L1: Staphylococcus & Streptococcal infections Flashcards
(31 cards)
What is the shape and stain of staphylococci?
Gram-positive cocci
What is the arrangement of staphylococci?
In Clusters
Are staphylococci motile?
No
Are staphylococci spore-forming?
No
What is the capsule of staphylococci?
- S. aureus forms microcapsule
- Other species may produce slime layer
What are staphylococci classified into?
Coagulase positive: S. aureus.
Coagulase-negative: S. epidermidis and S. saprophyticus.
What is the habitat of Steph aureus?
- It is a major pathogen for humans.
- Present as normal flora on skin & upper respiratory tract (10-40% of normal individuals carry S. aureus in the anterior nares).
What are the virulence factors of Staphylococcus aureus?
- Cell wall-associated proteins and polymers
- Enzymes and toxins produced by S. aureus
What are the cell wall-associated proteins and polymers of staphylococcus aureus?
Capsular polysaccharide:
Inhibits phagocytosis and chemotaxis
Protein A:
The major protein in the cell wall.
It binds to the Fc portion of IgG.
It inhibits opsonization
What are the enzymes of staphylococcus aureus?
“CCS - OTHER ENZYMES”
Coagulase: Causes clotting plasma to clot by converting prothrombin to thrombin which converts fibrinogen to fibrin coats the bacterial cell interferes with opsonization and phagocytosis.
Staphylokinase.
Catalase: Inactivates toxic H2O2 inside phagocytes enhance survival of S. aureus in phagocytes.
Other enzymes: Proteases, lipase, and DNase.
What are staphylococcus aureus exotoxins?
Membrane damaging toxins: Haemolysin and leucocidin.
Enterotoxins: Cause food poisoning when ingested, Heat and acid-stable
Toxic shock syndrome toxin (TSST-1): Causes toxic shock
syndrome.
Epidermolytic (exfoliative) toxin: Causes scalded skin syndrome in neonates.
What are the types of diseases caused by Staphylococcus aureus?
Suppurative (pus-forming) infections
Toxigenic diseases
What are suppurative infections caused by Staph aureus?
✓ Skin lesions: Boils, styes and furunculosis.
✓ Invasive infections: Pneumonia, meningitis, UTI, bacteremia, endocarditis, etc.
What are toxigenic diseases caused by staph Aureus
Scalded skin syndrome
Food poisoning
Toxic shock syndrome (TSS)
What is a clinical picture of scaled skin syndrome?
Widespread blistering and loss of the epidermis in neonates.
What is the type of food that causes food poisoning with staph aureus? And what is the incubation period?
Diary milk products, 1-6 hours
What is the clinical picture of food poisoning with staph aureus?
Nausea, vomiting, and slight diarrhea, but no fever.
Where does toxic shock syndrome occur?
Occurs in young females on using tampons
What is the clinical picture of toxic shock syndrome?
Fever, vomiting, diarrhea, desquamation of skin and
hypotension.
What are the samples used for the diagnosis of staphylococcus aureus?
Differ according to the site of infection (swabs, pus, sputum, C.S.F, blood, …).
Direct film of Staph aureus
Stained with Gram stain for characteristic morphology.
Culture of staph aureus
- Facultative anaerobes, optimum temperature: 37 °C, normal atmospheric CO2
- Can grow on ordinary media.
- S. aureus produce golden yellow endopigments.
- S. aureus causes β-hemolysis on blood agar.
- Selective media is mannitol salt agar, where S.aureus ferments mannitol and salt inhibits other normal flora.
How are colonies of staph aureus identified?
- Film stained by Gram for characteristic morphology.
- Biochemical reactions:
✓ All staphylococci are catalase positive.
✓ S. aureus is coagulase and DNAase positive and ferment mannitol.
What are the methods of diagnosis of Staph aureus?
- Direct film
- Culture
- Identification of colonies
- Antimicrobial susceptibility: to select the effective drug.
