S1B5 - Staphylococcus Flashcards
Toxic shock syndrome:
- commonly caused by which S. aureus exotoxin?
- classically associated with …?
- 4 S/Sx of S. aureus-mediated toxic shock syndrome?
- *TSST-1** causes toxic shock syndrome and is classically associated with tampon use and foreign bodies (eg, nasal packing for a nose bleed or status post surgery). S/Sx of toxic shock syndrome:
1. Fever
2. Hypotension → dizziness, multi-organ failure
3. Nausea and severe vomiting
4. Generalized rash—diffuse erythema that starts on the trunk and spreads to extremities, erythema of palms and soles, conjunctival hyperemia, strawberry tongue → 1-2 weeks later the rash will desquamate, especially on the palms and soles
Name 2 diseases caused by exfoliative toxin. What is the mechanism of this toxin?
In contrast to the superantigen mechanism of TSST-1 and enterotoxin, exfoliatin is a proteolytic exotoxin that cleaves the desmoglein (Dsg) 1, causing a blister just below the stratum corneum. This may allow bacteria to proliferate, leading to bullous impetigo and staphylococcal scalded-skin syndrome.
In this picture of Tellurite-Glycine agar plates, which is S. epidermidis and which is S. aureus?
Identification of S. aureus
- selective due to glycine
- differential due to tellurite reduction to tellurium
S. saprophyticus can be normally found where in the body?
S. saprophyticus is part of the normal flora of the rectum and vagina in up to 10% of females.
Name at least 3 diseases commonly caused by Staphylococcus aureus infection which are not necessarily mediated by exotoxins.
Tissue/organ infections:
- Pneumonia
- Osteomyelitis (S. aureus is the most common cause)
- Acute endocarditis, classically in an IV drug abuser (S. aureus is the most common cause of acute endocarditis—vs. subacute endocarditis, which is most commonly caused by Viridans Streptococcus)
Foreign bodies (eg, prosthetic valves, pacemakers, central lines, hemodialysis catheters, etc)—although S. aureus can colonize foreign bodies, S. epidermidis is more classically associated with colonization of foreign bodies and iatrogenic infection
What patients are at a higher risk of S. aureus infection?
- Newborn (Scalded Skin Syndrome),
- Young children with poor hygiene (Skin infections)
- Menstruating woman (Toxic Shock Syndrome)
- Patients with intravascular catheters (bacteremia & IE)
- Surgery & burn patients, patients with LRT disease.
Cultures from a superficial abscess grow gram-positive cocci in clusters. Which is the most likely virulence factor associated with this organism?
A) M protein
B) IgA protease
C) Protein A
D) Hemagglutinin
E) Streptokinase
Protein A
Gram-positive cocci in clusters causing superficial abscesses are most likely to be Staphylococcus aureus. A main virulence factor for S. aureu is Protein A, which binds Fc region of IgG to prevent complement fixation and phagocytosis.
IgA protease is a virulence factor for Streptococcus pneumoniae.
M protein and streptokinase are virulence factors for Streptococcus pyogenes.
Hemagglutinin initiates cellular infection by influenza viruses.
What are the symptoms caused by S. aureus enterotoxin? Is onset of symptoms rapid or slow? What foods is it associated with?
Enterotoxin is heat-resistant (stable at 100°C for 1h) and causes rapid-onset food poisoning. Symptoms arise within 1-6 hours of ingesting contaminated food and predominantly include nausea and vomiting/retching; watery diarrhea may also occur.
Foods classically associated with S. aureus enterotoxin-mediated food poisoning:
- meat
- poultry
- mayonnaise-containing foods (especially salads—potato salad, macaroni salad, tuna salad)
- milk/egg/dairy products
- cream-filled pastries (eg, custards)
This is a classic picture of what family of bacteria?
Staphylococci family
They are gram-positive cocci that cluster together like grapes. Staphylé = bunch of grapes
Is Staphylococcus epidermidis sensitive or resistant to Novobiocin?
Is Staphylococcus saprophyticus sensitive or resistant to Novobiocin?
S. epidermidis is novobiocin sensitive. Contrast this with S. saprophyticus, which is novobiocin resistant.
What is S. aureus’ main virulence factor? What is its function?
Major virulence factor is protein A, which binds Fc region of IgG, thereby inhibiting complement fixation and phagocytosis.
What are these pictures of? What is the most common cause?
S. aureus - Diseases
Scalded Skin Syndrome (Ritter’s Disease)
- ETA/ETB break desmoglein-1 in desmosomes
- Skin cells become unstuck.
- Abrupt onset of perioral erythema (covers entire body in 2d)
- Large bullae/blisters (like burn/scald), desquamation
- Blisters have fluid (no organism)
What is shown in the picture? What is the most common cause of this?
S. aureus - diseases
- Folliculitis: pyogenic infection of hair follicles
- Furuncle (boil): several hair follicles & the adjacent tissue
- S. aureus is the most common cause
- Good skin care and personal hygeine reduces risk
Is S. saprophyticus nitrate reductase negative or positive?
S. saprophyticus is nitrate reductase negative, so a dipstick test will produce a false-negative result. The nitrate reductase test is used to differentiate between bacteria based on their ability to reduce nitrate to nitrite using anaerobic respiration.
What is this a picture of? What is the most common cause?
S. aureus - Diseases
-
Bullous Impetigo (localized form of SSSS)
- Erythema does not extend beyond the borders of the blisters
- Unlike SSSS, bacteria can be isolated from the skin lesion
What microorganism is most likely in these pictures?
S. epidermidis
- White non-hemolytic colonies on blood agar
- Gram positive clusters of cocci
An indwelling central venous catheter is removed for infection despite several weeks of vancomycin treatment. Cultures grow catalase positive, coagulase negative gram positive cocci. What is the organism?
S. epidermidis is capable of generating a sticky, protective biofilm, facilitating colonization and infection of prosthetic devices including prosthetic heart valves and indwelling urinary catheters. Prosthetic valve endocarditis during the initial year after surgery is almost exclusively caused by S. epidermidis.
What is the primary laboratory diagnostic test sequence for Staphylococcus saprophyticus for genitourinary infections?
Laboratory diagnostics include urinalysis and culture (collect sample midstream and culture using blood agar).
Describe the mechanism of S. aureus TSST-1 and enterotoxin.
TSST-1 and enterotoxin are both superantigens.
Mechanism:
- Superantigens cross-link the α chain of MHC-II on the antigen-presenting cell (e.g. macrophage, dendritic cell) with the Variable region of the β-chain (Vβ) of T-cell receptors on CD4+ TH cells → polyclonal (antigen-independent) TH cell activation → increased IL-2 and IFN-γ, which activate macrophages → increased proinflammatorycytokines IL-1, IL-6, TNF-α
- Superantigens do not activate all CD4+ TH cells. Different superantigens are specific for different Vβ chains and activate different subsets of CD4+ TH cells
Development of toxic shock syndrome is associated with which type of immune cells?
A) Monocytes
B) Neutrophils
C) CD8+ T-cells
D) CD4+ T-cells
E) Natural killer cells
CD4+ T-cells
TSST-1 is a superantigen that causes polyclonal CD4+ Th-cell activation via MHC-II binding; it is associated with tampon use.
A woman eats a macaroni salad that was sitting on the counter overnight. Two hours later, she develops nausea, vomiting and retching, soon followed by severe, nonbloody diarrhea. Which of the following accurately describes the cause of her symptoms?
A) It is a lipopolysaccharide
B) It is a heat-resistant superantigen
C) It functions as an N-glycosidase to inhibit protein synthesis
D) It causes polyclonal natural killer cell activation
E) It is an endotoxin released by S. aureus
It is a heat-resistane superantigen
Mayonnaise-containing foods, especially salads (eg, potato salad, macaroni salad, tuna salad) are classically associated with S. aureus enterotoxin-mediated food poisoning. The presentation is dominated by nausea, vomiting and retching, sometimes followed by nonbloody diarrhea. Enterotoxin is heat-resistant, is a member of the superantigen class, and is secreted by certain strains of S. aureus. It is not an endotoxin, doesn’t cause polyclonal NK activation (it activates T-cells), it is protein-based, not a lipopolysaccharide (Gram-negative bacteria cell wall), and doesn’t inhibit protein synthesis (Shiga toxin).
What test is this a picture of? Which is positive and which is negative?
Coagulase test
- Incubation of S. aureus cells in plasma causes coagulation - positive test
- All other staphylococci (e.g. S. epidermidis) are coagulase negative
What test is shown in the picture? Which is positive and which is negative?
This is the catalase test. Bubbles are positive.
- Staphylococcus aureus*:
- gram stain?
- catalase?
- coagulase?
- hemolysis pattern?
- appearance of colonies on agar plate?
Staphylococcus aureus:
Gram +, cocci in clusters, catalase +, coagulase +, β-hemolytic, forms yellow/golden colonies (aurei=”golden”)
A 23 year old IV drug abuser is brought to the emergency room complaining of recent onset spiking fevers and chills. He is found to have a new murmur on cardiac exam. Transthoracic echocardiography demonstrates large vegetations on his tricuspid valve. Which pathogen is most likely?
A) Staphylococcus aureus
B) Candida albicans
C) Streptococcus mutans
D) Aspergillus fumigatus
E) Streptococcus pneumoniae
Staphylococcus aureus
Endocarditis can be acute or subacute. Acute endocarditis tends to be more severe and is most commonly caused by S. aureus. It commonly occurs in IV drug abusers.
Although C. albicans and S. mutans can both cause subacute endocarditis, these etiologies are less frequently seen. Moreover, C. albicans on the whole remains a rare cause of any form of endocarditis.
What is the morphology, gram staining, catalase-positive/negative, urease status, and oxygen requirement of Staphylococcus saprophyticus?
Staphylococcus saprophyticus is a facultative anaerobe, urease-positive, gram-positive, catalase-positive coccus.
What is this a picture of? What is the most common cause?
Septic arthritis
- S. aureus is the most common cause (N. gonorrhoeae in sexually active group)
- Painful (on movement), pus in aspirated fluid
- ~50% knee joints (wrists, ankles, & hips also commonly affected)
What is this a picture of?
CA-MRSA skin infection in a patient after obtaining a tattoo.
- Staphylococcus epidermidis*:
- Catalase positive or negative?
- Coagulase positive or negative?
- Urease positive/negative? - Hemolysis pattern and colony color? - Gram stain and morphology?
Staphylococcus epidermidis is a catalase-positive, coagulase-negative, urease-positive, non-hemolytic with white colonies, gram-positive cocci that grows in clusters.
What are the treatment options for methicillin-susceptible Staphylococcus epidermidis? What is the treatment option for methicillin-resistant Staphylococcus epidermidis?
Empiric treatment for S. epidermidis is vancomycin for assumed methicillin-resistance. Nafcillin or oxacillin can be given for methicillin-susceptible species.
What diseases are commonly caused by S. aureus?
S. aureus is the most common cause of:
- Hospital-acquired bacterial pneumonia
- Ventilator-associated bacterial pneumonia (Pseudomonas aeruginosa is the 2nd most common cause)
- Cellulitis
- Osteomyelitis
- Impetigo
- Acute infective endocarditis
What is the mechanism of action whereby the exfoliative toxin of S. aureus causes bullous impetigo and scalded skin syndrome?
A) Denaturing of claudins
B) Tight junction disruption
C) Desmoglein cleavage
D) Immune complex deposition
E) Activation of matalloproteinases
Desmoglein cleavage
Desmoglein is a protein component of desmosomes, which are responsible for joining cells together. Exfoliative toxin cleaves desmoglein, causing bullous impetigo and scalded skin syndrome. Desmoglein is also targeted in pemphigus vulgaris. Tight junctions prevent paracellular transport/movement of molecules and play no role in bullous impetigo and scalded skin syndrome. Claudins are a key component of tight junctions. Immune complexes aren’t formed by exfoliative toxin and metalloproteinases are also not activated.
In this Novobiocin resistance test, which side is S. epidermidis and which is S. saprophyticus?
- S. saprophyticus* is Novobiocin-resistant. It is on the right.
- S. epidermidis* is not Novobiocin resistant. It is on the right.
What is this a picture of and what is the most common causative agent?
S. aureus - Diseases
-
Oseomyelitis (OM)
- Most common cause of acute OM - pain and fever
- Hamatogenous spread or extension from a S/C infection
- In children - usually in metaphyseal area of long bones
- In adults, occurs in vertebra; rarely affects the long bones
Is Staphylococcus saprophyticus novobiocin sensitive or resistant?
S. saprophyticus is novobiocin resistant. Contrast this with S. epidermidis, which is novobiocin sensitive.
What is this a picture of and what is the most common cause?
S. epidermidis diseases
-
Prosthetic valve endocarditis - high mortality rate (~60%)
- prosthetic valve endocarditis with circumferential infection and dihiscence of the valve posteriorly
How can the morphology, catalase, and coagulase properties of Staphylococcus aureus be used to differentiate from other gram-positive species?
These three things together tell you it is staphylococcus aureus:
- gram-positive cocci in clusters
- catalase positive
- coagulase positive
What diseases does S. saprophyticus cause?
S. saprophyticus causes urinary tract infections and cystitis.
What is the most likely microorganism with the test results in this picture?
S. aureus
- Coagulase positive
- Catalase positive
- Clusters of cocci
- Grows on:
- Tellurite-Glycine agar
- Mannitol-salt agar
How is S. saprophyticus transmitted and what individuals are most commonly affected?
S. saprophyticus infections spread via bodily fluids and often occurs in sexually active young women.
Is S. saprophyticus coagulase negative or positive?
S. saprophyticus is coagulase negative.
What is the typical presentation of a S. saprophyticus infection?
Patients typically present with cystitis (symptoms include dysuria, polyuria, and abdominal pain/discomfort).
What are three practices/characteristics that increase risk of S. saprophyticus infection?
Practices and characteristics that increase risk of infection include unprotected sex, short urethra, and open cuts or sores. Infection is more common between the summer and fall seasons.
What is this a picture of? What is the most common cause of this?
S. aureus - Diseases
- Carbuncle (group of furuncles)
- Furincles coalesce and extend to deeper S/C tissue
- More severe than folliculitis/furuncle, multible sinus tracts
- Usually on the nape of the neck, upper back, or buttocks
- Initially tight, erythematic skin that later effaces & releases pus.
- Unlike folliculitis & furuncle, patient may have chills & fever
What are these pictures of? What are the likely causative agents?
S. aureus - Diseases
-
Impetigo
- Contagious and common in children (on face & limb)
- Starts with a small macule - becomes a pustule
- A honey brown crusting - when the pustule ruptures
- ~80% S. aureus; ~20% S. pyogenes
Where does S. epidermidis normally reside? What is a common result of this?
S. epidermidis is considered normal skin flora and is notorious for contaminating blood cultures.
A patient with an abdominal incision undergoes evacuation of a hematoma. Wound swabs sent for gram stain demonstrate coagulase negative gram-positive cocci in clusters. Which most accurately describes the likely organism?
A) Colonizes catheters without using biofilm
B) Common cause of endocarditis
C) Common cause of post-surgical soft tissue infections
D) Normal skin flora that is catalase negative
E) Normal skin flora that is catalase positive
Normal skin flora that is catalase positive
Gram-positive cocci in clusters indicates staphylococcal species. Coagulase negative status indicates S. epidermidis, where as S. aureus is coagulase positive. S. epidermidis is catalase positive (staph makes catalase). S. epidermidis is usually a contaminant and does not commonly cause post-surgical soft tissue infections. While S. epidermidis does colonize prosthetic devices like catheters, it usually generates a biofilm that forces removal of the prosthetic device.
A blood agar culture and gram stain microscopy of a bacteria present as shown in the pictures. What is the most likely microorganism?
Identification of S. aureus
- Hemolytic golden yellow colonies on blood agar (left)
- grape like cluster (right)
- gram positive
Which toxin-based disease is least likely to be caused by gram-positive cocci in clusters?
A) Skalded skin syndrome
B) Food poisoning
C) Bullous impetigo
D) Toxic Shock Syndrome
E) Scarlet fever
Scarlet fever
Pathogenic gram-positive cocci in clusters are most likely to be Staph. aureus. Its toxin-mediated diseases include: toxic shock syndrome (TSST-1), scalded skin syndrome (exfoliatin), and food poisoning (enterotoxin).
Bullous impetigo can be caused by S. pyogenes as well as S. aureus. However, scarlet fever is mediated by the erythrogenic toxin produced by certain strains of S. pyogenes (gram-positive chains/pairs) and has no association with S. aureus. Scarlet fever is therefore the least likely to be caused by gram-positive cocci in clusters.
What 4 toxin-mediated diseases does S. aureus cause?
S. aureus exotoxin-mediated diseases include
- Toxic shock syndrome
- Scalded skin syndrome
- Food poisoning
- Bullous impetigo
What are these pictures of? What is the most common cause?
S. aureus - diseases
- Staphylococcal Toxic Shock Syndrome
- Markedly red tongue - common finding in TSS
- Whole body rash
- Desquamation especially on the palms & soles (2-3 wk after the onset)
