Flashcards in L22 Staphylococci and Related Gram Positive Cocci Deck (73):
What family does staphylococcus belong to?
Compare and contrast micrococcus and staphylococcus.
1. Both are commonly recovered from the environment or as commensals inhabiting skin and mucus membranes.
2. Certain staphylococci are pathogenic for humans.
3. Micrococci are not typically associated with disease.
What are 4 important species of staphylococci?
1. S. aureus
2. S. epidermidis
3. S. lugdunensis
4. S. saprophyticus
A 26 year old marine recruit presents to the base medic. Exam reveals large, painful, pus-filled lesions surrounded by erythema on both legs. What is the diagnosis?
Cellulitis/abscess of the lower legs caused by S. aureus (in this case)
Where is S. aureus normally found?
1. External environment
2. Skin and mucous membranes (anterior nares, intertriginous skin folds, perineum, axillae, vagina)
What is a very common mucous membrane location of S. aureus?
Anterior nares (20-40% of adults)
What are the major sites of infection for S. aureus?
Sites in which the organism is part of normal flora: skin, nose/throat, GI tract, urethra, vagina
What are 5 infections associated with S. aureus in the skin?
5. Post-surgical wound infections
What are 5 infections associated with S. aureus in the nose and throat?
2. Peritonsillar abscesses
5. Staphylococcal pneumonia
What are 6 infections associated with S. aureus in the GI tract, urethra, and vagina?
6. Pelvic abscess
What is a skin disease caused by infection of hair follicles, resulting in localized accumulation of pus and dead tissue?
Describe a furuncle.
Red, pus-filled, tender, warm, extremely painful, yellow or white point at center when ready to drain
What is an abscess larger than a boil, usually with one or more openings draining pus onto the skin?
Where are carbuncles most common?
On the back and the nape of the neck
What are the three broad strategies by which organisms cause disease?
What are the strategies used by staphylococcus to adhere, evade, and destroy?
1. Adhere: cell surface features (adhesins)
2. Evade: protein A
3. Destroy: cytotoxins and enzymes
What is a significant evasion virulence factor of S. aureus and what does it do?
Protein A - interferes with opsonization and ingestion of organisms by neutrophils (phagocytosis)
Describe how Protein A functions.
Normally, Ab recognize staphylococcus and then bind to a phagocyte via the Fc receptor. Protein A binds to the Fc portion of the Ab, preventing it from binding to the phagocyte.
What type of cytotoxin is utilized by S. aureus for destruction of other cells and what do they do?
Hemolysins; lyse RBCs and leukocytes
What are the 4 hemolysin used by S. aureus and what are their functions?
1. Alpha-hemolysin: pore formation
2. Beta-hemolysin: sphingomyelinase
3. Delta-hemolysin: surfactant (disrupt cell membranes)
4. Gamma-hemolysin: pore formation
Which of the 4 hemolysins used by S. aureus are most significant and associated with tissue damage?
What type of enzyme is used by S. aureus to destroy and what is its function?
Coagulase - converts fibrinogen to fibrin
What three enzymes are used by S. aureus to spread from cell to cell?
1. Fibrinolysin (break down fibrin clots)
2. Hyaluronidase (hydrolyze intercellular matrix of connective tissue)
3. Lipase (survive in sebaceous material)
An 18 year old girl presents to the ED 3 weeks after a knee injury while playing baseball. The knee "feels like jelly" and is extremely painful. She has a history of 'spider bite cellulitis' on her shin 1 year ago. What is the diagnosis?
Septic arthritis and osteomyelitis of the knee caused by S. aureus; note that the infection spread hematogenously (through the bloodstream)
Answer the following questions regarding the osteomyelitis case:
1. Patients often present with ___ and ___.
2. A blood culture is positive in ___% of cases.
3. In sexually active persons, what infection predominates?
1. Pain; fever
3. N. gonorrhoeae
What are important differential diagnoses of skin and soft tissue infections (SSTI's) that are compatible with S. aureus infection?
1. Abscesses, pustular lesions, boils
2. "Spider bites"
What are important differential diagnoses of severe diseases compatible with S. aureus infection?
1. Sepsis syndrome
3. Necrotizing pneumonia
4. Septic arthritis
5. Necrotizing fasciitis
What 5 factors predispose someone to serious infection with S. aureus?
1. Defects in leukocyte chemotaxis (congenital or acquired)
2. Defects in opsonization by Abs secondary to congential or acquired hypogammaglobulinemias or complement component
3. Defects in intracellular killing of bacteria following phagocytosis due to inability to activate the membrane bound oxidase system
4. Skin injuries (burns, surgical incisions, eczema, sports injuries)
5. Presence of foreign bodies (sutures, IV lines, prosthetic devices)
What are some congenital causes of defective leukocyte chemotaxis?
1. Wiskott-Aldrich syndrome
2. Down's syndrome
3. Job's syndrome
4. Chediak-Higashi syndrome
What are some acquired causes of defective leukocyte chemotaxis?
1. Diabetes mellitus
2. Rheumatoid arthritis
A 3 month old baby presents to her pediatrician. Over the past three days, her mother has seen erythema spread from around her mouth to cover her entire body. Slight pressure displaces the skin and large cutaneous blister (bullae) form soon after, following by sloughing off of the skin layer (desquamation of the epithelium). The blisters contain clear fluid. What is the diagnosis?
Staphylococcal scalded skin syndrome (caused by S. aureus)
What patient population is affected by staphylooccal scalded skin syndrome?
Neonates and young children
When does the epithelium heal in staphylococcal scalded skin syndrome?
7-10 days later, when Ab are present
How is staphylococcal scalded skin syndrome diagnosed?
A culture cannot be done from the fluid, as no bacteria or leukocytes will be present; however, the organisms are often found around the eyes, nose, and mouth, and could be recovered here for a culture.
What S. aureus toxin causes staphylcoccal scalded skin syndrome and what is its function?
Exfoliatins or exfoliative toxins; dissolves the mucopolysaccharide matrix of the epidermis, causing separation of skin layers
18 people attending a retirement party became ill approximately 3-4 hours after eating. Symptoms included nausea, vomiting, and diarrhea. Relatively few people had headache or fever. Symptoms lasted 24 hours and were associated with eating ham. What is the diagnosis?
Staphylococcal food poisoning (S. aureus)
What are the symptoms of staphylococcal food poisoning?
Severe vomiting, nausea, diarrhea, abdominal cramping, headache, but NOT fever
What is the incubation period of staphylococcal food poisoning?
Why is culture not helpful in staphylococcal food poisoning?
Contamination does not mean colonization (the illness is caused by intoxication, not infection) - nothing will appear in the culture.
What S. aureus toxin causes staphylococcal food poisoning?
Why does re/heating food not prevent staphylococcal food poisoning?
Enterotoxins are heat-stable - re-heating kills bacteria, but not toxin
True or false - staphylococcal food poisoning is not produced by ingested staphylococcus.
True - it is produced by the enterotoxin
A 15 year old girl was admitted to the hospital with a 2 day history of pharyngitis and vaginitis. She has vomiting and watery diarrhea. She is febrile and hypotensive on admission. She has a diffuse erythematous rash over the entire body, as well as disseminated intravascular coagulation (DIS) and severe thrombocytopenia. Chest radiograph showed bilateral in infiltrates. What is the diagnosis/
Staphylococcal toxic shock syndrome (S. aureus)
What are the symptoms of staphylococcal toxic shock syndrome?
Multi-organ toxicity, fever, hypotension, rash, entire skin desquamation
What S. aureus toxin causes toxic shock syndrome and how?
Toxic shock syndrome toxin-1 (TSST-1); the toxin non-specifically activates T-cells (MHC class II), leading to polyclonal T-cell proliferation and massive cytokine release
A 21 year old female presents to an urgent care clinic. Her symptoms include burning and pain during urination (dysuria). She is sexually active in a new relationship. What is the diagnosis?
Urinary tract infection caused by S. saprophyticus
What is a common cause of acute urinary tract infection in young women?
S. saprophyticus is the second most common cause of ___ (after E. coli) among women of college and child-bearing age.
How is S. saprophyticus identified?
Negative coagulase, resistant to novobiocin (indicator in the lab)
A 36 year old cocaine user presents to the ED with acute onset weakness of the right extremity. She had 10 weeks of fever, chills, malaise, and shortness of breath. On admission, she had tachycardia, hypotension, fever, and a pansystolic murmur. There are multiple positive blood cultures. What is the diagnosis?
Native valve endocarditis caused by S. lugdunensis
What are the key features of native valve endocarditis caused by S. lugdunensis?
1. Persistent bacteremia
2. Positive blood cultures
Where does S. lugdunensis colonize?
The human inguinal area
Which staph species is most commonly associated with native valve infection?
What is the most frequently isolated clinically significant coagulase-negative staphylocci and what is it associated with?
S. epidermidis; infections of indwelling devices
The virulence of S. epidermidis is related to the production of a ___.
What 5 lab tools can be used to identify staphylococci in the lab?
1. Gram stain
2. Surface structures
3. Protein A
4. The "a-ses"
Describe the gram stain of S. aureus.
Gram positive cocci in clusters
Describe the macroscopic culture of S. aureus.
Golden beta-hemolytic colonies; S. aureus carries hemolysins that lyse RBCs. When there is complete hemolysis (beta-hemolysis), the colony is golden and the plate can be seen through.
S. aureus grow well on ___ plates.
Salt (turn the plate yellow)
Describe the coagulase #1 test for S. aureus.
Involves unbound coagulase, which binds to prothrombin and catalyzes conversion of fibrinogen to fibrin, creating a clot in a test tube.
True or false - rare S. aureus may be coagulase-negative and some animal isolates may be tube coagulase-positive.
Describe the coagulase #2 test for S. aureus.
Invovles bound coagulase (clumping factors), which directly covnerts fibrinogen to fibrin, creating a positive slide coagulation
What is a latex agglutination assay used for?
Detecting protein A and coagulase, monoclonal Ab
What is responsible for hospital associated and community-associated staph infections?
Methicillin resistant S. aureus (MRSA)
MRSA is resistant to all ___ antibiotics.
How is MRSA resistance to beta-lactam antibiotics?
The mecA gene is carried on a mobile genetic element called staphylococcal cassette chromosome mec (SCCmec). This encodes for altered penicillin-binding protein 2a (PBP2a), which has decreased affinity for beta-lactam antibiotics and allows peptidoglycan synthesis even in the presence of beta-lactams.
Staph infections, including MRSA, occur most frequently among...
...patients in hospital and healthcare facilities who have weakened immune dsystems.
Healthcare-associated MRSA infections include what 5 things?
1. Surgical wound infections
3. Bloodstream infections
5. Central venous catheter line infections
In hospitals, the most important reservoirs of MRSA are ___.
Colonized or infected patients
True or false - hospital personnel can serve as a link for transmission between colonized or infected patients.
True or false - both infected and colonized patients contaminate hospital environments with the same relative frequency.
What can be done to control MRSA?
1. Hand hygiene
2. Contact/droplet precautions
3. Effective cleaning of the environment
4. Clean shared/dedicated equipment
5. Appropriate antibiotic use