Microbiology Chapter 7

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Staphylococci and other catalase positive gram positive cocci
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1

Staphylococcaceae Family

Staphylococcus, Gemella, Macrococcus, Salinococcus

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Micrococcaceae Family

Micrococcus

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Micrococcus species

Gram positive cocci that appear as tetrads and are usually larger than Staphylococcus. Obligate aerobes and Catalase positive. colonies are yellow

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Staphylococcus genus

Gram positive cocci that appear as clusters or tetrads. Facultative anaerobes and Catalase positive

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Catalase Reaction

Hydrogen peroxide is added to bacterial colony. Rapid bubbling indicates a positive reaction

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Mannitol Salt Agar (MSA)

Contains 7.5%-10% NaCl, mannitol, phenol red indicator. Isolates salt tolerant bacteria like staphylococcus. Yellow indicates mannitol fermentation

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S. aureus

Coagulase is most reliable method of identification. Growth and fermentation on MSA, DNAse positive, and sometimes exhibits beta hemolysis

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DNase agar

Contains DNA and the dyes toluidine blue and methyl green. Activity detected by clearing of the green color when DNA is degraded.

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Coagulase Test

Fibrinogen is converted into a fibrin clot. Performed on both slide (Bound) and in tube (Free)

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Latex agglutination test

Carrier particles coated with human fibrinogen and IgG will react with clumping factor and protein A of S. aureus

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Chromogenic Agars

Isolate and identify staphylococci by inhibiting other bacteria and utilizing chromogens that bind with specific enzymes and form colored compounds.

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HardyCHROM agar

A chromogenic agar that will form deep pink to fuchsia colored colonies if populated with S aureus.

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Specimen Collection

Specimens are collected from wounds, sputum, blood, urine, and abscess and are isolated on Sheep blood agar and Colistin-nalidixic acid (CNA).

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S. aureus pathogenicity

Production of many extracellular toxins and compounds, antibiotic resistance, and ubiquitous. Invades tissues and can disseminate through blood to tissues and organs.

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S.aureus infections

Toxic-Shock syndrome, food-borne illnesses, pneumonia, and septicemia. Causes folliculitis, boils, furuncles, impetigo in skin and soft tissues

16

S. aureus virulence factors

Protein A inhibits complement fixation and phagocytosis, Capsular polysaccharide enables organism to resist phagocytosis. Peptidoglycan and techoic acid permits organism to sustain environmental stress

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S.aureus extracellular toxins (Exotoxins)

Coagulase, Fibrinolysin-Staphylokinase, Lipase, Hyaluronidase, Nucleases, Exfoliatins.

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Coagulase

Fibrin formation, antiphagocytic property

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Fibrinolysi-Staphylkinase

Lyses and dissolves fibrin clots; enables infections to spread

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Lipase

Hydrolyzes lipids in plasma and skin; allows for colonization and initiation of skin infections

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Hyaluronidase

Lyse hyaluronic acid and spreads infection

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Nucleases

Deoxyribonuclease (DNase); degrades DNA

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Exfoliatins

Hydrolyze tissue; Staphylococcal Scalded Skin Syndrome

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S. aureus superantigens

Enterotoxins and other pyrogenic toxin such as toxic-shock syndrome toxin 1 (TSST-1); promote cytokine release and progression of toxic shock syndrome

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S. aureus antibiotic resistance

Beta lactamase (penicillinase), methicillin resistance, and vancomycin resistance

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Beta lactamase (penicillinase)

Enzyme that provides resistance to penicillin and other beta lactam antibiotics. Can be detected with iodometric, acidometric, or chromogenic assays

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Methicillin Resistant S. aureus (MRSA)

Isolated with Cefoxitin screen tests, chromogenic agars, and latex agglutination for PBP2a

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Coagulase negative Staph (CoNS) species

S. epidermidis, S. saprophyticus, S. hominis, S. hemolyticus

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Coagulase negative Staph (CoNS) characteristics

Catalase positive, GPC clusters, growth on Mannitol Salt Agar, but do not ferment mannitol, DNase negative, and coagulase negative

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S. epidermidis

Normal flora on skin and mucous membrane. Increasingly being found as and opportunistic pathogen in health care associated infections

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S. epidermidis characteristics

Gram-positive cocci in clusters, nonhemolytic, positive growth on CNA, Growth on MSA but no fermentation, coagulase negative, DNase negative, and susceptible to novobiocin

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S. saprophyticus

Associated with urinary tract infections (UTI), resistant to novobiocin, coagulase negative, DNase negative, grows on MSA with variable fermentation

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Novobiocin Test

Used to differentiate S. epidermis (susceptible) from S. saprophyticus (resistant).