1- Bacterial structure and bacterial genetics.pdf

Ninevah University Medicine College Microbiology Department 2023 - 2024 Dr. Saba Abdul Salam Hamid Al-Sultan [email protected] P:1  1. Bacterial structure & Bacterial genetics The lectu...

Ninevah University Medicine College Microbiology Department 2023 - 2024 Dr. Saba Abdul Salam Hamid Al-Sultan [email protected] P:1  1. Bacterial structure & Bacterial genetics The lecture objectives (aims) Study the bacterial structure & Bacterial genetics The lecture overview Shape & Size of Bacteria Structure of Bacteria ( Cell Wall , Cytoplasmic Membrane ,Cytoplasm , Structures outside the Cell Wall , Bacterial Spores) Bacterial genetics and Horizontal Gene Transfer in Bacteria P:2  Shape & Size Bacteria have three shapes: cocci (spheres), bacilli (rods), and spirochetes (spirals). 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 microscopic appearance of a bacterium is one of the most important criteria used in its identification Cocci are arranged in three patterns: pairs ( diplococci), chains(streptococci), and clusters (staphylococci). P:3 The arrangement of rods and spirochetes is medically less important and is not described in this lecture P:4  Shape & Size The size most of bacteria ranges from 1 to 3 µm. Mycoplasma, the smallest bacteria ( and therefore the smallest cells), are 0.2 µm. some bacteria, such as Borrelia, are as long as 10 µm; that is, they are longer than a human red blood cell, which is 7 µm in diameter. P:5 P:6  Structure of bacteria Two basic Cell types: 9/20/2023 7 P:7 A bacterial cell shows a typical prokaryotic structure. The cytoplasm is enclosed by three layers, the outermost slime or capsule, the middle cell wall and inner cell membrane. The major cytoplasmic contents are nucleoid, plasmid, ribosome, mesosome etc., and the cell is devoid of endoplasmic reticulum, mitochondria, centrosome and Golgi bodies. P:8  Bacterial 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 peptidoglycan provides structural support and maintains the characteristic shape of the cell. P:9  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 P : 10 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 penicillin’s, cephalosporins, and vancomycin, inhibit the synthesis of peptidoglycan by inhibiting the transpeptidase that makes the cross-links between the two adjacent tetrapeptides. P : 11  Periplasmic s pace, which is the site, in some species, of enzymes called ß lactamases that degrade penicillin's and other ß -lactam drugs. 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. 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. P : 12 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. In view of their importance, three components of the cell wall (i.e., peptidoglycan, lipopolysaccharide, and teichoic acid) are discussed in in this lecture. P : 13  Peptidoglycan Due to differences in the thickness of a peptidoglycan layer in the cell membrane between Gram positive and Gram negative bacteria, Gram positive bacteria (with a thicker peptidoglycan layer) retain crystal violet stain during the decolorization process, while Gram- negative bacteria, because they have an outer lipid-containing membrane and thin peptidoglycan, lose the purple dye when treated with acetone-alcohol. They become colorless and then stain pink when exposed to a red dye such as safranin. Not all bacteria can be visualized using Gram stain. Some important human pathogens, such as the bacteria that cause tuberculosis and syphilis, cannot be seen using this stain P : 14 The Gram stain is useful in two ways: ( 1) In the identification of many bacteria. (2) In influencing the choice of antibiotic because, in general, gram positive bacteria are more susceptible to penicillin G than are gram negative bacteria. Lysozyme: an enzyme present in human tears, mucus, and saliva, can cleave the peptidoglycan backbone by breaking its glycosyl bonds, thereby contributing to the natural resistance of the host to microbial infection P : 15 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. Teichoic Acid Teichoic acids are fibers located in the outer layer of the gram-positive cell wall and extend from it. They are composed of polymers of either glycerol phosphate or ribitol phosphate. The medical importance of teichoic acids lies in their ability to induce septic shock when caused by certain gram-positive bacteria. Teichoic acids also mediate the attachment of staphylococci to mucosal cells. Gram-negative bacteria do not have teichoic acids. P : 16 Cytoplasmic Membrane Just inside the peptidoglycan layer of the cell wall lies the cytoplasmic membrane, which is composed of a phospholipid bilayer. 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, and ( 4) secretion of enzymes and toxins. P : 17 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. 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, whereas eukaryotic ribosomes are 80S in size,. 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. P : 18 Granules The cytoplasm contains several different types of granules that serve as storage areas for nutrients and stain characteristically with certain dyes. For example, Metachromatic granules are a characteristic feature of Corynebacterium diphtheriae, the cause of diphtheria. Nucleoid The nucleoid is the area of the cytoplasm in which DNA is located. The DNA of prokaryotes is a single, circular molecule and contains about 2000 genes. (By contrast, human DNA has approximately 100,000 genes.) Because the nucleoid contains no nuclear membrane, no nucleolus, no mitotic spindle, and no histones, there is little resemblance to the eukaryotic nucleus. One major difference between bacterial DNA and eukaryotic DNA 1s that bacterial DNA has no introns, whereas eukaryotic DNA does. P : 19 Plasmids Plasmids are extra chromosomal, 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 plasmids can exist in one cell: (1) Transmissible plasmids : can be transferred from cell to cell by conjugation. They are large, since they contain about a dozen genes responsible for synthesis of the sex pilus and for the enzymes required for transfer. They are usually present in a few (1-3) copies per cell. (2) Nontransmissible plasmids are small, since they do not contain the transfer genes; they are frequently present in many ( 10-60) copies per cell. P : 20 Plasmids carry the genes for the following functions and structures of medical importance: ( 1) Antibiotic resistance, which is mediated by a variety of enzymes. (2) Resistance to heavy metals, such as mercury, and silver (3) Resistance to ultraviolet light, which is mediated by DNA repair enzymes. ( 4) Pili (fimbriae), which mediate the adherence of bacteria to epithelial cells. (5) Exotoxins, including several enterotoxins. Other plasmid-encoded products of interest are as follows: ( 1) Bacteriocins are toxic proteins produced by certain bacteria that arelethal for other bacteria. (2) Several antibiotics produced by Streptomyces. (3) A variety of degradative enzymes that are produced by Pseudomonas and are capable of cleaning up environmental hazards such as oil spills and toxic chemical waste sites. P : 21 Transposons Transposons are pieces of DNA that move readily from one site to another either within or between the DNAs of bacteria, plasmids, and bacteriophages. Because of their unusual ability to move, they are nicknamed "jumping genes." Some transposons move by replicating their DNA and inserting the new copy into another site (replicative transposition), whereas others are excised from the site without replicating and then inserted into the new site ( direct transposition). Transposons can code for drug-resistant enzymes, toxins, or a variety of metabolic enzymes and can either cause mutations in the gene into which they insert or alter the expression of nearby genes. P : 22 P : 23 Structure Outside the cell wall 1. Capsule The capsule is a gelatinous layer covering the entire bacterium. It is composed of polysaccharide. The sugar components of the polysaccharide vary from one species of bacteria to another and frequently determine the serologic type (serotype) within a species. For example, there are 84 different serotypes of Streptococcus pneumoniae. 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. bacteria that have lost the ability to produce a capsule are usually nonpathogenic. (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. For example, the purified capsular polysaccharides of 23 types of S. pneumoniae are present in the current vaccine. The capsule may play a role in the adherence of bacteria to human tissues, which is an important initial step in causing infection. P : 24 2. Flagella Flagella are the organelles for bacterial locomotion made up of protein sub unit called flagellin. Flagella are medically important for two reasons: (1) Some species of motile bacteria ( e.g., E. coli and Proteus species) are common causes of urinary ·tract infections. Flagella may play a role in pathogenesis by propelling the bacteria up the urethra into the bladder. (2) Some species of bacteria (e.g., Salmonella species) are identified in the clinical laboratory by the use of specific antibodies against flagellar proteins. P : 25 25 3. Pili (Fimbriae) Pili are hairlike filaments that extend from the cell surface. They are shorter and straighter than flagella and are composed of subunits of pilin. They are found mainly on gram-negative organisms. Pili have two important roles: (1) They mediate the attachment of bacteria to specific receptors on the human cell surface, which is a necessary step in the initiation of infection for some organisms. Mutants of Neisseria gonorrhoeae that do not form pili are nonpathogens. (2) A specialized kind of pilus, the sex pilus, forms the attachment between the male ( donor) and the female (recipient) bacteria during conjugation. P : 26 4. Glycocalyx (Slime Layer) The glycocalyx is a polysaccharide coating that is secreted by many bacteria. It covers surfaces like a film and allows the bacteria to adhere firmly to various structures (e.g., skin, heart valves, prosthetic joints, and catheters). The glycocalyx is an important component of biofilm. The medical importance of the glycocalyx is illustrated by the finding that it is the glycocalyx-producing strains of Pseudomonas aeruginosa that cause respiratory tract infections in cystic fibrosis patients, and it is the glycocalyx- producing strains of Staphylococcus epidermidis and viridans streptococci that cause endocarditis. The glycocalyx also mediates adherence of certain bacteria, such as Streptococcus mutans, to the surface of teeth. This plays an important role in the formation of plaque. P : 27 Bacterial Spores These highly resistant structures are formed in response to adverse conditions by two genera of medically important gram-positive rods: the genus Bacillus, which includes the agent of anthrax, and the genus Clostridium, which includes the agents of tetanus and botulism. Spore formation (sporulation) occurs when nutrients, such as sources of carbon and nitrogen, are depleted.The spore forms inside the cell and contains bacterial DNA, a small amount of cytoplasm, cell membrane, peptidoglycan, very little water, and most importantly, a thick, keratin like coat that is responsible for the remarkable resistance of the spore to heat, dehydration, radiation, and chemicals. P : 28  Bacterial Genetics Bacterial genetics deals with the study of heredity and variation seen in bacteria. All hereditary characteristics of the bacteria are encoded in their DNA which is present in chromosome as well in extrachromosomal genetic material as plasmid. Type of Plasmids based on function: I Fertility or F-plasmids 2. Resistance (R) plasmids 3. Col plasmids 4. Virulence plasmids 5. Metabolic Plasmids Plasmid as vector: By their ability to transfer DNA from one cell to another, plasmids , have become important vectors in genetic engineering. Plasmids contain certain sites where genes can be inserted artificially by recombinant DNA technology. Such plasmids can be used for protein production, gene therapy, P : 29 Horizontal Gene Transfer in Bacteria Gene transfer in bacteria can be broadly divided into: Vertical gene transfer (transmission of genes from parents to offspring) Horizontal gene transfer (transmission of genes from one bacterium to another bacterium). This occurs by: 1. Transformation Transformation is the process of random uptake of free or naked DNA fragment from the surrounding medium by a bacterial cell and incorporation of this molecule into its chromosome in a heritable form. It has been studied so far only in certain bacteria: Streptococcus, Bacillus, Haemophilus, Neisseria, Acinetobacter and Pseudomonas. The Griffith experiment (1928) on mice using pneumococci strains provided the direct evidence of transformation. P : 30 Griffith experiment P : 31 2.Transduction Transduction is defined as transmission of a portion of DNA from one bacterium to another by a bacteriophage. Types of transduction 1. Generalized transduction: 2. It involves transfer of any part of the donor bacterial genome into the recipient bacteria. 2. Restricted or specialized transduction: Here, only a particular genetic segment of the bacterial chromosome that is present adjacent to the phage DNA is transduced. Role of transduction In addition to chromosomal DNA, transduction is also a method of transfer of episomes and plasmids. Drug resistance, e.g. plasmid coded penicillin resistance in staphylococci. Treatment: As a method of genetic engineering in the treatment of some inborn metabolic defects. P : 32 Lysogenic Conversion During the temperate or lysogenic life cycle, the phage DNA remains integrated with the bacterial chromosome as prophage. The prophage acts as an additional chromosomal element which encodes for new characters to the daughter cells. P : 33 3. Conjugation Conjugation refers to the transfer of genetic material from one bacterium (donor or male) to another bacterium (recipient or female) by mating with each other and forming the conjugation tube. Conjugation plays an important role in the transfer of plasmids coding for antibacterial drug resistance [ resistance transfer factor (RTF)] and bacteriocin production [Colicinogenic (Col) factor] P : 34 P : 35 Dr. Saba Al-Sultan P : 36