microbiology cheat sheet, Cheat Sheet of Microbiology

Download microbiology cheat sheet and more Cheat Sheet Microbiology in PDF only on Docsity! For more study tools, check out www.biochem4boards.com Microbiology Cheat Sheet Warning: Don’t go into the exam without knowing these facts! Some may have zero implication on your future as a dentist, but for whatever reason, these 20 tidbits tend to show up on the exam! Obligate anaerobic microorganisms in the oral cavity are normal flora and opportunistic. Respiratory syncytial virus (RSV) infection is different from influenza because RSV causes disease mostly in infants. Aflatoxin is made by Aspergillus. Facultative streptococci are the most numerous group of microorganisms in the oral cavity. Histoplama capsulatum is a fungus that causes systemic disease, mostly in the lungs, and is characterized by its production of tuberculate chlamydospores in culture. Along with Neisseria meningitidis, Strep pneumoniae can cause meningitis. Shigella are the causative microorganism of bacillary dysentery. Most bacterial endotoxins are made up of lipoprotein-polysaccharide complexes. Rubella and toxoplasmosis both have teratogenic potential. Chronic, bilateral ulcerations at the corners of the mouth are usually linked to Candida. Clostridium perfringens produce lecithinases. Coxsackievirus causes the oral lesions of herpangina and hand-foot-and-mouth disease. Epstein-Barr virus is associated with Burkitt’s lymphoma and nasopharyngeal carcinoma. For most people, the initial infection with herpes simplex virus cause subclinical disease. The microorganisms in a histoplasmosis infection reside in reticuloendothelial cells. Behcet’s causes oral, ocular, and genital lesions. Rhinovirus doesn’t cause a persistent infection. HIV can’t infect CD8 lymphocytes. The different types of Strep pneumoniae are distinguished by different capsular polysaccharides. Staph aureus is the most common cause of osteomyelitis. MID 1 Author: Frank Lowy Bacterial Classification, Structure and Function Introduction The purpose of this lecture is to introduce you to terminology used in microbiology. The lecture will: 1. Cover different classification schemes for grouping bacteria, especially the use of the Gram stain 2. Describe the different types of bacteria 3. Discuss bacterial structure and the function of the different bacterial components 4. Discuss the distinguishing characteristics of Gram positive and Gram negative bacteria. For this lecture you should focus on the major concepts and not on the names of the different bacteria. They are mentioned as illustrations of different principles. You will see them all again as the course progresses. Classification Systems The classification of bacteria serves a variety of different functions. Because of this variety, bacteria may be grouped using many different typing schemes. The critical feature for all these classification systems is an organism identified by one individual (scientist, clinician, epidemiologist), is recognized as the same organism by another individual. At present the typing schemes used by clinicians and clinical microbiologists rely on phenotypic typing schemes. These schemes utilize the bacterial morphology and staining properties of the organism, as well as O2 growth requirements of the species combined with a variety of biochemical tests. For clinicians, the environmental reservoir of the organism, the vectors and means of transmission of the pathogen are also of great importance. The classification schemes most commonly used by clinicians and clinical microbiologists are discussed below. Scientists interested in the evolution of microorganisms are more interested in taxonomic techniques that allow for the comparison of highly conserved genes among different species. As a result of these comparisons a phylogenetic tree can be developed that displays the degree of relatedness of different organisms. A relatively new application of this technology has been the recognition and characterization of noncultivatable pathogens and the diseases that they cause. Phenotypic classification systems: There is a chart at the end of these lecture notes on the general phenotypic classification of many of the clinically important bacteria. This is provided as a reference. By the end of the course you will be able to recognize most of these microorganisms. Gram stain and bacterial morphology: Of all the different classification systems, the Gram stain has withstood the test of time. Discovered by H.C. Gram in 1884 it remains an important and useful technique to this day. It allows a large proportion of clinically important bacteria to be classified as either Gram positive or negative based on their morphology and differential staining properties. Slides are sequentially stained with crystal violet, iodine, then destained with alcohol and counter-stained with safranin. Gram positive bacteria stain blue-purple and Gram negative bacteria stain red. The difference between the two groups is believed to be due to a much larger peptidoglycan (cell wall) in Gram positives. As a result the iodine and crystal violet precipitate in the MID 1 2) Capsule: This polysaccharide outer coating of the bacterial surface often plays a role in preventing phagocytosis of bacteria. 3) Peptidoglycan (cell wall) Provides bacterial shape and rigidity. The cell wall consists of alternating units of N-acetylglucosamine and N-acetylmuramic acid. The polysaccharide chains are cross-linked by a peptide bridge. It is a primary target of antimicrobial therapy – because it is specific to prokaryotes. Assembly of the peptidoglycan: This is a critical step for bacterial survival. The sequence of events is outlined below. i. Synthesis begins with formation of a water soluble, nucleotide-linked precursor (N-acetylmuramic acid - NAM) also carrying a pentapeptide in the cytoplasm. ii. The precursor is then linked to a lipid-like carrier in the cell membrane (bactoprenol) and N-acetyl glucosamine (NAG) is added to the NAM. This complex is mobilized across the cytoplasm iii. The disaccharide subunit (NAM-NAG) is then added to the end of a glycan strand. iv. The final step is the transpeptidation reaction catalyzed by a transpeptidase enzyme (also called penicillin binding proteins) that crosslinks the growing strand with others. 4) Cytoplasmic membrane: This is a phospholipid bilayer that assumes many of the functions of eukaryotic organelles such as the biosynthetic processes. 5) Flagella: These provide bacteria with the capacity for locomotion. They vary in number and location. 6) Pili: These structures project from the cell surface enabling bacteria to adhere to host tissue surfaces. Based on their amino acid structure their affinity for particular host tissue surfaces can be remarkably specific. 6) Secreted products: There are a variety of these products including exotoxins that are proteins grouped into A-B toxins (such as those elaborated by vibrio, the cause of cholera), membrane damaging toxins (e.g., hemolysins) and hydrolytic enzymes capable of destroying host tissues and extracellular matrices. Distinguishing Features between Gram Positive and Negative Bacteria Gram positive bacteria have a large peptidoglycan structure. As noted above, this accounts for the differential staining with Gram stain. Some Gram positive bacteria are also capable of forming spores under stressful environmental conditions such as when there is limited availability of carbon and nitrogen. Spores therefore allow bacteria to survive exposure to extreme conditions and can lead to re-infection (e.g., pseudomembranous colitis from Clostridium difficle) Gram negative bacteria have a small peptidoglycan layer but have an additional membrane, the outer cytoplasmic membrane. This creates an additional permeability barrier and results in the need for transport mechanisms across this membrane. A major component of the cytoplasmic membrane that is unique to Gram negatives is endotoxin. This component is essential for bacterial survival. Endotoxin has three components: the lipid A moiety, the highly conserved core polysaccharide, and the species specific O antigen (also polysaccharide). In contrast with the secreted exotoxins, endotoxin is cell-associated but MID 1 can be released during cell division or cell death. The Lipid A moiety of endotoxin is responsible for sepsis which may be fatal. Sepsis is characterized clinically by confusion, fever, drop in blood pressure and ultimately multi-organ failure. Endotoxin (also known as lipopolysaccharide-LPS): MID 1 General Phenotypic Classification of Bacteria Gram Positive Bacteria Name Morphology O2 Require-ments Commens- al Reservoirs / Sites of colonization, Transmission Types of Infections Staphylococci Cocci in grape- like clusters facultative anaerobe Yes Skin, nares / endogenous, direct contact, aerosol Soft tissue, bone, joint, endocarditis, food poisoning Streptococci Cocci in pairs, chains facultative anaerobe Some species Oropharynx, skin / endogenous, direct contact, aerosol Skin, pharyngitis, endocarditis, toxic shock Pneumococci Diplococci, lancet shaped facultative anaerobe ± Oropharynx, sinus / aerosol Pneumonia, otitis, sinusitis, meningitis Enterococci Cocci in pairs, chains facultative anaerobe Yes GI tract / endogenous, direct contact UTI, GI, catheter- related infections Bacilli Rods, spore- forming aerobic ± Soil, air, water, animals / aerosol, contact Anthrax, food poisoning, catheter-related infections Clostridia Rods, spore formers anaerobic Some species GI tract, soil / Breach of skin, endogenous, ingestion Tetanus, diarrhea, gas gangrene, botulism Corynebac- terium Rods, nonspore forming facultative anaerobe Some species Skin Catheter-related infections, diphtheria Listeria Rods, nonspore formers facultative anaerobe No Animals, food products / Ingestion Meningitis Actinomyces Irregular, filamentous, form sulfur granules anaerobic Yes GI tract / endogenous Skin, soft tissue