Staphylococcus Flashcards
Catalase test –> positive or negative indicates what?
Positive catalase test = Micrococcaceae family -> Staphylococcus or Micrococcus genus
Negative catalase test = Streptococcaceae family -> Streptococcus genus
How do you further differentiate Catalase Positive bacteria?
With a Nitrofurantoin or Lysostaphin Susceptibility test or with a Dextrose fermentation test
Catalase positive= Staphylococcus or Micrococcus
Nitrofurantoin or Lysostaphin susceptible = Staphylococcus
Positive Dextrose fermentation = Staphylococcus
Not susceptible = Micrococcus
Negative Dextrose fermentation = Micrococcus
How do you differentiate Staphylococcus species?
With a coagulase test
Coagulase positive: Staphylococcus aureus
S. aureus produces both exocoagulase and endocoagulase
exocoagulase: binds to serum immunoglobulins and this exocoagulase-Ig complex cleaves fibrinogen to fibrin. detected with coagulase tube test.
endocoagulase: aka clumping factor is bound permanently on the bacterial surface, and cleaves fibrinogen to fibrin diretly. detected by slide agglutination (cultured bacteria sample mixed w/ blood drop on a slide) or latex-agglutination
Coagulase negative: S. epidermidis, S. haemolyticus, S. saprophiticus, S. hominis
Morphology and Culture of Staphylococcus aureus.
Gram positive staphylococci, grape clusters
1uM cocci
Catalase+, Coagulase+
Faculattive anaerobic
Agar: 2-3mm wide golden colonies (au=gold), due to a non-diffusible fat soluble pigment they create which does not diffuse into the medium.
Blood agar: yellow golden colonies that are beta-hemolysing.
Can be enriched in 7.5% NaCl agar medium (note that L. monocytogenes is also salt resistant up to 10%)
Staph aureus virulence factors (10)
Polysaccharide capsule
Secreted polysaccharide slime layer
Protein A (of Staph A): A cell wall protein that binds Fc regions of IgG, inhibiting complement binding and activation. Inhibits opsonization of the bacterium.
Teichoic acid and lipoteichoic acid: Bind to mucosal cells, mediate mucosal adherence
Coagulases: exocoagulase, and endocoagulase aka clumping factor Generates Fibrin and clotting
Hyaluronidase: Spreading factor, allows ECM penetration
Staphylokinase aka Fibrinolysin: another spreading factor
Ctyolysins: alpha, beta, gamma, and delta toxins. Lysis of RBCs, as well as lysis of other cell types.
Leukocidin: causes lysis of leukocytes
Superantigens (3): 1. Exfoliative toxin 2. Toxic shock syndrome toxin TSST 3. Enterotoxins A-F
What is the mechanism of action of superantigens
Superantigens bind to MHC-2 molecules on APCs, (remember the class 2 is on APCs, and tho it is variable it is much less variable than the class 1s).
then bind to to the variable regions of the alpha or beta chains (or both) of the TCR on a T-helper cell, CD4+. They display broad specificity, activating as many as 20-40% of the T helper population.
Causes massive release of cytokines IFN-gamma, IL-2, TGF-beta,
Septic shock
What are the virulence factors of Staphylococcus aureus
What is the definition of a virulence factor?
Virulence factors are molecules produced by pathogens that aid in:
colonization of a niche in the host (this includes attachment to cells)
immunoevasion, evasion of the host’s immune response
immunosuppression, inhibition of the host’s immune response
cell entry and exit (if the pathogen is an intracellular one)
obtain nutrition from the host
and factors that cause damage to the host
What is the habitat of Staphylococcus aureus?
What is the means of transmission?
What are the 3 broad categories of clinical diseases it causes?
S. aureus is carried as a member of the normal flora of nasopharynx, mucous membranes, and skin.
S.a causes pyogenic infections, invasive infections, and exotoxin mediated syndromes
What are the types of skin (dermis and epidermis) infections caused by Staphylococcus aureus
Folliculitis - infection of an individual hair follicle or a patch of multiple separate hair follicles
Paronychia - infection of the fingernail or toenail beds.
Impetigo - patchy red sores in the epidermis which quicly burst and form a hard yellow crust. typically on the face. bullous impetigo - involves larger deepr boils filled with pus non bullous - the crusty dry kind.
Furuncle aka a Boil- a deeper, larger infection around a hair follicle, which penetrates into the subcutaneous/subdermal tissue
Carbuncle -Multiple Furuncles overlapping with one another forming an overlapping cluster which may go very deep and wide, down to the bone even.
What are the sub-epidermal infections caused by Staphylococcus aureus?
cellulitis - infection of the sub-dermal fat (cellulite)
phlegmone - a deep, sub-dural purulent infection causing swelling from pus buildup in the deep tissue layers.
dactylitis - infection of toe or finger around the nail
What are the deep pyogenic infections caused by S. aureus
septic arthritis (pus filled ankle/other joint)
osteomyelitis
mediastinitis
peritonitis
bacterial meningitis
plueritis
abscess formation in any organ parenchyma, brain, lung, renal abcess(except heart, it causes endocarditis, but the patient would die before it could progress to a myocardial abcess)
What types of pyogenic infections can be caused by S. aureus
Epidermal
Sub-epidermal
Deep pyogenic infections: bones, any of the serous membranes, parenchymal infections of all of the organs.
Eye infections
Ear infections
Respiratory tract infections
Urogenital infections
Cardiovascular infections: Vasculitis, endocarditis thrombophlebitis (vein inflammation related to thrombus)
Generalized infection: Bacteremia, Sepsis.
What are the exotoxin mediated syndromes caused by S. aureus
Food poisoning - vopmiting, diahhrea due to secreted enterotoxins exotoxins, which increase GI secretions
Exfoliative dermatitis - massive skin peeling/flaking due to exfoliative toxin
Toxic epidermal necrolysis (TEN) - severe/lethal problem causes separation of the epidermis from the dermis and then total death of the epidermal layer, leaving the patient severely exposed to infection and sepsis
Toxic shock syndrome: toxic shock syndrome toxins, the superantigens
