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Bacteriology and Mycology > Staphylococcus > Flashcards

Staphylococcus Flashcards

1
Q

Properties of Staph

A
  • Gram staining
  • -Gram (+) appear purple because of thick peptidoglycan layer in the cell well
  • -Gram (-) appear pink because thin peptidoglycan layer does not hold onto the stain
  • Coccus v. cluster
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2
Q

Staphylococcus sp.

A
  • Gram-positive cocci
  • Formation (from broth or tissue)
  • -Irregular “grape-like” clusters
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3
Q

Streptococcus sp.

A

-Form chains or coccobacillus

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4
Q

Staph colony morphology

A
  • Can grow on blood agar and mannitol salt agar
  • Most are facultative anaerobes (but should ask for aerobic culture for diagnostic purposes)
  • Morphology:
  • -White/off-white
  • -Golden in S. aureus - due to carotenoid pigments
  • -Smooth
  • -Butyrous “butter-like”
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5
Q

Hemolysis

A
  • Alpha toxin present in Staph always for pore formation

- -Results in complete lysis (beta-hemolysis) of the RBCs in blood agar

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6
Q

Biochemical characteristics

A
  • Salt tolerance (ex. MSA - 7.5-10%)
  • Catalase positive
  • -Helps to differentiate between Staph (+) and Strep (-)
  • -Exceptions: S. aureus ssp. anaerobius and S. saccharolyticus are catalase negative; also , Streptococcus didelphis is catalase positive
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7
Q

Species level identification

A
  • Can be challenging
  • Sugar fermentation tests (not as commonly used anymore)
  • Other tests: MALDI (mass spec captures the protein profile) and sequencing
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8
Q

Virulence factors

A
  • Capsules
  • Protein A
  • Surface proteins (adhesins)
  • Superantigens
  • Hemolysins (ex. Alpha toxin)
  • Exfoliative toxins
  • Biofilm formation
  • Coagulase
  • Other enzymes
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9
Q

Capsules

A
  • Form of evasion from the host defenses
  • Most medically important strains are encapsulated
  • More heavily encapsulated strains appear mucoid
  • Capsular polysaccharides inhibit phagocytosis
  • Colonies may appear extra shiny
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10
Q

Protein A

A
  • Form of evasion from the host defenses

- Binds Fc fragment of IgG –> inhibits phagocytosis

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11
Q

Surface proteins (adhesins)

A
  • Used for adhesion
  • Examples:
  • -Elastin-binding protein
  • -Collagen-binding protein
  • -Fibronectin-binding protein
  • -Clumping factor - adheres to fibrinogen
  • -Bap (biofilm associated protein)
  • –Almost exclusively animal isolates
  • –Esp. bovine mastitis
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12
Q

Superantigens

A
  • Involved in damage (immune-mediated)
  • TSS toxin 1 (toxic shock syndrome)
  • Significant in humans
  • Superantigens cause non-specific T-cell activation, resulting in massive cytokine release
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13
Q

Alpha toxin (hemolysin)

A
  • Alpha toxin is a pore-forming toxin
  • -Possible role in escape from the phagosome
  • Associated with gangrenous mastitis in cattle
  • Causes the beta-hemolysis we see in culture
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14
Q

Exfoliative toxins (damage)

A
  • Serine proteases that bind desmoglein-1
  • Clinical syndrome examples:
  • -S. aureus –> human scalded skin syndrome
  • -S. hyicus –> greasy pig disease
  • -S. pseudintermedius –> canine pyoderma
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15
Q

Biofilms

A
  • Include more than Staph spp.
  • Complex anatomy
  • -Bacterial aggregate
  • -Exopolysaccharides - matrix formation
  • Stages
  • -Adhesion –> maturation
  • Quorum sensing
  • -Cell to cell talk via small molecules
  • -Regulates gene expression
  • Resistant to host immune clearance and antibiotics
  • See them on devices (such as IV catheters, urinary catheters, and implants) and directly on the patient (teeth, wounds, and ears/skin)
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16
Q

Coagulase

A
  • In the lab, used for ID of colonies - correlates with pathogenicity
  • Converts fibrinogen –> fibrin (protects the bacterium from phagocytosis)
  • Clots rabbit plasma in vitro
  • -Tube test - detects ‘bound’ coagulase (clumping factor)
  • -Slide test - detects ‘free’ coagulase - rapid screen, but not definitive
  • Staph epidermidis is coag negative
17
Q

Other enzymes that have a role in pathogenesis via direct damage to the host tissues

A
  • Staphylokinase (–> fibrinolytic)
  • Hyaluronidase (hydrolysis of extracellular matric)
  • -“Spreading factor” Know this
  • Lipase
  • Collagenase
  • Protease
  • Nuclease
  • Urease
18
Q

Commensal to pathogen

A
  • Staphylococci are commensals of skin, upper respiratory tract, mucous membranes
  • A break in host defenses is required for disease
  • -A breech in the epithelial barrier –> suppurative infections and septicemia
  • -Immunosuppression
19
Q

Staph species of veterinary interest

A
  • S. aureus
  • S. pseudintermedius
  • S. schleiferi
  • S. hyicus
20
Q

S. aureus - “Bumblefoot”

A
  • Pododermatitis
  • Guinea pigs and avian species
  • Abscess/pyogranuloma
  • Affects joints
  • Painful
  • Predisposing factors:
  • -Trauma
  • -Poor husbandry (ex: wet flooring, sharp materials)
21
Q

S. aureus - bovine mastitis

A
  • Contagious mastitis - NOT normal flora
  • Gangrenous mastitis (chronic form; alpha toxin)
  • If S. aureus is cultured from the bulk tank, individual cow milk cultures are recommended to find the culprit
  • If positive:
  • -Milk last or with dedicated milking unit
  • -Cull
  • -Can pasteurize the milk
  • Other agents of contagious mastitis:
  • -Strep agalactiae
  • -Mycoplasma sp. (usually M. bovis)
22
Q

S. aureus - tick pyemia

A
  • Lambs in the UK (5-29% affected); 2-5 weeks old
  • Predisposed by heavy infestation with the tick Ixodes ricinus
  • Septicemia –> death or failure to thrive
  • -Arthritis, meningitis, abcesses may be found in any organ
  • Economic impact
23
Q

S. pseudintermedius

A
  • Normal flora of skin
  • Previously classified as S. intermedius
  • Mostly dogs, but also in cats and horses
  • Clinical signs:
  • -Pyoderma, otitis, UTIs, and pyometra
24
Q

S. schleiferi

A
  • S. schleiferi ssp. scheiferi and S. schleiferi ssp. coagulans can be eitheer coag positive or negative
  • Clinical presentation are the same as S. pseudintermedius
  • ADDL reports “S. schleiferi coag positive” or “S. schleiferi coag negative”
  • -Why? Because coag positive strains are more likely to be pathogenic
25
Q

S. hyicus - greasy pig disease

A
  • Exudative epidermitis
  • Usually young (<6 weeks)
  • -Cutaneous erythema, painful
  • -Greasy, gray-brown exudate
  • The bacteria/toxin spreads systemically to other areas of skin
  • Peracute cases: usually die
  • Less severe cases survive if treated
  • Has exfoliative toxin
26
Q

Other Staph species

A
  • Even coagulase negative strains can cause disease if in the wrong place at the wring time (or right place at right time)
  • Usual presentations: abscesses (suppurative conditions), can be systemic
27
Q

Diagnosis

A
  • Submit appropriate sample for culture:
  • -Urine
  • -Abscess material in red top tube
  • -Section of abscessed organ from necropsy in sterile container
  • -Swab of pustule
  • Remember: Staph are commensals of skin and mucous membranes, carefully interpret culture results from these sites
  • Request aerobic culture
  • Antimicrobial susceptibility testing
  • -Don’t rely on empirical drug choices
  • -Evidence based medicine
28
Q

Control and prevention

A
  • Vaccination - not effective
  • -Antibodies - short-lived, not protective
  • -Vaccine for bovine mastitis, decreases severity of clinical disease
  • Control
  • -Good husbandry: sanitation, reduction of stressors, biosecurity
  • -Address underlying conditions
  • –Dog with atopy and staph pyoderma
  • –Lamb with tick infestation (UK)
  • Treatment
  • -Antibiotics (systemic and/or topical)
  • -Wound management: surgical debridement, drain placement, flushing
  • -Physically disrupt biofilms: bathe skin, clean ears, flush wounds
29
Q

Resistance to treatment

A
  • Biofilms
  • Beta-lactamase
  • MRSA/MRSP
30
Q

Biofilms

A
  • Mechanically disrupt (clean ear, wound, etc.)

- Remove implant

31
Q

Beta-lactamase

A
  • Enzyme
  • Widespread among all staph species
  • Destroys beta-lactam antibiotics such as penicillin and amoxicillin
  • Solution: potentiated beta-lactams
  • -Ex: amoxicillin + clavulanic acid (beta-lactamase inhibitor)
  • –Helps preserve the drug so it can fight the bacteria (rendering it ineffective)
32
Q

Methicillin resistance

A
  • Beta-lactam antibiotics MOA:
  • -Bind to the Penicillin binding protein on staphylococci to inhibit crosslinking of peptidoglycan during cell wall formation
  • -No cell wall –> dead bacteria (therefore bactericidal)
  • Methicillin-resistance:
  • -Bacteria produces an “altered penicillin binding protein” (PBP2a) with low affinity for beta-lactams
  • -Beta-lactam antibiotics cannot bind to PBP2a –> bacteria live
  • PBP2a is encoded by mecA; can be shared between pathogenic and commensal staphylococci via a mobile element
33
Q

MRSA/MRSP

A
  • MIC panel: oxacillin resistance
  • Often resistant to multiple drug classes
  • PCR available to detect mecA
  • -Usually reserved for VTH and large hospitals for epidemiology data
  • -Should confirm with PCR if it is available
  • Can be hospital acquired or community acquired
  • -Common misconception is that you need to get rid of your dog if they are positive
  • -Dogs can be transient carriers (not the primary carrier), they might actually get it from other people and transmit it to you
  • ID asymptomatic carriers
  • -May be reasonable to screen animals who live with humans diagnosed with MRSA
  • -Sample swabs from distal nares, perianal area, and any suspicious lesions (if present)
  • -Humans can also be screened to see if they are carriers

Bacteriology and Mycology (6 decks)

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