Gram-Negative Bacteria

7 Key excerpts on "Gram-Negative Bacteria"

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  Ocular Infection

  Investigation and Treatment in Practice

  • David V. Seal, Uwe Pleyer(Authors)
  • 2007(Publication Date)
  • CRC Press

    (Publisher)

  TWO Microbiology BACTERIOLOGY Cell Wall Bacteria are divided into Gram-positive and Gram-negative groups based on their cell wall structure. The bacterial cell wall is a rigid structure surrounding a flexible cell membrane. The cell wall maintains the shape of the cell: its rigid wall compensates for the innate flexibility of the phospholipid membrane and also maintains the cell’s integrity when the intracellular osmotic gradient is unfavorable. The wall is also responsible for attachment: teichoicacids attached to the outer surface of the wall serve as attachment sites for bacteriophages. Flagella, fimbriae, and pili all emanate from the wall. The composition of the Gram-positive cell wall is 90% peptidoglycan polymer made of alternating sequences of N-acetylglucosamine (NAG) and N-acetyl-muraminic acid (NAMA) with each layer cross-linked by an amino acid bridge. The peptidoglycan polymer imparts thickness to the Gram-positive cell wall. In contrast, peptidoglycan makes up only 20% of the Gram-negative cell wall. Periplasmic space and an outer membrane also diffe]rentiate the Gram-negative organism and contain proteins that destroy potentially dangerous foreign matter. The outer membrane, composed of lipid, protein, and lipopolysaccharide (LPS), is porous because porin proteins allow free passage of small molecules. The lipid portion of LPS also contains lipid A, a toxic substance, which imparts the pathogenic virulence associated with some Gram-Negative Bacteria. Gram stain was the innovation of Hans Christian Joaquim Gram, a Danish physicist, who sought to distinguish bacterial organisms based on their different cell wall structures. The crystal violet primary stain in Gram stain preferentially binds peptidoglycan. Because the cell wall of Gram-Negative Bacteria is low in peptidoglycan content and high in lipid content, the primary crystal violet stain is washed out when the decolorizer (acetone) is added.

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  Medical Microbiology and Infection

  • Tom Elliott, Anna Casey, Peter A. Lambert, Jonathan Sandoe(Authors)
  • 2012(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Part 1 Basic Microbiology Passage contains an image Chapter 1 Basic Bacteriology Peter Lambert Aston University, Birmingham, UK

  Bacterial Structure

  Bacteria are single-celled prokaryotic microorganisms, and their DNA is not contained within a separate nucleus as in eukaryotic cells. They are approximately 0.1–10.0 μm in size (Figure 1.1 ) and exist in various shapes, including spheres (cocci), curves, spirals and rods (bacilli) (Figure 1.2 ). These characteristic shapes are used to classify and identify bacteria. The appearance of bacteria following the Gram stain is also used for identification. Bacteria which stain purple/blue are termed Gram-positive, whereas those that stain pink/red are termed Gram-negative. This difference in response to the Gram stain results from the composition of the cell envelope (wall) (Figure 1.3 ), which are described below.

  Figure 1.1 Shape and size of some clinically important bacteria.

  Figure 1.2 Some bacterial shapes.

  Figure 1.3 A section of a typical bacterial cell.

  Cell Envelope

  Cytoplasmic Membrane

  A cytoplasmic membrane surrounds the cytoplasm of all bacterial cells and are composed of protein and phospholipid; they resemble the membrane surrounding mammalian (eukaryotic) cells but lack sterols. The phospholipids form a bilayer into which proteins are embedded, some spanning the membrane. The membrane carries out many functions, including the synthesis and export of cell-wall components, respiration, secretion of extracellular enzymes and toxins, and the uptake of nutrients by active transport mechanisms.

  Mesosomes

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  Microbial Food Safety

  A Food Systems Approach

  • Charlene Wolf-Hall, William Nganje(Authors)
  • 2017(Publication Date)
  • CAB International

    (Publisher)

  6 Gram-Positive Bacteria

  Key Questions

  •   Which Gram-positive bacteria are of most concern for microbial food safety?

  •   What are the mechanisms by which these Gram-positive bacteria cause illness?

  •   What are the hazards these Gram-positive bacteria present to consumers?

  •   What controls are available to prevent foodborne illness due to these Gram-positive bacteria?

  The Difference Between Gram-Positive and Gram-Negative Bacteria

  Hans Christian Gram was the Danish scientist who, in 1884, published a method for a staining technique to help better see bacteria in tissue samples under the microscope. An unanticipated result of this technique was a way to differentiate two major groups of bacteria based on their cell wall compositions.

  Bacterial cell membranes that contain thick layers of peptidoglycan are able to retain the crystal violet stain used in the method, resulting in purple- or violet-stained cells that are described as Gram positive. Bacteria that contain less peptidoglycan in their cell membranes are unable to retain the crystal violet stain after the destaining step of the procedure, and as a result of counter-staining with safranin dye appear red or pink, and are described as Gram negative. As with all microbiological testing methods, there are limitations and some bacterial species may produce Gram-variable results, indicating an ability to stain with either reaction result and not provide a clear distinctive result.

  Gram staining is a preliminary test used on bacterial cultures to give clues to the identity of the species. The Gram reaction is a first clue, and then other clues like the microscopic cell morphology or shape and placement can provide other clues. The remainder of this chapter will focus on those bacterial pathogens of most concern in foods that fall under the category of Gram positive, and Chapter 7

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  Veterinary Microbiology and Microbial Disease

  • P. J. Quinn, B. K. Markey, F. C. Leonard, P. Hartigan, S. Fanning, E. S. Fitzpatrick(Authors)
  • 2011(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Also known as fimbria (plural, fimbriae). Thin, straight, thread-like structures present on many Gram-Negative Bacteria. Two types exist, attachment pili and conjugation pili Chromosome DNA Single circular structure without nuclear membrane Ribosome RNA and protein Involved in protein synthesis Storage granules or inclusions Chemical composition variable Present in some bacterial cells; may be composed of polyphosphate (volutin or metachromatic granules), poly-beta-hydroxybutyrate (reserve energy source), glycogen

  The outer membrane of Gram-Negative Bacteria is a protein-containing asymmetrical lipid bilayer. The structure of the inner surface of the membrane resembles that of the cytoplasmic membrane, whereas that of the outer surface is composed of lipopolysaccharide (LPS) molecules. Low molecular weight substances such as sugars and amino acids enter through specialized protein channels, known as porins, in the outer membrane. The outer membrane LPS, the endotoxin of Gram-Negative Bacteria, is released only after cell lysis. The major components of LPS molecules are core polysaccharides bound to lipid A and long external polysaccharide side chains. The polysaccharide side chains of the LPS molecules stimulate antibody production and correspond to the somatic (O) antigens used for serotyping of Gram-negative cells. Lipid A is the molecular component in which endotoxic activity resides. On account of its composition, the outer membrane excludes hydrophobic molecules and renders Gram-Negative Bacteria resistant to some detergents which are lethal to most Gram-positive bacteria. Comparative features of the cell walls of Gram-positive and Gram-Negative Bacteria are illustrated in Fig. 7.3 .

  The mycoplasmas comprise an important group of bacteria without cell walls. Conventional bacteria, exposed to the action of antibiotics such as penicillin, or other substances which interfere with the synthesis of peptidoglycan, cannot produce cell walls and are termed L forms.

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  Infection Management for Geriatrics in Long-Term Care Facilities

  • Thomas T. Yoshikawa, Joseph G. Ouslander, Thomas T. Yoshikawa, Joseph G. Ouslander(Authors)
  • 2006(Publication Date)
  • CRC Press

    (Publisher)

  25 Gram-Negative Bacteria Vinod K. Dhawan Division of Infectious Diseases, Department of Internal Medicine, Charles R. Drew University of Medicine and Science, Martin Luther King, Jr.–Charles R. Drew Medical Center, and UCLA School of Medicine, Los Angeles, California, U.S.A. KEY POINTS 1. Gram-Negative Bacteria are common causes of infection among the residents of long-term care facilities. Emergence of antimicro- bial resistance in the Gram-Negative Bacteria has been a growing problem in nursing homes. Bacterial resistance to antimicrobial agents negatively impacts the outcome of infections. 2. Mechanisms of resistance to antibiotics in gram-negative bacilli include the production of b-lactamase by the organism, decreased membrane permeability, and the efflux mechanisms that pump the antimicrobial agent out of the cell. 3. Risk factors for acquisition of gram-negative bacilli in the elderly include poor functional status; increased patient morbidity; prior antibiotic use; presence of wounds, foreign bodies, and percuta- neous devices; mechanical ventilation; emergency abdominal surgery; and longer hospital stay. 4. Appropriate therapy of infections due to antibiotic resistance gram-negative bacilli is critical to patient survival. Antimicrobial therapy should take into account the prevalent resistance patterns of organisms and the antimicrobial susceptibility data. Carbape- nems are highly effective in the treatment of bacteria containing extended-spectrum b-lactamases. Trimethoprim–sulfamethoxazole 427 is the treatment of choice for infections due to Stenotrophomonas maltophilia. 5. Measures for control of antibiotic resistance in the long-term care facilities include appropriate planning for identifying, isolating, and treating patients. Better staffing, hand washing, and optimal use of antimicrobials are important components of such a strategy.

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  Complement Infectious Diseases

  • Douglas P. Fine(Author)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  Many bacteria can be segregated on the basis of the Gram stain. Gram-positive organisms retain the dye crystal violet; in contrast, the dye is washed out of Gramnegative bacteria by acetone-alcohol. This seemingly trivial difference reflects profound differences in biological behavior between Gram-positive and Gram-Negative Bacteria. To a great extent, these differences are due to the structure of the respective cell walls.

  1. Gram-Positive Bacteria

  Cell walls of Gram-positive bacteria are thick, poorly defined structures, comprised mostly of peptidoglycan. The nonpeptidoglycan portion includes a wide variety of polysaccharides and teichoic acids, which are covalently bound to the peptidoglycan.5

  Teichoic acid is the collective name for water-soluble polymers of ribitol or glycerol phosphate linked by phosphodiester bonds. Variability among teichoic acids is conferred by side chains. Glycerol teichoic acids are found intracellularly associated with plasma membranes of all Gram-positive bacteria as well as in the cell walls. Ribitol teichoic acids are limited to cell walls. Though apparently not critical to structural integrity of the cell wall, the teichoic acids may be important in ion transport. Because the teichoic acids are highly antigenic, they contribute greatly to serological differences among Gram-positive bacteria.7

  Table 1 AN ORGANIZED LIST OF BACTERIAL GENERA PERTINENT TO THIS MONOGRAPH1

  2. Gram-Negative Bacteria

  Whereas the cell walls of Gram-positive bacteria are somewhat amorphous and composed largely of peptidoglycan, the cell walls of Gram-Negative Bacteria are highly structured and peptidoglycan is a relatively minor constituent. A thin peptidoglycan layer lies just outside the plasma membrane; the two may be linked in some way. Outside and covalently linked to the peptidoglycan are two layers of lipopolysaccharides and lipoproteins. These three layers and the intervening spaces can be readily demonstrated by electron microscopy.5

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  Microbiology and Molecular Diagnosis in Pathology

  A Comprehensive Review for Board Preparation, Certification and Clinical Practice

  • Audrey Wanger, Violeta Chavez, Richard Huang, Amer Wahed, Amitava Dasgupta, Jeffrey K. Actor(Authors)
  • 2017(Publication Date)
  • Elsevier

    (Publisher)

  Chapter 6

  Overview of Bacteria

  Abstract

  This chapter will deal with all common Gram-positive and Gram-Negative Bacteria, as well as Enterobacteriaceae, anaerobes, and mycobacteria and their role in infective patients. Drugs which are used in treating such infections will also be addressed.

  Keywords

  Bacterial infections; Gram positive; Gram negative; Enterobacteriaceae; anaerobes; mycobacteria

  Contents

  Introduction  76

  Diagnostic Approach  77

  Gram-Positive Bacteria  77

  Coagulase positive Staphylococcus  

  78

  Staphylococcus species Not Staphylococcus aureus  

  78

  Streptococci  79

  Mutans group (e.g., Streptococcus mutans ) 

  83

  Salivarius group (e.g., Streptococcus salivarius ) 

  83

  Anginosus group (e.g., Streptococcus anginosis ) 

  83

  Catalase negative gram-positive cocci  83

  Vancomycin resistant gram-positive cocci  84

  Gram-Positive Bacilli  84

  Gram positive, endospore-forming bacilli  87

  Aerobic gram positive, nonendospore-forming bacilli  87

  Weakly acid fast bacteria  88

  Nocardia spp. 

  89

  Streptomyces  

  89

  Rhodococcus spp. 

  89

  Mycobacteria spp. 

  89

  Nontuberculous mycobacteria  91

  Photochromogens  91

  Scotochromogens  92

  Nonphotochromogens  92

  Rapid growers—Mycobacterium fortuitum , Mycobacterium chelonae/Mycobacterium abscessus, Mycobacterium mucogenicum  

  92

  Skin mycobacteria—Mycobacterium marinum, Mycobacterium haemophilum , and Mycobacterium ulcerans  

  93

  Gram-Negative Bacteria  93

  Lactose fermenters  95

  Slow/nonlactose fermenters  96

  Nonenterobacteriaceae  97

  Neisseria spp. 

  98

  Gram-Negative Bacilli  99

  Pseudomonas aeruginosa  

  99

  Burkholderia spp. 

  99

  Vibrio  

  100

  Aeromonas  

  100

  Plesiomonas shigelloides  

  100

  Fastidious gram-negative rods  101

  HACEK organisms: Aggregatibacter

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