General Microbiology I - Lecture Three - Classification of Bacteria (Part One) PDF

General Microbiology I Undergraduate Course Lecture Three- Classification 1 Department of Biology 24-25 of Bacteria (Part One) Classification of Bacteria in different 9 ways Bacteria are classified and identified to distingu...

General Microbiology I Undergraduate Course Lecture Three- Classification 1 Department of Biology 24-25 of Bacteria (Part One) Classification of Bacteria in different 9 ways Bacteria are classified and identified to distinguish one organism from another and to group similar organisms by criteria of interest to microbiologists or other scientists. The classification of bacteria serves a variety of different functions. Because of this variety, bacteria may be grouped using many different typing schemes. The grounds for the classification commonly used may be: - Morphologic Characteristics - Growth Characteristics - Antigens and Phage Susceptibility - Biochemical Characteristics 1. Classification on the basis of Gram Stain and Bacterial Cell Wall 2. Classification of Bacteria on the Basis of Shape Others 3. Classification of Bacteria on the Basis of Mode of Nutrition 4. Classification of Bacteria on the Basis of Temperature Requirement 5. Classification of Bacteria on the Basis of Oxygen Requirement 6. Classification of Bacteria on the Basis of pH of Growth 7. Classification of Bacteria on the Basis of Osmotic Pressure Requirement 8. Classification of Bacteria on the Basis of Number of Flagella 9. Classification of Bacteria on the basis of Spore Formation Morphologic Characteristics Both wet-mounted and properly stained bacterial cell suspensions can yield a great deal of information. These simple tests can indicate the Gram reaction of the organism, whether it is acid-fast, its motility, the arrangement of its flagella, the presence of spores, capsules, and inclusion bodies, and, of course, its shape. This information can often allow the identification of an organism at the genus level or minimize the possibility that it belongs to one or another group. Growth Characteristics A primary distinguishing characteristic is whether an organism grows aerobically, anaerobically, facultatively (i.e., in either the presence or absence of oxygen), or microaerobically (i.e., in the presence of a less than atmospheric partial pressure of oxygen). The proper atmospheric conditions are essential for isolating and identifying bacteria. Other important growth assessments include the incubation temperature, pH, nutrients required, and resistance to antibiotics. For General Microbiology I Undergraduate Course Lecture Three- Classification 2 Department of Biology 24-25 of Bacteria (Part One) example, one diarrheal disease agent, Campylobacter jejuni, grows well at 42° C in the presence of several antibiotics; another, Y. enterocolitica, grows better than most other bacteria at 4° C. Legionella, Haemophilus, and some other pathogens require specific growth factors, whereas E. coli and most other Enterobacteriaceae can grow on minimal media. Antigens and Phage Susceptibility Cell wall (O), flagellar (H), and capsular (K) antigens are used to aid in classifying certain organisms at the species level, to serotype strains of medically important species for epidemiologic purposes, or to identify serotypes of public health importance. Serotyping is also sometimes used to distinguish strains of exceptional virulence or public health importance, for example with V. cholerae (O1 is the pandemic strain) and E. coli (enterotoxigenic, enteroinvasive, enterohemorrhagic, and enteropathogenic serotypes). Phage typing (determining the susceptibility pattern of an isolate to a set of specific bacteriophages) has been used primarily as an aid in epidemiologic surveillance of diseases caused by Staphylococcus aureus, mycobacteria, P. aeruginosa, V. cholerae, and S. Typhi. Susceptibility to bacteriocins has also been used as an epidemiologic strain marker. In most cases recently, phage and bacteriocin typing have been supplanted by molecular methods. Biochemical Characteristics Most bacteria are identified and classified largely on the basis of their reactions in a series of biochemical tests. Some tests are used routinely for many groups of bacteria (oxidase, nitrate reduction, amino acid degrading enzymes, fermentation or utilization of carbohydrates); others are restricted to a single family, genus, or species (coagulase test for staphylococci, pyrrolidonyl arylamidase test for Gram-positive cocci). 1. Classification on the basis of Gram Stain and Bacterial Cell Wall General Microbiology I Undergraduate Course Lecture Three- Classification 3 Department of Biology 24-25 of Bacteria (Part One) Of all the different classification systems, the Gram stain has withstood the test of time. Discovered by H.C. Gram in 1884 it remains an important and useful technique to this day. It allows a large proportion of clinically important bacteria to be classified as either Gram-positive or negative based on their morphology and differential staining properties. Slides are sequentially stained with crystal violet and iodine, then destained with alcohol and counter-stained with safranin. Gram-positive bacteria stain blue-purple and Gram-negative bacteria stain red. The difference between the two groups is believed to be due to a much larger peptidoglycan (cell wall) in Gram positives. As a result, the iodine and crystal violet precipitate in the thickened cell wall and are not eluted by alcohol, in contrast with the Gram-negatives, where the crystal violet is readily eluted from the bacteria. As a result, bacteria can be distinguished based on their morphology and staining properties. Some bacteria, such as mycobacteria, are not reliably stained due to the large lipid content of the peptidoglycan. Alternative staining techniques (Kinyoun or acid-fast stain) are therefore used to take advantage of the resistance to destaining after lengthier initial staining. 2. Classification of Bacteria on the Basis of Shape In the year 1872 scientist Cohn classified bacteria into FOUR major types depending on their shapes as follows: A) Cocci: These types of bacteria are unicellular, spherical, or elliptical in shape. They may either remain as a single cell or aggregate together for various configurations. They are as follows: General Microbiology I Undergraduate Course Lecture Three- Classification 4 Department of Biology 24-25 of Bacteria (Part One) Monococcus:– they are also called micrococcus and represented by single, discrete round Example: Micrococcus flavus. Diplococcus:– the cell of the Diplococcus divides in a particular plane, and after division, the cells remain attached to each other. Example: Diplococcus pneumonia. Streptococcus: – here, the cells divide repeatedly in one plane to form a chain of cells. Example: – Streptococcus pyogenes. Tetracoccus: This consists of four round cells divided into two planes at right angles. Example: – Gaffkya tetragena. Staphylococcus: – here, the cells are divided into three planes, forming a structure like bunches of grapes, giving an irregular configuration. Example: – Staphylococcus aureus. Sarcina: In this case, the cells divide into three planes, but they form a cube-like configuration consisting of eight or sixteen cells. They have a regular shape. Example: –Sarcina lutea. B) Bacilli: These are rod-shaped or cylindrical bacteria that either remain singly or in pairs. Example: –Bacillus cereus. C) Vibro: – The Vibro is the curved, comma-shaped bacteria represented by a single genus. Example: – Vibro cholerae. D) Spirilla: – These types of bacteria are spiral or spring-like with multiple curvatures and terminal flagella. Example: –Spirillum volutans. Others Actinomycetes are branching filamentous bacteria, so called because of a fancied resemblance to the radiating rays of the sun when seen in tissue lesions (from actis meaning ray and mykes meaning fungus). Mycoplasmas are bacteria that are cell wall deficient and, hence, do not possess a stable morphology. They occur as round or oval bodies and as interlacing filaments. 3. Classification of Bacteria on the Basis of Mode of Nutrition 1. Phototrophs: Those bacteria gain energy from light. Phototrophs are further divided into two groups based on the source of the electron. Photolithotrophs: these bacteria gain energy from light and use reduced inorganic compounds such as H2S as an electron source. Eg. Chromatium okenii. Photoorganotrophs: these bacteria gain energy from light and use organic compounds such as succinate as an electron source. 2. Chemotrophs: General Microbiology I Undergraduate Course Lecture Three- Classification 5 Department of Biology 24-25 of Bacteria (Part One) Those bacteria gain energy from chemical compounds. They cannot carry out photosynthesis. Chemotrophs are further divided into two groups on the basis of source of electron. Chemolithotrophs: they gain energy from the oxidation of chemical compounds and reduce inorganic compounds such as NH3 as an electron source. Eg. Nitrosomonas. Chemoorganotrophs: they gain energy from chemical compounds and use organic compounds such as glucose and amino acids as sources of electrons. eg. Pseudomonas pseudoflava. 3. Autotrophs: Those bacteria use carbon dioxide as the sole source of carbon to prepare their own food. Autotrophs are divided into two types based on the energy utilized to assimilate carbon dioxide. ie. Photoautotrophs and chemoautotrophs. Photoautotrophs: they utilized light to assimilate CO2. They are further divided into two groups on the basis of electron sources. i.e. Photolithographic autotrophs and Photoorganotrophic autotrophs Chemoautotrophs: They utilize chemical energy for the assimilation of CO2. 4. Heterotrophs: Those bacteria use organic compounds as carbon sources. They lack the ability to fix CO2. Most of the human pathogenic bacteria are heterotrophic in nature. Some heterotrophs are simple because they have simple nutritional requirements. However, there are some bacteria that require special nutrients for their growth, known as fastidious heterotrophs. 4. Classification of Bacteria on the Basis of Temperature Requirement Bacteria can be classified into the following major types on the basis of their temperature response as indicated below: 1. Psychrophiles: Bacteria that can grow at 0°C or below but the optimum temperature of growth is 15 °C or below and the maximum temperature is 20°C are called psychrophiles Psychrophiles have polyunsaturated fatty acids in their cell membrane, which gives a fluid nature to the cell membrane even at lower temperatures. Examples: Vibrio psychroerythrus, vibrio marinus, Polaromonas vaculata, Psychroflexus. 2. Psychrotrophs (facultative psychrophiles): General Microbiology I Undergraduate Course Lecture Three- Classification 6 Department of Biology 24-25 of Bacteria (Part One) Those bacteria can grow even at 0°C, but the optimum temperature for growth is (20-30)°C 3. Mesophiles: Those bacteria can grow best between (25-40) o C, but the optimum temperature for growth is 37C Most of the human pathogens are mesophilic in nature. Examples: E. coli, Salmonella, Klebsiella, Staphylococci. 4. Thermophiles: Those bacteria can best grow above 45 o C. Thermophiles capable of growing in the mesophilic range are called facultative thermophiles. True thermophiles are called Stenothermophiles, they are obligate thermophiles, Thermophiles contain saturated fatty acids in their cell membrane, so their cell membrane does not become too fluid even at higher temperatures. Examples: Streptococcus thermophiles, Bacillus stearothermophilus, Thermus aquaticus. 5. Hyperthermophiles: Those bacteria that have an optimum temperature of growth above 80C. Mostly, Archeobacteria are hyperthermophiles. The monolayer cell membrane of Archeobacteria is more resistant to heat, and they adapt to grow in higher temperatures. Examples: Thermodesulfobacterium, Aquifex, Pyrolobus fumari, Thermotoga. General Microbiology I Undergraduate Course Lecture Four- Classification 1 Department of Biology 24-25 of Bacteria (Part Two) Classification of Bacteria on the Basis of Oxygen Requirement Obligate Aerobes: (A) Require oxygen to live. Example: Pseudomonas, a common nosocomial pathogen. Facultative Anaerobes: (C) It can use oxygen but can grow in its absence. They have a complex set of enzymes. Examples: E. coli, Staphylococcus, yeasts, and many intestinal bacteria. Obligate Anaerobes: (B) Cannot use oxygen and are harmed by the presence of toxic forms of oxygen. Examples: Clostridium bacteria that cause tetanus and botulism. Aerotolerant Anaerobes: (E) Cannot use oxygen, but tolerate its presence. Can break down toxic forms of oxygen. Example: Lactobacillus carries out fermentation regardless of oxygen presence. Microaerophiles: (D) Requires oxygen, but at low concentrations. Sensitive to toxic forms of oxygen. Example: Campylobacter. General Microbiology I Undergraduate Course Lecture Four- Classification 2 Department of Biology 24-25 of Bacteria (Part Two) Classification of Bacteria on the Basis of pH of Growth 1. Acidophiles: These bacteria grow best at an acidic pH. The cytoplasm of these bacteria is acidic in nature. Some acidophiles are thermophilic in nature; such bacteria are called Thermoacidophiles. 2. Alkaliphiles: These bacteria grow best at an alkaline pH. Example: Vibrio cholerae optimum ph of growth is 8.2. 3. Neutrophiles: These bacteria grow best at neutral pH (6.5-7.5). Most of the bacteria grow at neutral pH. Example: E. coli Classification of Bacteria on the Basis of Osmotic Pressure Requirement Halophiles: Require moderate to large salt concentrations. The cell membrane of halophilic bacteria is made up of glycoprotein with a high content of negatively charged glutamic acid and aspartic acids. So high concentration of Na+ ion concentration is required to shield the –ve charge. Ocean water contains 3.5% salt. Most such bacteria are present in the oceans. Archeobacteria, Halobacterium, Halococcus. Extreme or Obligate Halophiles: Require very high salt concentrations (20 to 30%). Bacteria in the Dead Sea, brine vats. Facultative Halophiles: Do not require high salt concentrations for growth, but tolerate upto 2% salt or more. Classification of Bacteria on the Basis of Number of Flagella On the basis of flagella, the bacteria can be classified as: 1. Atrichos: – These bacteria have no flagella. Example: Corynebacterium diptherae. 2. Monotrichous: – One flagellum is attached to one end of the bacteria cell. Example: – Vibro cholerae. 3. Lophotrichous: A bunch of flagella is attached to one end of the bacteria cell. Example: Pseudomonas. 4. Amphitrichous: A bunch of flagella arising from both ends of the bacteria cell. Example: Rhodospirillum rubrum. General Microbiology I Undergraduate Course Lecture Four- Classification 3 Department of Biology 24-25 of Bacteria (Part Two) 5. Peritrichous: – The flagella are evenly distributed surrounding the entire bacterial cell. Example: Bacillus. Classification of Bacteria on the basis of Spore Formation 1. Spore-forming bacteria: Those bacteria produce spores during unfavorable conditions. These are further divided into two groups: i) Endospore-forming bacteria: Spore is produced within the bacterial cell. Examples. Bacillus, Clostridium. ii) Exospore forming bacteria: Spore is produced outside the cell. 2. Non sporing bacteria: Those bacteria that do not produce spores. Eg. E. coli, Salmonella. Method of Obtaining Bacterial Growth Curve A population growth curve for any particular species of bacterium may be determined by growing a pure culture of the organism in a liquid medium at a constant temperature. Samples of the culture are collected at fixed intervals (e.g., every 30 minutes), and the number of viable organisms in each sample is determined. The data are then plotted on logarithmic graph paper. The logarithm of the number of bacteria per milliliter of medium is plotted against time. The bacterial growth curve shows the following four distinct phases: 1. Lag phase: General Microbiology I Undergraduate Course Lecture Four- Classification 4 Department of Biology 24-25 of Bacteria (Part Two) After a liquid culture broth is inoculated, the multiplication of bacteria does not start immediately. It takes some time to multiply. The time between inoculation and the beginning of multiplication is known as the lag phase. In this phase, the inoculated bacteria become acclimatized to the environment, switch on various enzymes, and adjust to the environmental temperature and atmospheric conditions. During this phase, there is an increase in size of bacteria but no appreciable increase in number of bacterial cells. The cells are active metabolically. The duration of the lag phase varies with the bacterial species, nature of the culture medium, incubation temperature, etc. It may vary from 1 hour to several days. 2. Log phase: This phase is characterized by rapid exponential cell growth (i.e., 1 to 2 to 4 to 8, and so on). The bacterial population doubles during every generation. They multiply at their maximum rate. The bacterial cells are small and uniformly stained. The microbes, such as antibiotics and other antimicrobial agents, are sensitive to adverse conditions. The growth rate is the greatest during the log phase. The log phase is always brief unless the rapidly dividing culture is maintained by constant addition of nutrients and frequent removal of waste products. When plotted on logarithmic graph paper, the log phase appears as a steeply sloped straight line. 3. Stationary phase: After the log phase, the bacterial growth almost stops completely due to a lack of essential nutrients, lack of water oxygen, changes in pH of the medium, etc., and accumulation of their own toxic metabolic wastes. During this phase, the culture is at its greatest population density. However, the Death rate of bacteria exceeds the rate of replication of bacteria. Endospores start forming during this stage. Bacteria become Gram variable and show irregular staining. Many bacteria start producing exotoxins. 4. Decline phase: During this phase, the bacterial population declines due to the death of cells. The decline phase starts due to (a) accumulation of toxic products and autolytic enzymes and (b) exhaustion of nutrients. When these involuted forms are inoculated into a fresh nutrient medium, they usually revert to the original shape of the healthy bacteria. General Microbiology I Undergraduate Course 1 Lecture five Department of Biology 24-25 Classification of Bacteria based morphology Structure of Bacterial Cells Bacteria are classified by:  Bacteria range in size from about 0.2 to 5 μm. The smallest bacteria(Mycoplasma) are about the same size as the largest viruses (poxviruses).The longest bacteria rods are the size of some yeasts and human red blood cells (7 μm).  shape into three basic groups: cocci, bacilli, and spirochetes. The cocci are round, the bacilli are rods, and the spirochetes are spiral-shaped. Some bacteria are variable in shape and are said to be pleomorphic (many-shaped). The shape of a bacterium is determined by its rigid cell wall.  The arrangement of bacteria is important. For example, certain cocci occur in pairs (diplococci), some in chains(streptococci), and others in grapelike clusters (staphylococci). These arrangementsare determined by the orientation and degree of attachment of the bacteria at the time of cell division. Bacterial cell structure: Cell Wall The cell wall is the outermost component common to all bacteria (except Mycoplasma species, which are bounded by a cell membrane, not a cell wall). Some bacteria have surface features external to the cell wall, such as a capsule, flagella, and pili, which are less common components. The cell wall is located external to the cytoplasmic membrane and is composed of peptidoglycan. The structure, chemical composition, and thickness of the cell wall depending upon the bacteria type. The peptidoglycan provides structural support and maintains the characteristic shape of the cell. The cell wall has several other important properties: 1. In gram-negative bacteria, it contains endotoxin, a lipopolysaccharide 2. Its polysaccharides and proteins are antigens that are useful in laboratory identification. 3. Its porin proteins play a role in facilitating the passage of small, hydrophilic molecules into the cell. General Microbiology I Undergraduate Course 2 Lecture five Department of Biology 24-25 Classification of Bacteria based morphology Porin proteins in the outer membrane of gram-negative bacteria act as a channel to allow the entry of essential substances such as sugars, amino acids, vitamins, and metals as well as many antimicrobial drugs such as penicillin's. Peptidoglycan The term peptidoglycan is derived from the peptides and the sugars (glycan) that make up the molecule. peptidoglycan are murein and mucopeptide. Peptidoglycan is a complex, interwoven network that surrounds the entire cell and is composed of a single covalently linked macromolecule. It is found only in bacterial cell walls.  It provides rigid support for the cell.  Is important in maintaining the characteristic shape of the cell.  allows the cell to with stand media of low osmotic pressure, such as water. The other important component in this network is the peptide cross-link between the two tetrapeptides. The cross-links vary among species. Because peptidoglycan is present in bacteria but not in human cells, it is a good target for antibacterial drugs. Several of these drugs, such as penicillins Teichoic acid These fibers composed of polymers of either glycerol phosphate or ribitol phosphate are located in the outer layer of the gram-positive cell wall and extend from it. Some polymers of glycerol teichoic acid penetrate the peptidoglycan layer and are covalently linked to the lipid in the cytoplasmic membrane, in which case they are called lipoteichoic acid; The medical importance of teichoic acids lies in their ability to induce septic shock when caused by certain gram-positive bacteria; i.e., they activate the same pathways as does endotoxin (LPS) in gram-negative bacteria. Teichoic acids also mediate the attachment of staphylococci to mucosal cells. Gram-negative bacteria do not have teichoic acids. Lipopolysaccharide The lipopolysaccharide (LPS) of the outer membrane of the cell wall of gram negative bacteria is endotoxin. It is responsible for many of the features of disease, such as fever and shock (especially hypotension), caused by these organisms. It is called endotoxin because it is an integral part of the cell wall, in contrast to exotoxins, which are actively secreted from the bacteria. The symptoms caused by the endotoxin of one gram negative bacteria is similar to another, but the severity of the symptoms can differ greatly. In contrast, the symptoms caused by exotoxins of different bacteria are usually quite different General Microbiology I Undergraduate Course 3 Lecture five Department of Biology 24-25 Classification of Bacteria based morphology Cell Walls Of Gram-positive And Gram-negative Bacteria The structure, chemical composition, and thickness of the cell wall differ in gram positive and gram-negative bacteria. (1) The peptidoglycan layer is much thicker (multilayer )in gram-positive than in gram negative bacteria. Many gram-positive bacteria also have fibers of teichoic acidwhereas gram-negative bacteria do not have teichoic acids. (2) In contrast, the gram-negative bacteria have a complex outer layer consisting of lipopolysaccharide, lipoprotein, and phospholipid. Lying between the outer membrane layer and the cytoplasmic membrane in gram-negative bacteria is the periplasmic space Cell Walls of Acid-fast Bacteria Mycobacteria (e.g., Mycobacterium tuberculosis) have an unusual cell wall, resulting in their inability to be Gram-stained. These bacteria are said to be acid fast because they resist decolorization with acid–alcohol after being stained with carbolfuchsin. This property is related to the high concentration of lipids, called mycolic acids, in the cell wall of mycobacteria Cytoplasmic Membrane Just inside the peptidoglycan layer of the cell wall lies the cytoplasmic membrane ,which is composed of a phospholipid bilayer similar in microscopic appearance to that in eukaryotic cells. They are chemically similar, but eukaryotic membranes contain sterols, whereas prokaryotes generally do not. The only prokaryotes that have sterols in their membranes are members of the genus Mycoplasma. The membrane has four important functions: (1) active transport of molecules into the cell. (2) energy generation by oxidative phosphorylation. (3) synthesis of precursors of the cell wall. (4) secretion of enzymes and toxins. General Microbiology I Undergraduate Course 4 Lecture five Department of Biology 24-25 Classification of Bacteria based morphology Cytoplasm The cytoplasm has two distinct areas when seen in the electron microscope: 1. An amorphous matrix that contains ribosomes, nutrient granules, metabolites, and plasmids. 2. An inner, nucleoid region composed of DNA. Mesosome Mesosomes are respiratory sites of bacteria. The mesosomes are attached to the bacterial chromosomes and is involved in DNA segregation during cell division. They are predominant in Gram positive bacteria. Ribosomes Bacterial ribosomes are the site of protein synthesis as in eukaryotic cells, but they differ from eukaryotic ribosomes in size and chemical composition. Bacterial ribosomes are 70S in size, with 50S and 30S subunits, whereas eukaryotic ribosomes are 80S in size, with 60S and 40S subunits. The differences in both the ribosomal RNAs and proteins constitute the basis of the selective action of several antibiotics that inhibit bacterial, but not human, protein synthesis Plasmids Plasmids are extrachromosomal, double-stranded, circular DNA molecules that are capable of replicating independently of the bacterial chromosome. Although plasmids are usually extrachromosomal, they can be integrated into the bacterial chromosome. Plasmids occur in both gram-positive and gram-negative bacteria, and several different types of plasmid can exist in one cell : 1-Transmissible plasmid s can be transferred from cell to cell By conjugation. 2-Nontransmissible plasmids are small. General Microbiology I Undergraduate Course 5 Lecture five Department of Biology 24-25 Classification of Bacteria based morphology Plasmids carry the genes for the following functions and structures of medical importance: (1) Antibiotic resistance (2) Resistance to heavy metals (3) Resistance to ultraviolet light. (4) Pili (fimbriae), which mediate the adherence of bacteria to epithelial cells. (5) Exotoxins, including several enterotoxins. Granules  The cytoplasm contains several different types of granules that serve as storage areas for nutrients and stain characteristically with certain dyes Structures Outside the Cell Wall Capsule The capsule is a gelatinous layer covering the entire bacterium. It is composed of polysaccharide, except in the anthrax bacillus, which has a capsule of polymerized d-glutamic acid. The sugar components of the polysaccharide vary from one species of bacteria to another The capsule is important for four reasons: 1) It is a determinant of virulence of many bacteria since it limits the ability of phagocytes to engulf the bacteria 2) Specific identification of an organism can be made by using antiserum against the capsular polysaccharide 3) Capsular polysaccharides are used as the antigens in certain vaccines because they are capable of eliciting protective antibodies. For example, the purified capsular polysaccharides of 23 types of S. pneumoniae are present in the current vaccine. 4) The capsule may play a role in the adherence of bacteria to human tissues, which is an important initial step in causing infection General Microbiology I Undergraduate Course 6 Lecture five Department of Biology 24-25 Classification of Bacteria based morphology Flagella Threadlike, locomotor appendages extending outward from plasma membrane and cell wall o Functions  motility and swarming behavior  attachment to surfaces  may be virulence factors Three Parts of Flagella Filament - extends from cell surface to the tip -hollow, rigid cylinder of flagellin protein Hook - links filament to basal body Basal body - series of rings that drive flagellar motor Pili and fimbria Pili are hair-like microfibrils, 0.5 to 2 µm in length and 5 to 7 nm in diameter. They are thinner, shorter and more numerous than flagella. They are present only on gram negative cells. They are composed of protein known as pillin. They are unrelated to motility and are found on motile and non-motile cells. Fimbriae and pili, these two terms are used interchangeably but they can be distinguished. Fimbriae can be evenly distributed over the entire surface of the cell or they occurs at the poles of the bacterial cell. Each bacteria possess 100 to 200 fimbriae. Pili are usually longer than fimbriae and number only one or two per cell. Function: Pili play an important role in attachment to surfaces. Hence pili is also called organ of adhesion. General Microbiology I Undergraduate Course 7 Lecture five Department of Biology 24-25 Classification of Bacteria based morphology Spores The process of endospore formation is known as sporulation and it may take 4 to 8 hours in a vegetative cell. Endospores are thick-walled, highly refractile bodies that are produced one per cell. Each bacterial spore on germination forms a single vegetative cell. Therefore, sporulation in bacteria is a method of preservation and not reproduction. Spores are extremely resistant to dessication, staining, disinfecting chemicals, radiation and heat. They remain viable for centuries and help bacteria to survive for long period under unfavorable environment. Endospore can remain dormant for thousand of years. Spores of all medically important bacteria are destroyed by moist heat sterilization ( autoclave ) at 121 °C for 20 minutes.