Bacteria Lecture Notes PDF

University of Kerbala Title of the course: General Biology College of Applied Medical Sciences Level: 1st Class, 1st Semeste Department of Medical Physics Assist.Pro.Dr. Noor Abdulameer Oudah...

University of Kerbala Title of the course: General Biology College of Applied Medical Sciences Level: 1st Class, 1st Semeste Department of Medical Physics Assist.Pro.Dr. Noor Abdulameer Oudah Lecture 9: Bacteria Bacteria (singular: bacterium) are relatively simple, singlecelled (unicellular) organisms, microscopic living organisms (ranged from 0.5-2.0 micron in diameter) it can be seen just under light microscope with the aid of oil immersion lenses (100x). Because their genetic material is not enclosed in a special nuclear membrane, bacterial cells are called prokaryotes (pro¯-KAR- e-o¯ts), from Greek words meaning prenucleus. Bacteria could be found everywhere. They can be either - Beneficial bacteria, as in the process of fermentation (such as in vinegar and dairy production) , also many bacteria play important role in decomposition. or - Pathogenic bacteria that could cause a disease when enters any living body (human or animal) and it can spread through water, air, soil and through physical contact Classification of Bacteria Bacteria can be classified into various categories based on their features and characteristics. The classification of bacteria is mainly based on the following: Classification of bacteria based on Shape Type of Classification Examples Escherichia coli (E. coli) Bacillus (Rod-shaped) Helicobacter pylori Spirilla or spirochete (Spiral) Streptococcus pneumoniae Coccus (Sphere) Vibrio (Comma-shaped) Vibrio cholerae 1 Fig. 1. Different morphological types of the bacteria. Classification of bacteria based on the Composition of the Cell Wall Type of Classification Examples Peptidoglycan cell wall Gram-positive bacteria Lipopolysaccharide cell wall Gram-negative bacteria Fig. 2. Gram staining reactions of the different bacterial isolates 2 Classification of bacteria based on the Mode of Nutrition Type of Classification Examples Autotrophic Bacteria Cyanobacteria Heterotrophic Bacteria All disease-causing bacteria Classification of bacteria based on the Mode of Respiration Type of Classification Examples Aerobic Bacteria Bacillus, Pseudomonas Anaerobic Bacteria Clostridium Facultative anaerobes Escherichia coli Microaerophiles Helicobacter pylori Nomenclature The system of nomenclature (naming) for organisms in use today was established in 1735 by Carolus Linnaeus. Scientific names are latinized because Latin was the language traditionally used by scholars. Scientific nomenclature assigns each organism two names—the genus (plural: genera) is the first name and is always capitalized; the specific epithet (species name) follows and is not capitalized. The organism is referred to by both the genus and the specific epithet, and both names are underlined or italicized. By custom, after a scientific name has been mentioned once, it can be abbreviated with the initial of the genus followed by the specific epithet. Scientific names can , among other things, describe an organism, honor a researcher, or identify the habitat of a species. 3 For example, consider Staphylococcus aureus, a bacterium commonly found on human skin. Staphylo- describes the clustered arrangement of the cells; -coccus indicates that they are shaped like spheres. The specific epithet, aureus, is Latin for golden, the color of many colonies of this bacterium. The genus of the bacterium Escherichia coli is named for a physician, Theodor Escherich, whereas its specific epithet coli, reminds us that E. coli live in the colon, or large intestine. Structure and Functions of Bacterial Cell Envelope The outer layer or cell envelope provides a structural and physiological barrier between the protoplasm (inside) of the cell and the external environment. The cell envelope protects bacteria against osmotic lysis and gives bacteria rigidity and shape. The cell envelope primarily consists of two components: a cell wall and plasma membrane. (Fig. 3) Fig.3. Schematic diagram of structure of a bacteria. Cell Wall Prokaryotic cells almost always are bounded by a fairly rigid and chemically complex structure present between the cell membrane and capsule/slime layer called the cell wall. Peptidoglycan is the main component of the cell wall and is responsible for the shape and strength of the cell. It is a disaccharide and contains two sugar derivatives—N acetyl glucosamine and N acetyl muramic acid—joined together by short peptide chains. 4 Most bacteria are classified as Gram-positive or Gram negative according to their response to the Gram-staining procedure. This procedure was named for the histologist Hans Christian Gram, who developed this differential staining procedure in an attempt to identify bacteria in infected tissues. The Gram-stain depends on the ability of certain bacteria (the Gram-positive bacteria) to retain a complex of crystal violet (a purple dye) and iodine after a brief wash with alcohol or acetone. Gram-negative bacteria do not retain the dye– iodine complex and become translucent, but they can then be counterstained with safranin (a red dye). Thus, Gram-positive bacteria look purple under the microscope, and Gram-negative bacteria look red. The distinction between these two groups turns out to reflect fundamental differences in their cell envelopes.. ◗ Gram-positive cell wall The Gram-positive cell wall is thick (15–80 nm) and more homogenous than that of the thin (2 nm) Gram-negative cell wall. The Gram-positive cell wall contains large amount of peptidoglycan present in several layers that constitutes about 40–80% of dry weight of the cell wall (Fig. 4). The Gram-positive cell wall consists primarily of teichoic and teichuronic acids. These two components account for up to 50% of the dry weight of the wall and 10% of the dry weight of the total cell. ◗ Gram-negative cell wall The Gram-negative cell wall is much more complex than the Gram- positive cell wall. Peptidoglycan content in the Gram-negative cell wall is significantly less than the Gram positive cell wall. Only 1–2 layers of peptidoglycan (2–8 nm) are present just outside the cell membrane (Fig. 4). The Gram-negative cell wall outside the peptidoglycan layer contains three main components— (a) lipoprotein layer. (b) outer membrane. (c) lipopolysaccharides 5 Fig.4. Molecular representation of the envelope of a Gram-positive and Gram-negative bacterium. Periplasmic space Periplasmic space is a distinct space between cell membrane and outer membrane (innermost layer of Gram-negative cell wall) in Gram-negative bacteria. This space is filled with a loose layer of peptidoglycan matrix. The periplasmic space of Gram negative bacteria contains many proteins that participate in nutrient acquisition, and many hydrolytic enzymes, beta lactamases binding proteins, and enzymes that participate in the peptidoglycan synthesis. Polymers of D- glucose, called membrane-derived oligosaccharides, appear to play a role in osmoregulation. The periplasmic space is less distinct in Gram positive cell walls. Capsule and Slime Layer Many bacteria, both Gram-positive and Gram-negative, possess a gel-like layer outside the envelope when growing in their natural environments. When a gel-like layer forms a well defined condensed layer around the bacterial envelope, it is called a capsule and is demonstrable by a light microscope. When this gel-like layer is narrower, detectable only by indirect serological methods or by electron microscope but not by light microscope, it is called a microcapsule. An amorphous viscid colloidal material secreted by some bacteria extracellularly is termed as loose or free slime or glycocalyx. 6 1-Capsule The capsule is mostly made up of polysaccharides, often referred to collectively as exopolysaccharides. However, Bacillus anthracis has a capsule comprising of polyamino acids, such as D-glutamic acid. The D-glutamic acid is probably analogous to the negatively charged polysaccharide capsule. 2-Slime layer Slime layer (S-layer) is a structured para crystalline protein layer shown by electron microscopy. These are generally composed of a single kind of protein molecule, sometimes with carbohydrates attached. They are resistant to proteolytic enzymes and protein-denaturing agents. The slime layer protein protects the cell from wall-degrading enzymes and bacteriophages. It plays an important role in the maintenance of cell shape, and it may be involved in cell adhesion to host epidermal surfaces. FIG. 5. Morphology of bacteria Surface Appendages The surface appendages of the bacteria include flagella and fimbriae or pili. 1-Flagella Bacterial flagella are thread-like appendages intricately embedded in the cell envelope. These structures are responsible for conferring motility to the bacteria. The arrangement of flagella varies between different bacterial species. Depending on the arrangement, flagella can be of the following types: 7  Monotrichous (single polar flagellum), e.g., Vibrio cholerae. Lophotrichous (multiple polar flagella), e.g., Spirilla. Peritrichous (flagella distributed over the entire cell), e.g., Salmonella Typhi, E. coli, etc. Amphitrichous (single flagellum at both the ends) (Fig. 6). FIG. 6. Arrangement of the bacterial flagella Function: Flagella have the following functions: They are primarily responsible for motility of bacteria by chemotaxis. They may play a role in bacterial survival and pathogenesis. 2-Pili (fimbriae) Pili or fimbriae are synonymous for most purposes. They are hair-like filaments that extend from cell surface and are found almost exclusively on Gram-negative bacteria. They are composed of structural protein subunits termed pilins. Minor proteins termed adhesins are located at the tips of pili and are responsible for the attachment properties. Structure: The pili are shorter and straighter than flagella, although the basic structure is same. Like flagella, it consists of helics of protein called pilins, arranged around a hollow core but without a motor. Sex pili: A specialized kind of pili called sex pili is responsible for the attachment of donor and recipient cells in bacterial conjugation. 8 Sporulation Sporulation is a primitive process of differentiation with formation of endospores, a highly resistant resting phase of some of the bacteria (e.g., spores of aerobic Bacillus spp. and anaerobic Clostridium spp.). The organism survives in spores, a dormant state, for longer period of starvation and other adverse conditions. Sporulation process begins in nutrition deprived conditions. During the process of sporulation, each cell forms a single internal spore; the spore germinates to produce a single vegetative cell (Fig. 7). FIG. 7. Schematic diagram showing process of bacterial sporulation. Properties of spores: Bacterial spores are resistant to ordinary boiling, disinfectants, and heating. Spores of all medically important bacteria are destroyed by autoclaving at 121 C for 15 minutes. The process of conversion of a spore into vegeta tive cell under suitable conditions is known as germination. The germination process occurs in three stages: activation, initiation, and outgrowth. 9 Normal Human Microbiota The term “normal microbial flora” denotes the population of microorganisms that inhabit the skin and mucous membranes of healthy normal persons. Previous estimates suggested that the microorganisms that live inside and on humans (now referred to as the normal microbiota). Research has shown that the “normal microbiota” provides a first line of defense against microbial pathogens, assists in digestion, plays a role in toxin degradation, and contributes to maturation of the immune system. Bacterial Diseases in Humans There are a variety of bacteria that cause diseases in humans. These include: Bacterial Diseases in Humans Causative Agent Pulmonary Tuberculosis Mycobacterium tuberculosis Diptheria Corynebacterium diptheriae Cholera Vibrio cholerae Leprosy Mycobacterium leprae Pertussis Bordetella pertussis Tetanus Clostridium tetani Plague Yersinia pestis Gonorrhoea Neisseria gonorrhoeae Salmonellosis Salmonella enteritis 10