Microbiology: Skin Infection Staph and Strep Flashcards
Why do gram positive bacteria stain blue?
Thick murein layers (peptidoglycan)
A teenager comes to see you with acne. Where is the most likely source of bacteria that caused her acne?
Resident skin flora. Staph epidermidis to be exact.
Who are the transient bacterial bums of our skin?
Staph aureus and staph pyogenes
A child comes to see you with a bacterial infection around their mouth confined to the epidermis. What spreading infection may the child have?
Impetigo from strep or staph
Lab results indicate your patient has a spreading bacterial infection of the dermal lymphatics. What might your patient have?
Erysipelas from strep
Your cousin cuts his leg and gets a spreading bacterial infection in the subcutaneous fat layer. What would you call this infection?
Cellulitis from strep or staph
After shaving you notice razor burn bumps that form little abscesses across your face. What do you call this?
Folliculitis from staph
What bacteria do you suspect when you see pus and pyogenic skin infections?
Staph and strep
Characteristics of pyogenic cocci
Invasive, pus-filled lesions, obligate extracellular bacteria, anti-phagocytic mechanisms
Can strep and staph be lysed by antibody and complement?
No, only gram negative bacteria can be lysed. Antibody and complement aid in phagocytosis of gram positive bacteria.
Characteristics of streptococci
Gram +, come in pairs or chains, and are catalase negative (can’t make water from a dilute solution of hydrogen peroxide)
Bacteria is placed in a dilute solution of hydrogen peroxide. Bubbles are observed, what is the likely bacteria?
Staph. It is a catalase positive bacteria.
What helps the clinical laboratory determine what type of strep has been isolated?
Beta-hemolytic pattern around the colony on blood agar. Antigenic composition. Also using physiological traits (can the organism grow in salt).
What type of strep is always beta hemolytic?
Group A strep
What are rapid group A strep tests looking for?
Group A carbohydrate antigen on cell wall. All group A strep have this antigen.
How can you test for group A strep with antibiotics?
Apply bacitracin to a plate of the bacteria and incubate over night
Lab tests come back for gram-positive, chain forming, catalase negative, bacitracin/PYR sensitive, beta hemolytic bacteria. What is the likely bacteria?
Group A streptococcus
Why does our body have a hard time recognizing strep capsules?
It is made of hyaluronic acid, which we also have in our body.
What is the major anti-phagocytitic component of group A strep?
M protein extending out of the bacterial capsule
Why do some people get endocarditis after a group A strep infection?
The M protein of group A strep is similar to the cells in the heart
What regulates production of bacterial pyrogenic exotoxins?
Transduction of a bacterial phage and expression of toxin gene. These toxins can produce super antigens
How do super toxins work?
Super activation of T cells.
What effects stem from a bacteria that produces toxic shock-like symptoms?
The bacteria also becomes systemic and symptoms are systemic.
This bacterial product causes RBC lysis on blood plates and we make antibodies against it.
Streptolysin O
How does strep escape destruction by PMNs?
DNAses chop up the DNA that entangles them from the PMNs
What strep product is useful during surgery?
Streptokinase. It lyses blood clots which can help bacteria spread but can also be used in surgery.
What strep product helps it escape complement opsinization?
C5a peptidase. It degrades C5a so PMNs are not attracted.
What are the suppurative strep diseases?
Pharyngitis, Pyoderma and TSS
What sub diseases are pyodermas?
Impetigo, Erysepelas, Cellulitis and Necrotizing fasciitis
What are examples of non-suppurative strep diseases?
Acute glomerular nephritis and rheumatic fever due to antibody production for M protein
A patient presents with strawberry tongue, rash and desquamation of the skin. What might this patient have and how are these symptoms being produced?
Scarlet fever. It is a staph infection that causes systemic symptoms but the bacteria stays in the same place.
A patient comes to your clinic with symptoms of rheumatic fever following a sore throat. Why should the sore throat have been treated more seriously in the first place?
Group A strep infections have the M protein which mimics heart tissue. If not treated quickly, the body will develop anti-M antibodies and these antibodies will attack the heart.
A patient comes to your clinic with symptoms of acute glomerulonephritis after a sore throat. Why should the sore throat have been treated more seriously in the first place?
The body made anti-M antibodies and the antigen-antibody complex has deposited in the kidney.
What syndrome follows a strep A infection that affects neurological function in pediatrics?
PANDAS
What must people with rheumatic fever be treated with for the rest of their lives?
Penicillin G. This is so they don’t have another autoimmune response against the heart.
How does s. aureus differ from a strep?
They form clusters instead of pairs and lines. It is still hemolytic but catalase it positive (makes H2O2 bubble).
Which type of staph is not beta-hemolytic?
S. epidermidis
How would an agar plate of s. aureus and s. epidermidis differ?
S. aureus is yellow and hemolytic. S. epidermidis is white an non hemolytic
How does s. aureus differ from s. epidermidis in coagulase tests?
S. aureus is positive and s. epidermidis is negative
What happens if you mix S. aureus with mammalian plasma?
You get a fibrin clot. This is the coagulase test used to test for S. aureus which turns fibrinogen to fibrin.
A patient presents with a sty on the eye. How would you classify this subcutaneous skin infection?
Furuncle
A patient with a furuncle loses the medication for it and the sty gets worse and begins to produce pus. What is it classified as now?
Carbuncle. The furuncles coalesce and begin producing pus as they recruit neutrophils.
A breast feeding mother gets mastitis. What are possible sources of infection?
Her normal flora staph or the hospital
Where does all of this patient’s acne come from?
Staph in his nose. It is a chronic furuncluosis
What does a staph infection progress to after a carbuncle?
Cellulitis or impetigo
What are possible consequences of staph infections going deep?
Osteomyelitis, septic arthritis, meningitis or pneumonia.
What are possible consequences of staph getting into the blood stream (bacteremia)?
Endocarditis, meningitis, pneumonia, pyelonephritis and septic shock
Why are diabetics at higher risk for staph aureus infections?
They have poor circulation and decreased neutrophil response to bacteria.
What is the body’s #1 defense against gram positive bacteria?
Neutrophils. This is why complement C3b is crucial.
How does staph aureus defend itself against phagocytes?
Capsule, Protein A (binds Fc end of antibodies to decrease specific antibody opsinization), Catalase breaks down toxic by products from O2 burst, Leukocidin pokes holes inside neutrophils.
How are staph aureus infections so good at making abscesses?
Initiates conversion of fibrinogen to fibrin and surrounds the infection site with fibrin.
How does staph aureus induce toxic shock?
Techoic acid induces TLR2 action
How does staph aureus make it easier for itself as far as moving through subcutaneous tissue?
They produce hyaluronidase, which breaks down ECM hyaluronic acids.
What cytotoxins are produced by staph aureus and what do they do?
Alpha hemolysin: pore-former, Beta and Delta Toxin: disrupts cell membranes, Gamma toxin and leukocidins put holes in neutrophils and macrophages
What are the toxin-mediated diseases caused by staph aureus?
Bullous impetigo, scaled skin syndrome, staphylococcal scarlet fever, TSS and food poisoning
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Bullous impetigo caused by exfoliatin toxin from staph
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Scaled skin syndrome. Caused by exfoliatin toxin from staph.
How do exfoliatins work?
They split desmosomes in the stratum granulosum
What causes toxic shock syndrome?
TSST-1. Nonspecifically binds T cells with APCs.
How can you confirm what strain of bacteria someone has?
Pulsed Field Gel Electrophoresis (PFGE) chops up chromosomes into fragments with restriction enzymes. The fragments are run on a gel and compared. Bacteria is also immune to its own phages and phage typing can be used.
How are MRSA bacteria resistant to antibiotics?
It has a mecA gene which codes for an altered PCN binding protein that won’t bind antibiotics.
How are VRSA bacteria resistant to vancomycin?
They have the vanA gene changes the peptidoglycan terminus to D-ala-D-lactate instead of D-ala-D-ala which is what vancomycin acts on and inhibits peptidoglycan chain production.
Why do you wipe someone’s arm with alcohol before a shot?
To prevent s. epidermidis infection
Why is s. epidermidis particularly good at nosocomial infections?
They form biofilms that can mix with other bacteria.
