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
2
Q
Staphylococcus sp.
A
- Gram-positive cocci
- Formation (from broth or tissue)
- -Irregular “grape-like” clusters
3
Q
Streptococcus sp.
A
-Form chains or coccobacillus
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”
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
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
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
8
Q
Virulence factors
A
- Capsules
- Protein A
- Surface proteins (adhesins)
- Superantigens
- Hemolysins (ex. Alpha toxin)
- Exfoliative toxins
- Biofilm formation
- Coagulase
- Other enzymes
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
10
Q
Protein A
A
- Form of evasion from the host defenses
- Binds Fc fragment of IgG –> inhibits phagocytosis
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
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
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
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
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)
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
