Lecture 1 - Bacterial Classification PDF

Microbial World and You; Bacterial Classification and Structure Natia Tamarashvili, PhD  What is Microbiology Microbiology – study of microorganisms (simple forms of life visible only with a microscope) Microorganisms may be Normal flora...

Microbial World and You; Bacterial Classification and Structure Natia Tamarashvili, PhD  What is Microbiology Microbiology – study of microorganisms (simple forms of life visible only with a microscope) Microorganisms may be Normal flora Pathogenic There are > 5 x 1030 types Microbes in the world Humans have intimate relation with Microbes > 90% of the cells in our Body are Microbes Nearly 2,000 different microbes cause diseases 10 B infections/year worldwide 13 M deaths from infections/year worldwide Microorganisms Nomenclature Non-cellular organism Virus Prokaryotes Bacterium Eukaryotes Fungi Others Prions Viroid Characteristics of microbes  Structur e The eukaryotic cell has: ▪ a true nucleus with multiple chromosomes surrounded by a nuclear membrane ▪ and uses a mitotic apparatus to ensure equal allocation of the chromosomes to progeny cells. The nucleoid of a prokaryotic cell consists of ▪ a single circular molecule of loosely organized DNA, ▪ lacking a nuclear membrane and mitotic apparatus Viruses have an inner core of genetic material (either DNA or RNA) but no cytoplasm, ▪ they depend on host cells to provide the machinery for protein synthesis and energy generation. Eukaryotes Vs. Prokaryotes Eukaryotic cells contain organelles, such as mitochondria and lysosomes, and larger (80S) ribosomes, whereas prokaryotes contain no organelles and smaller (70S) ribosomes. Most prokaryotes have a rigid external cell wall that contains peptidoglycan, a polymer of amino acids and sugars, as its unique structural component. Eukaryotes on the other hand, do not contain peptidoglycan. Either they are bound by a flexible cell membrane, or, in the case of fungi, they have a rigid cell wall with chitin, a homopolymer of N-acetylglucosamine, typically forming the framework. The eukaryotic cell membrane contains sterols, whereas no prokaryote, except the wall-less Mycoplasma, has sterols in its membranes. Method of replication Prokaryotic cells (e.g., bacteria) replicate by binary fission whereas eukaryotic cells replicate by mitosis. In contrast, viruses disassemble, produce many copies of their nucleic acid and protein, and then reassemble into multiple progeny viruses. Reproductio n Prokaryotic cell division is binary fission. Single DNA molecule that first replicates. Attaches each copy to a different part of the cell membrane. Cell begins to pull apart. There are then two cells of identical genetic composition. Structure of Bacterial Cells Shape & Size of Bacteria Structure of Bacteria Cell Wall Cytoplasmic Membrane Cytoplasm Structures Outside the Cell Wall Bacterial Spores Shape & Size of Bacteria Bacteria are classified by 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 microscopic appearance of a bacterium is one of the most important criteria used in its identification. The arrangement of bacteria certain cocci occur in pairs (diplococci) some in chains (streptococci), and others in grapelike clusters (staphylococci). These arrangements are determined by the orientation and degree of attachment of the bacteria at the time of cell division. SHAPE & SIZE OF BACTERIA 22 Sizes of representative bacteria, viruses, yeasts, protozoa, and human red cells STRUCTURE OF BACTERIA  Cell wall Peptidoglycan N acetyl glucosamine & N acetyl Muramic acid Protect the cell from osmotic changes Rigidity Multilayered in Gram positive Mono to bi layered in Gram negative Peptid o glycan N acetyl glucosamine (NAG) & N acetyl muramic acid (NAM). Peptidoglyca n 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, cephalosporins, and vancomycin, inhibit the synthesis of peptidoglycan by inhibiting the transpeptidase that makes the cross-links between the two adjacent tetrapeptides. Cell Walls of Gram-Positive and Gram-Negative Bacteria Cell Walls of Gram-Positive and Gram-Negative Bacteria  GRAM STAIN Gram stain is the most important staining procedure. Gram positive bacteria stain purple, whereas gram-negative bacteria stain pink. This difference is due to the ability of gram-positive bacteria to retain the crystal violet–iodine complex in the presence of a lipid solvent, usually acetone–alcohol. 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.  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. The Gram stain Note that it takes approximately 100,000 bacteria/mL to see 1 bacterium per microscopic field using the oil immersion (100×) lens. So the sensitivity of the Gram stain procedure is low. This explains why a patient’s blood is rarely stained immediately but rather is incubated in blood cultures overnight to allow the bacteria to multiply. One important exception to this is meningococcemia in which very high concentrations of Neisseria meningitidis can occur in the blood. Medically Important Bacteria That Cannot Be Seen in the Gram Stain  Special components of Gram positive cell wall Teichoic Acid Polymers containing ribitolor glycerol residues there are 2 types of teichoic acids a) wall teichoic acid b) membrane teichoic acid (lipoteichoic acid) linked to membrane glycolipid The medical importance of teichoic acids lies in their ability to induce septic shock when caused by certain gram-positive bacteria; that is, 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  The periplasmic space The space between the inner and outer membranes, called the periplasmic space Contains: the peptidoglycan layer and a gel-like solution of proteins. The periplasmic space  Cytoplasmic Membrane Lipid bi layer Selective permeability Site of ATP production Cytoplas m 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. Structures inside Cytoplasm Ribosomes Granules Nucleoid Plasmids Transposons  The cytoplasm of a prokaryote contains numerous 70s ribosomes; ribosomes consist of rRNA and protein. Protein synthesis occurs at ribosomes; it can be inhibited by certain antibiotics. The difference between prokaryotic (70s) and eukaryotic (80s) ribosomes allows antibiotics to selectively target the prokaryotic ribosomes while sparing eukaryotic ribosomes. Dr. Dalia Mohsen Associate prof. of microbiology Nucleoi d The nucleoid is the area of the cytoplasm in which DNA is located. The DNA of prokaryotes is a single, circular molecule that contains about 2000 genes. One major difference between bacterial DNA and eukaryotic DNA is that bacterial DNA has no introns, whereas eukaryotic DNA does. Plasmid s Plasmids are extrachromosomal, double-stranded, circular DNA molecules that are capable of replicating independently of the bacterial chromosome (1) Transmissible plasmids can be transferred from cell to cell by conjugation. (2) Nontransmissible plasmids do not contain the transfer genes; 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, the active component of some antiseptics and silver, which is mediated by a reductase enzyme. (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  Structures Outside the Cell Wall Capsule - a gelatinous layer covering the entire bacterium. It is composed of polysaccharide 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. 4) The capsule may play a role in the adherence of bacteria to human tissues. Capsules Streptococcus pneumoniae Klebsiella pneumoniae Haemophilus influenzae Bacillus anthracis Streptococcus mutans K. pneumoniae  Flagell aLong (3 to 12 µm), filamentous surface appendages Organs of locomotion Chemically, composed of proteins called flagellins The number and distribution of flagella on the bacterial surface are characteristic for a given species - hence are useful in identifying and classifying bacteria Flagella may serve as antigenic determinants (e.g. the H antigens of Gram-negative enteric bacteria)  Pili (Fimbriae) Pili are hair like filaments that extend from the cell surface. 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. (2) A specialized kind of pilus, the sex pilus, forms the attachment between the male (donor) and the female (recipient) bacteria during conjugation. Glycocalyx (Slime Layer) The glycocalyx is a polysaccharide “slime layer” 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 biofilms.  Biofilm s the attachment of bacteria to the cell surface. Production of biofilm is initiated after Biofilm is polysacchride and protein in nature and protect bacteria from both antibiotics and host immune defenses such as antibodies and neutrophils.  Additional Spores Organelles Highly resistant resting stages formed during adverse environment (depletion of nutrients). Formed inside the parent cell, hence called Endospores. Very resistant to heat, radiation and drying and can remain dormant for hundreds of years. Formed by bacteria like Clostridia, bacillus. Keratin like coat is responsible for the remarkable resistance of the spore to heat, dehydration,  Identification of Bacteria Morphology Biochemical properties References Gerard J. Tortora, Berdell R. Funke, Christine L. Case - Microbiology_ an introduction-Pearson (2018)