Medical Microbiology Dr. Oruba Lec. No.1

Medical Microbiology is the study of microorganisms: Bacteria Fungus Parasites Viruses Most can only be seen with the microscope!

Bacteriology Bacteriology means the study of bacteria . Bacteria are prokaryotic cells (unicellular organisms with no a nucleus) with simple structures that typically range in size from about 0.5 to 20 micrometers.

Bacteria Characteristics The bacterium cell is a prokaryotic cell. Bacterium cell are very small (approximately 0.1to 10.0 μm) Bacteria are widely distributed. It found in soil, air, water, and living bodies. Some bacteria cause diseases for animals and plants. Some bacteria live in our bodies as normal flora which harmless.

Eukaryotic Eukaryotic cells characteristics: Have nuclear envelope and cellular organelles. Have nucleolus. Have complex enzyme system. Reproduce by simple and complex processes. All the living cells are eukaryotic cells except that of bacteria.

Bacterial Structure

General Bacterial Cell Structure

Important Chemical Components of Surface Structures; Cell Wall Peptidoglycans: Unique features of almost all prokaryotic cells (except for Halobacterium halobium and mycoplasmas) Both Gram-positive and Gram-negative bacteria possess cell wall peptidoglycans, which confer the characteristic cell shape and provide the cell with mechanical protection. Peptidoglycans are unique to prokaryotic organisms and consist of a glycan backbone of muramic acid and glucosamine (both N-acetylated), and peptide chains highly cross-linked with bridges in Gram-positive bacteria (e.g., Staphylococcus aureus) or partially cross-linked in Gram-negative bacteria (e.g. E. coli). The cross-linking transpeptidase enzymes are some of the targets for b-lactam antibiotics.

Diagrammatic representation of peptidoglycan structures with adjacent glycan strands cross-linked directly from the carboxyterminal D-alanine to the e-amino group of an adjacent tetrapeptide or through a peptide cross bridge ,N-acetylmuramic acid; N-acetylglucosamine. The number of the amino acids, their types and their way of attachment represent the type of the peptydoglycan. There are about 100 types of peptydoglycan known.

The ß-1,4 glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine is specifically cleaved by the bacteriolytic enzyme lysozyme. Widely distributed in nature, this enzyme is present in human tissues and secretions and can cause complete digestion of the peptidoglycan walls of sensitive organisms. When lysozyme is allowed to digest the cell wall of Gram-positive bacteria suspended in an osmotic stabilizer (such as20% sucrose), protoplasts are formed. These protoplasts are able to survive and continue to grow on suitable media in the wall-less state. Gram-negative bacteria treated similarly produce spheroplasts, which retain much of the outer membrane structure. The dependence of bacterial shape on the peptidoglycan is shown by the transformation of rod-shaped bacteria to spherical protoplasts (spheroplasts) after enzymatic breakdown of the peptidoglycan. The mechanical protection afforded by the wall peptidoglycan layer is evident in the osmotic fragility of both protoplasts and spheroplasts

Comparison of the thick cell wall of Gram-positive bacteria with the comparatively thin cell wall of Gram-negative bacteria. Note the complexity of the Gram-negative cell envelope (outer membrane, its hydrophobic lipoprotein anchor; periplasmic space).

Gram Reactions Gram-positive Gram-positive bacteria have a relatively thick layer of Peptidoglycan and can retain a crystal violet-iodine complex when treated with acetone or alcohol, and appear blue-black or purple when stained using Gram's method.

Gram Reactions (Cont’d) Gram-negative Gram-negative bacteria have a thin layer of Peptidoglycan and cannot retain the crystal violet-iodine complex.

Bacteria Shapes There are three basic shapes: Coccus: An average coccus is about 0.5-1.0 micrometer (µm) in diameter. Rod or bacillus: An average bacillus is 0.5-1.0 µm wide by 1.0-4.0 µm long. Spiral: Spirals range in size from 1 µm to over 100 µm in length.

Bacteria Shapes Spiral Coccus Rod

Teichoic Acids: Teichoic acids are polyol phosphate polymers bearing a strong negative charge. They are covalently linked to the peptidoglycan in some Gram-positive bacteria. They are strongly antigenic, but are generally absent in Gram-negative bacteria. Lipoteichoic Acids: Lipoteichoic acids as membrane teichoic acids that are considered as bridges connecting cell membrane to cell wall in Gram positive bacteria. They are antigenic and cytotoxic. Lipopolysaccharides: A characteristic feature of Gram-negative bacteria is the possession of various types of complex macromolecular lipopolysaccharide (LPS).The LPS of all Gram-negative species are also called endotoxins, thereby distinguishing these cell-bound, heat-stable toxins from heat-labile, protein exotoxins secreted into culture media. Endotoxins possess an array of powerful biologic activities and play an important role in the pathogenesis of many Gram-negative bacterial infections. In addition to causing endotoxic shock, LPS is pyrogenic, can activate macrophages and complement, it is mitogenic for B lymphocytes, induces interferon production, causes tissue necrosis and tumor regression. It is a complex molecule; consists of 1- lipid A anchor, 2- polysaccharide core, and 3- chains of carbohydrates (mainly penta sugars). The number, types and the way of attachment of these sugars specifies the type of O-antigen

Periplasmic space: This is a layer found between the cell membrane and the cell wall, other authers say it lays between the peptydoglycan layer and the outer membrane layer of the gram negative bacterial cell wall, which plays a very important role in maintaining cell’s osmotic pressure, and also as reservoir for digestive enzymes before their secretion to the outside of the cell. Cytoplasmic Structures; Plasma Membrane: The bacterial plasma membrane is composed primarily of protein and phospholipid (about 3:1). It performs many functions, including transport, biosynthesis, and energy transduction.  

References: 1- Jawetz, Melnick, & Adelberg’s.( 2013). Medical Microbiology (Twenty-Sixth Edition). 2- Kenneth Todar. (2008).Todar’s Online Textbook of Bacteriology ,University of Wisconsin.