Medical Microbiology - BIOL 212 PDF

Medical Microbiology Linfeng Huang Associate Professor of Biology BIOL 212 Microbiology  What is “Medical microbiology”? Medical microbiology is the study of the interactions between animals (primarily humans) and microorganisms, such as b...

Medical Microbiology Linfeng Huang Associate Professor of Biology BIOL 212 Microbiology  What is “Medical microbiology”? Medical microbiology is the study of the interactions between animals (primarily humans) and microorganisms, such as bacteria, viruses, fungi, and parasites especially the diseases caused by these interactions. 2  Infection concepts Exposure of an individual to an organism can lead to (1) transiently colonize the person, (2) permanently colonize the person, or (3) produce disease (cause “infection” and damage to the human host). Exogenous infection: Diseases arise when a person is exposed to organisms from external sources. Endogenous infection: Most human diseases are produced by organisms in the person’s own microbial flora that spread to inappropriate body sites where disease can ensue. Nosocomial infections: infections acquired in the hospital. 3 Pathogen types based on propensity for causing disease Strict pathogens: Organisms always associated with human disease. E.g., Mycobacterium tuberculosis which causes tuberculosis (TB). Opportunistic pathogens: Organisms that are typically members of the patient’s normal microbial flora (e.g., Staphylococcus aureus, Escherichia coli, Candida albicans). These organisms do not produce disease in their normal setting but establish disease when they are introduced into unprotected sites (e.g., blood, tissues). If a patient’s immune system is defective, that patient is more susceptible to disease caused by opportunistic pathogens. 4 Bacterial classification by hemolysis Fastidious organisms (species that do not grow easily) requiring a rich media providing many nutrients and growth factors that are largely supplied by blood. 5  Blood agar plate (BAP) Blood agar plates (BAPs) contain enriched medium and mammalian blood (usually sheep or horse), typically at a concentration of 5–10%. Components: 0.5% Peptone 0.3% beef extract/yeast extract 1.5% agar 0.5% NaCl Distilled water pH should be from 7.2 to 7.6 (7.4) Autoclave the dissolved mixture at 121 °C for 15 minutes. Once the nutrient agar has been autoclaved, allow it to cool but not solidify. When the agar has cooled to 45-50 °C, add 5% (vol/vol) sterile defibrinated blood that has been warmed to room temperature and mix gently but well. 6  Chocolate agar Chocolate agar (CHOC) is a type of blood agar plate in which contains red blood cells that have been lysed by slowly heating to 80 °C. It is used for growing fastidious respiratory bacteria, such as Haemophilus influenzae. Haemophilus influenzae www.medical-labs.net 7  Hemolysis – lysis of erythrocytes Hemolyses of Streptococcus spp. (left) α-hemolysis (S. mitis); (middle) β-hemolysis (S. pyogenes); (right) γ-hemolysis = non- hemolytic (S. salivarius) α-hemolysis: incomplete hemolysis; β-hemolysis: activity will show lysis and complete digestion of red blood cell contents surrounding colony; γ-hemolysis: no hemolysis. Blood agar 8 Wikipedia  Skin and soft tissue infections (SSTIs) SSTIs are clinical entities of variable presentation, etiology and severity that involve microbial invasion of the layers of the skin and underlying soft tissues. Soft tissue refers to tissues that connect, support, or surround other structures and organs of the body, including muscles, tendons, ligaments, fascia, nerves, fibrous tissues, fat, blood vessels, and synovial (joint) membranes. 9  The affected area may also become dysfunctional (e.g. hands and legs) depending on the severity of the infection. The most common route is through a break in the barrier. Lacerations, bite wounds, scratches, instrumentation (e.g. needles), pre-existing skin conditions, wounds (e.g. chicken pox or ulcer), burns and surgery are the common mechanisms of compromising the skin barrier. The challenge of SSTIs is to efficiently differentiate those cases that require immediate attention and intervention, whether medical or surgical, from those that are less severe. 10  The structure of skin Epidermis: outermost layers of the skin; comprises keratinocytes (95%) and melanocyte. Dermis: connective tissues (fibers and glycoproteins); contains nerve endings, hair follicles, glands, blood vessels. Hypodemis (subcutaneous tissue): attach the skin to bone and muscle; contain fibroblasts, macrophages and adipocytes (fat). 11 http://www.eurostemcell.org/factsheet/skin-stem-cells-where-do-they-live-and-what-can-they-do Wikipedia http://www.webmd.com/beauty/wrinkles/cosmetic-procedures-overview-skin Natural defense systems of the skin Keratin (any of various sulfur-containing fibrous proteins that form the chemical basis of horny epidermal tissues such as hair and nails). Skin cell shedding and regeneration. Sebum (secreted the oily, waxy substance): low pH, high lipid. Sweat: low pH, high salt, and containing lysozyme, which digests peptidoglycan. Through its own normal microbe flora. 12  Skin normal microbial flora Propionibacterium acnes Staphylococci – Staphylococcus epidermidis – Staphylococcus aureus (in small numbers) Streptococci species Corynebacterium species Peptostreptococcus species Acinetobacter species Small numbers of others (Pseudomonas aeruginosa, Candida species, etc) 13  Streptococcus A diverse collection of gram-positive cocci typically arranged in pairs or chains (in contrast to the clusters formed by Staphylococcus). Most species are facultative anaerobes. Their nutritional requirements are complex, necessitating the use of blood- or serum-enriched media for isolation. Carbohydrates are fermented, resulting in the production of lactic acid and, unlike Staphylococcus species, streptococci are catalase-negative. Divided into two groups: (1) the β-hemolytic streptococci, which are classified by Lancefield grouping (e.g. S. pyogenes and S. agalactiae) and (2) the α-hemolytic and γ-hemolytic streptococci, which are classified by biochemical testing (e.g. S. pneumoniae and S. suis). 14  Streptococcus pyogenes The β-hemolytic streptococci are classified by group- specific cell wall antigens, most of which are carbohydrates - Lancefield groupings A to W. S. pyogenes is a rapidly growing gram-positive cocci arranged in chains; group-specific carbohydrate A antigen (so a GAS) and type-specific proteins (M protein) in cell wall. S. pyogenes, popularly called “flesh-eating” bacteria, results from life-threatening necrotic damage to muscle tissue. Gram stain 15