxa) or taxonomic group. Taxonomic Rank Although most macroscopic organisms can be readily classified into two kingdoms (Plantae and Animalia), microscopic organisms cannot. Following Van Leeuwenhoek’s discovery of microscopic life forms, many new organisms were identified that did not meet the criteria for either kingdom. One way to solve this problem was to establish a new kingdom. In 1866, Ernst Haeckel proposed a third kingdom be established which he called Protista. This kingdom would include all single-celled organisms and those multicellular forms not developing complex tissues. A diverse group of organisms including protozoa, algae, fungi, sponges and slime molds were to be classified within this kingdom, but their relatedness was minimal. In 1957, Roger Stainer and his associates used electron microscopy to demonstrate significant differences between prokaryotic and eukaryotic cells, suggesting more than three kingdoms were required. In 1969, R.H. Whittaker proposed a five-kingdom system to improve classification. This system included three kingdoms of more complex organisms based on three modes of nutrition. The Animalia ingest food, the Plantae make their own food via photosynthesis, and the Fungi (Myceteae) absorb food in a liquid form. The other two kingdoms, Protista and Monera, include organisms without complex structures that are separated based on their cell types. Monera are prokaryotic and Protista are eukaryotic. Although the Whittaker five-kingdom system is included in many modern textbooks, it is not without problems. Taxonomic Rank Recent studies based on biochemical analyses indicate considerable variation among eukaryotic microorganisms, and the need for multiple additional kingdoms. In 1978, Carl Woese and his associates using biochemical analyses demonstrated significant differences within the kingdom Monera; and prompted the addition of a new taxonomic rank called Domain above kingdoms in the taxonomic hierarchy. This work provided evidence that organisms now recognized as Archaea (formerly Archaeobacteria) have multiple important characteristics unlike either bacteria or eukaryotic organisms. The three domains of life currently accepted by most biologists include the Eukarya (all organisms with eukaryotic cells), the Bacteria and the Archaea. Adding domains to the previously established taxonomic ranks generates a slightly modified hierarchy as shown : Phylogeny Since understanding the phylogeny (evolutionary history) of life on earth is a major goal of taxonomists, numerous methods have been employed to determine the evolutionary relationships between organisms. Since the 1970s, computer technology and a method called cladistics have provided considerable information relative to evolutionary trends. In cladistics, specific features of organisms are used to determine relatedness. A feature that is common to several different types of organisms, but shows variation within them is assigned a value or form called a character state. Analysis of the character is then conducted to determine which state is primitive (ancestral) and which is derived (evolved from something else). Finally, the evolutionary relationships determined are portrayed as straight-line diagrams (evolutionary trees) called cladograms. Many changes in taxonomy, including the addition of the new taxonomic rank (domain), are due to studies involving cladistics. Although the primary features used to determine the relatedness between macroscopic organisms were initially based on morphology (the study of external features) and mode of reproduction, these are not as useful for the classification of microorganisms. New features such as types of nutrition and metabolism, temperature requirements, gas requirements, pH and salinity preferences, and biochemical properties have proven to be much more useful. Criteria Used in the Classification of Microorganisms 1. Morphology – Although highly valuable in the classification of multicellular organisms, morphology has limited usefulness when applied to prokaryotes. Many different types of bacteria form colonies and cells with similar morphology even when subjected to various stain techniques. 2. Mode of Reproduction – Variation in reproductive structures/methods is of primary consideration in the classification of plants, animals and fungi; but somewhat less useful in the classification of single-celled organisms. Most single-celled eukaryotes, bacteria and archaea reproduce by means of fission, i.e., one cell divides itself into two daughter cells. Criteria Used in the Classification of Microorganisms 3. Nutrition and Metabolism – All living organisms can be categorized on the basis of their nutritional requirements and type of metabolism. A. Nutritional categories are based on energy source and carbon source. Organisms can obtain the energy they require either from light or from chemicals. Those using light energy are called phototrophs (photo = light) and those using chemical energy are called chemotrophs (Chemo = chemical). Organisms can obtain the carbon they need either from inorganic or organic carbon compounds. Organisms using inorganic compounds as carbon sources are called autotrophs (auto = self) while those using pre-formed organic compounds as their source of carbon are called heterotrophs (hetero = different). Criteria Used in the Classification of Microorganisms By combining energy source and carbon source, we obtain four nutritional categories: Photoautotrophs = Organisms using light energy and inorganic compounds for carbon. Photoheterotrophs = Organisms using light energy and organic compounds for carbon. Chemoautotrophs = Organisms using chemical energy and inorganic compounds for carbon. Chemoheterotrophs = Organisms using organic compounds for both energy and carbon. Plants, algae and some bacteria are photoautotrophs, but only prokaryotic cells function as photoheterotrophs or chemoautotrophs. Animals (including humans), fungi, protozoa and many prokaryotes function as chemoheterotrophs, so this category is often subdivided. Saprotrophs = Chemoheterotrophs using dead or decaying organic materials for nutrients. These are sometimes called saprophytes or decomposers. Parasites = Chemoheterotrophs using living organisms as their source of nutrients (some living inside their host and others living outside). Hypotrophs = Obligate intracellular parasites, i.e., organisms able to grow and reproduce only when inside a living cell. Viruses, some protozoa and some bacteria are hypotrophs. Carnivores = Chemoheterotrophs obtaining nutrients from meat. Herbivores = Chemoheterotrophs obtaining nutrients from plant material. Omnivores = Chemoheterotrophs able to obtain nutrients from both meat and plant material. Criteria Used in the Classification of Microorganisms B. Metabolism Metabolism includes all the chemical reactions occurring within living organisms (anabolism and catabolism), and can be categorized as either fermentative or respiratory (oxidative). Fermentative organisms use organic compounds (usually pyruvic acid) as the final electron acceptors in their metabolic processes. Respiratory (oxidative) organisms use inorganic compounds (usually molecular oxygen) as the final electron acceptors in their metabolic processes. Criteria Used in the Classification of Microorganisms 4. Gas Requirements The gas requirements of organisms (based on oxygen utilization) can be useful in their classification as indicated below: Obligately aerobic organisms (obligate aerobes) = Organisms requiring molecular oxygen for growth and reproduction (metabolic processes). Obligately anaerobic organisms (obligate anaerobes) = Organisms unable to tolerate exposure to molecular oxygen; oxygen is often toxic to these, and they cannot grow in its presence. Facultatively anaerobic/aerobic organisms (facultative anaerobes/aerobes) = Organisms able to grow and reproduce with or without oxygen available to them. Microaerophiles = Organisms able to grow best in environments with limited oxygen, as might occur in the mud at the bottom of a pond, lake, sea, etc., or within the gastrointestinal tract. Although obligately aerobic organisms typically have a respiratory or oxidative metabolism and require oxygen as a final electron acceptor; not all obligately anaerobic organisms are fermentative. Many types of bacteria can use inorganic compounds other than molecular oxygen as final electron acceptors for their respiratory metabolic processes. Criteria Used in the Classification of Microorganisms 5. Temperature Requirements The temperatures required for optimum growth are variable and can be used to categorize microorganisms as follows: Psychrophiles = Psychrophiles are cold-loving organisms (psychro = cold, phil = love). These organisms grow best at cold temperatures (between –5 and 20 degrees C). Mesophiles = Mesophiles are moderate-loving organisms (meso = medium or intermediate) and grow best at moderate temperatures (between 20 and 45 degrees C). Thermophiles = Thermophiles are warm-loving organisms (thermo = warm) and grow best at warm temperatures (between 45 and 60 degrees C). Hyperthermophiles = Hyperthermophiles are hot-loving organisms and grow best at hot temperatures, e.g., above 60 degrees C. Hyperthermophiles living in hot springs grow at temperatures above 90 degrees C. Organisms can also be described relative to their temperature tolerance, i.e., ability to survive or tolerate exposure to temperature extremes. Organisms that can tolerate exposure to extreme cold are said to be psychroduric. They cannot grow at these temperatures, but do not die either. Most bacteria are psychroduric and can be maintained in a viable state at –70 degrees C. Organisms that can tolerate exposure to heat are said to be thermoduric. They cannot necessarily grow in hot environments, but are not killed by exposure to them. Endospores are thermoduric. Criteria Used in the Classification of Microorganisms 6. Acidity Vs Alkalinity or pH Requirements Although most organisms grow best in neutral environments (pH between 6.5 and 7.5), some prefer acidic environments, and others prefer alkaline. Many types of culture media contain buffers (substances that resist pH change) to help stabilize the pH or pH indicators (substances that change color in response to changes in pH) to indicate the presence of acidic or alkaline metabolic end products. Organisms that grow best in acidic environments are called acidophiles, but are relatively rare. Highly acidic or alkaline environments tend to inhibit microbial growth because cellular enzymes fail to function under these conditions. Criteria Used in the Classification of Microorganisms 7. Osmotic Pressure Requirements The effective osmotic pressure (tonicity) of an environment is influenced by the solute concentration present, and can significantly impact microbial growth. Isotonic environments (iso = same) contain solute levels similar to protoplasm, so cells placed into them will experience neither a net gain nor net loss of water. Hypotonic environments (hypo = under, beneath, less than or too little) contain lower levels of solute than protoplasm and will cause cells placed into them to gain water. Microorganisms equipped with cell walls (e.g., algae, fungi, bacteria and archaea) or contractile vacuoles (many types of fresh water protozoa) can live comfortably in hypotonic environments. Organisms lacking these protective structures will tend to take on water (via osmosis) until they explode. Hypertonic environments (hyper = over, above, too much or excessive) contain higher levels of solute than protoplasm and will cause cells placed into them to lose water. Hypertonic environments containing high levels of salt or sugar are often used to preserve foods, i.e., inhibit microbial growth within those foods. Organisms capable of growing and reproducing in environments containing high levels of salt are called halophiles. These may be categorized as extreme halophiles/obligate halophiles (those requiring high levels of salt for growth) or facultative halophiles (those capable of growing with or without salt). Criteria Used in the Classification of Microorganisms 8. Environmental Relationships The types of environmental relationships microorganisms form with other organisms can be useful as criteria for classification; however, these relationships are often not thoroughly documented nor understood. Symbiosis – Symbiosis is a condition or circumstance existing when two or more different types of organisms are living together in a close association. Although once thought to be unusual, symbiosis is now recognized as a common occurrence, essential to ecosystem function. Pathogen Vs Host – Pathogens growing within a host benefit from host resources, but the host is harmed, and sometimes killed. Microorganisms capable of causing infection and disease in humans, domestic animals and plants used in agriculture have been extensively studied, but represent an extremely small percentage of the total. Parasite Vs Host – Parasites also benefit from their hosts without giving in return. Organisms capable of parasitizing humans and other animals have been studied extensively because some cause disease and others serve as vectors involved in the transmission of disease-causing agents. Mutualistic relationships (mutualism), i.e., those involving organisms in mutually beneficial arrangements are the most common form of symbiotic relationships. Even pathogens and parasites can be considered beneficial in the sense that they help prevent population overgrowth and maintain balance within ecosystems, a concept foreign/repugnant to most humans. Criteria Used in the Classification of Microorganisms 9. Biochemical Analysis Biochemical analysis allows for a more technical evaluation of the relationships existing between organisms and has become the method of choice for the classification of bacteria and archaea. Various subcategories exist as follows: A. Enzymatic Testing – The types of enzymes organisms produce can be determined by testing their ability to catabolize various materials and/or to form specific end products. Enzymatic testing will be used extensively during the identification of Physiological Unknown No. 1. B. Chromatography – Various applications of chromatography can be used to identify specific chemical constituents of cells, e.g., cell wall lipid or amino acid content, membrane protein content, or the presence of specific pigments. C. Serology – Serology is the science or study of antibody and antigen interactions in vitro, and has multiple applications in the detection, identification and classification of microorganisms. Microorganisms are antigenic, i.e., are perceived by the body as foreign agents (antigens), and typically stimulate the production of immune proteins called antibodies. Because the interactions between antigens and antibodies are quite specific, and because antibodies can bind with antigens, it is possible to use known types of antibodies to detect or identify specific types of antigens. Several different types of serological reactions will be explained and demonstrated in the laboratory. D. Phage Typing – Phage typing (bacteriophage typing) involves the use of viruses called bacteriophages. Like antibodies, these will recognize and bind with specific types of bacteria; however, unlike antibodies, they cause infection typically resulting in cell death. Because these viruses are host-specific, known types of virus particles can be used to identify unknown types of bacteria. Phage typing will be explained and demonstrated in the laboratory Criteria Used in the Classification of Microorganisms E. Nucleic Acid Analysis The analysis of nucleic acids, DNA and RNA, can provide considerable information useful in the identification and classification of microorganisms. Techniques commonly used in nucleic acid analysis include: 1. Percent base composition (G + C or A + T) – Organisms with identical percentages in base composition may or may not be closely related, but organisms with very different percentages in base composition are not related. 2. Nucleic Acid Hybridization – Hybridization, the ability of two nucleic acid strands to form hydrogen bonds with one another, has multiple applications including PCR and DNA chip technology. 3. Polymerase Chain Reaction (PCR) – The polymerase chain reaction involves hybridization and can be used to amplify DNA or RNA in vitro. 4. Gel Electrophoresis – Gel electrophoresis can be used to separate DNA or RNA fragments on the basis of size by exposing them to an electric field. 5. DNA Fingerprinting or RFLP analysis – Fragments of DNA generated by restriction endonuclease digestion will form patterns when subjected to electrophoresis. These patterns are called DNA fingerprints or RFLP patterns. 6. Nucleotide sequencing – Determining the sequence of nucleotides in a strand of DNA or RNA can yield information highly significant to identification and classification. Criteria Used in the Classification of Microorganisms F. Protein analysis The analysis of proteins other than antibodies can also be useful in the identification and classification of microorganisms. Some methods involved include: 1. Gel electrophoresis – Similar to methods used with nucleic acids. 2. Amino acid sequencing – Determining the sequence of amino acids present in a protein can be useful in determining protein function and sometimes protein origin. For example, the origin of prions (infectious protein particles) was determined using amino acid sequencing in conjunction with nucleic acid analysis. Bacterial Taxonomy Although no universally accepted bacterial classification system is available, three main approaches are usually followed. These include (1) phylogenetic, (2) adansonian and (3)molecular and genetic classifications. Phylogenetic Classification: Phylogenetic classification is a type of hierarchical classification that presents a branching tree-like arrangement, with one characteristic being employed for divisions at each branch or level. It is called phylogenetic classification because it denotes an evolutionary arrangement of the species. This classification groups together the types that are related on an evolutionary basis where several groups are used such as Divisions, Classes, Orders, Families, Tribes, Genera and Species. Some characters of special importance, such as Gram staining properties, lactose fermentation, spore formation and so on, are used to differentiate between the major groups. Less important properties (nutritional requirements for growth of bacteria, production of certain enzymes by bacteria and soon) distinguish between the minor groups such as the genera and the species. Bacterial Taxonomy Adansonian Classification: The adansonian classification, the first classification, was pro-posed by Michael Adanson in the eighteenth century and considers the characteristics expressed at the time of the study. Hence, it is called a Phenetic system. This classification gives equal weight to all measurable features and groups of bacteria based on similarities of several characteristics. The availability of computer facilities has expanded the scope of phenetic classification. It allows for the comparison of a vast number of properties of several organisms simultaneously. A computer analysis of a large number of characteristics of a bacterium facilitates the identification of several broad sub-groups of bacterial strains that are further sub- divided into species and are represented in adendrogram This type of classification, based on a large number of properties, is known as numerical taxonomy. Bacterial Taxonomy Molecular and Genetic Classification: The molecular classification is based on the homology of the DNA base sequences of the microorganisms. First, DNA is extracted from the organism, and the DNA relatedness of the microorganisms is tested, and the nucleotide sequence of the DNA is studied using DNA hybridization or recombination methods. The degree of hybridization can be assessed by many methods, such as by using labelled DNA preparations. The genetic relatedness can also be assessed by studying the messenger RNA (mRNA). Also, ribosomal RNA (rRNA) analysis is of immense value. Evolutionary relationships among widely divergent organisms have been shown by studying the nucleotide sequence of 16S ribosomal RNA from different biologic sources. It has contributed to the understanding of new groups of bacteria such as the archaebacteria. In recent times, genetic classification is increasingly being used to study viruses. Nomenclature, Taxonomy & Classification of Viruses The initial classification of viruses was based on the sites of their isolation or the symptomatology of the disease. Since the formation of the International Committee on the Taxonomy of Viruses (ICTV) in 1966, a systematic classification of viral taxonomy and nomenclature was carried out. The ICTV has been introducing a systematic approach for the classification and nomenclature of the viruses. The ICTV has grouped viruses into families based on (1)the type of nucleic acid they possess, (2) their means of replication and (3) their morphology (e.g. membrane envelope). The suffix virus is used for genus names, viridae for family names and ales for order names. In formal usage, the family and genus names are used in the following manner: for example, family Rhabdoviridae, genus Lyssavirus, human rabies virus. A viral species is a group of viruses that share the same genetic information and ecological niche. These viral species are designated by descriptive common names, such as human herpes virus, with sub-species, if any, designated by a number(e.g.HHV-1). Depending on the type of nucleic acids that viruses possess, they are classified into two groups: deoxyriboviruses, which contain DNA (DNA virus), and riboviruses, which contain RNA (RNA virus). The DNA and RNA viruses associated with human diseases are divided into six and 13 families, respectively. Taxonomy & Classification of Fungi The members of the fungal kingdom are eukaryotes and although most of them are multi- cellular, some like the yeast have a single-cell structure. For a long time, fungi were classified under the plant kingdom, but later a separate kingdom was assigned to these organisms. The basis of fungal classification largely remains the morphology of the yeast or the mould, together with other features, particularly the mode of reproduction and the type of spores produced. Four phyla were initially described based on these features. Subsequently, the taxonomy of fungi underwent a state of flux, a situation which is being resolved in recent years by the application of genomic studies. Novel relationships between various fungal groups have been revealed by using the tools of molecular biology and through the sequencing of 18S ribosomal RNA. As per the latest understanding, nine phylum level groups have been described: Opisthosporidia, Chytridiomycota, Neocallimastigomycota, Blastocladiomycota, Zoopagomycota, Mucoromycota, Glomeromycota, Basidiomycota and Ascomycota. Taxonomy & Classification of Fungi Differentiating features of fungi and bacteria