Staphylococci (1/8) Flashcards
What are staphylococci?
Coagulase positive (S. aureus has enzyme coagulase that converts fibrinogen to fibrin) or negative (S. epidermis)
All are catalase positive (catalase negative = streptococcacae)
Nonsporulating & nonmotile gram positive cocci that grow in clusters like grapes
Can survive a variety of environmental stresses
Golden sheen of colonies that form on agar plate
What are the main structural components of staphylococci?
Cell wall, peptidoglycan, capsule + lipoteichoic acid
Surface proteins: MSCRAMMs aid in adherence to host cell surfaces i.e. fibronectin, fibrinogen, collagen of ECM & explain infections in certain places i.e. required for joint infection
Secreted proteins: TSST-1 (toxic shock syndrom toxin-1), Protein A (binds TNF receptor, antiphagocytic), alpha toxin (initiates sepsis-like syndrom sepsis, causes hypotension & mulitorgan system failure)
What are some enzymes that staphylococci secrete?
Catalase: H2O2 to H2O/O2
Coagulase: fibrinogen to fibrin only in S. aureus
Hyaluronidases: hydrolyze hyaluronic acids & contribute to tissue breakdown/spread across tissue barriers
Beta-lactamases: hydrolyze beta-lactam ring of penicillins –> antibiotics are useless
Lipases (associated with abscesses, soft tissue infections)
Which toxins do staphylococcus aureus secrete?
TSST-1 (cause of toxic shock syndrome)
Enterotoxins (food poisoning)
Exfoliative toxins serine proteases (responsible for scalded skin syndrome)
Membrane damaging toxins (leukocidin- soft tissue & necrotizing pulmonary infections)
Alpha toxin (cytotoxic to host cell membranes, initiates sepsis-like syndrome; its presence is one of the determinants to cause hypotension & multiorgan system failure)
How are staphylococci identified in the lab?
Gram stain to see gram positive cocci in grape-like clusters
On agar, form round, beta-hemolytic colonies
What is the natural reservoir of staphylococcus?
Humans; normally coagulase neg staph are found on normal skin flora & anterior nares
Infection is often due to autoinoculation or transmission from a carrier to a patient
Those at high risk of staphylococcal disease are increased carriers i.e. dialysis patients, diabetics, HIV infected subjects. Reducing colonization has shown to reduce infection
Which diseases can be caused by staphylococcus aureus?
Skin and soft tissue infection
Bacteremia: sepsis, metastatic seeding
Endocarditis
Musculoskeletal infections
Respiratory tract infections
Toxin-related diseases (TSST-1, scalded skin syndrome, food poisoning) – don’t require infection, just entry of toxin
What is the pathogenesis of staphylococcus aureus infections?
Colonization, infection, invasion (local/systemic, often can be metastatic), host response
Because they’re present on skin, both coag + and - bacteria often cause prosthetic device (i.e. IV catheters) related infections
Adherence to host tissue is facilitated by fibronectin-binding protein & collagen-binding protein.
How do regulatory genes relate to pathogenesis of infection?
They provide means for the bacteria to adhere when they’re exposed to the matrix molecules
They also help it spread to other sites once the infection has been established
What causes Toxic Shock Syndrome?
Vaginal colonization (not infection) –> toxin
TSS is a superantigen mediated disease: toxins bind antigen presenting cells’ MHC 2 molecule –> bind T cells –> huge lymphocyte response –> dramatic release of cytokines from the lymphocytes i.e. IL-1, IL-2, TNF, interferon gamma –> multiorgan disease similar to septic shock
What causes staphylococcal food poisoning?
Enterotoxin mediated disease: doesn’t require viable staphylococci
Results from the ingestion of heat stable eterotoxin, which stimulate the vagus nerve & CNS vomiting center & increase peristalysis
What is the most common coagulase negative staphylococci? What niche of diseases do they cause?
Staphylococcus epidermidis = most common
Relatively avirulent, part of normal skin flora
Unique niche for infections: prosthetic devices i.e. IV catheters, prosthetic heart valves
Frequent contaminant in cultures
