L1: Staphylococcus & Streptococcal infections

Question 1

What is the shape and stain of staphylococci?

Answer 1

Gram-positive cocci

Question 2

What is the arrangement of staphylococci?

Answer 2

In Clusters

Question 3

Are staphylococci motile?

Answer 3

No

Question 4

Are staphylococci spore-forming?

Answer 4

No

Question 5

What is the capsule of staphylococci?

Answer 5

• S. aureus forms microcapsule

- Other species may produce slime layer

Question 6

What are staphylococci classified into?

Answer 6

Coagulase positive: S. aureus.

Coagulase-negative: S. epidermidis and S. saprophyticus.

Question 7

What is the habitat of Steph aureus?

Answer 7

• 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).

Question 8

What are the virulence factors of Staphylococcus aureus?

Answer 8

• Cell wall-associated proteins and polymers

- Enzymes and toxins produced by S. aureus

Question 9

What are the cell wall-associated proteins and polymers of staphylococcus aureus?

Answer 9

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

Question 10

What are the enzymes of staphylococcus aureus?

Answer 10

“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.

Question 11

What are staphylococcus aureus exotoxins?

Answer 11

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.

Question 12

What are the types of diseases caused by Staphylococcus aureus?

Answer 12

Suppurative (pus-forming) infections

Toxigenic diseases

Question 13

What are suppurative infections caused by Staph aureus?

Answer 13

✓ Skin lesions: Boils, styes and furunculosis.

✓ Invasive infections: Pneumonia, meningitis, UTI, bacteremia, endocarditis, etc.

Question 14

What are toxigenic diseases caused by staph Aureus

Answer 14

Scalded skin syndrome

Food poisoning

Toxic shock syndrome (TSS)

Question 15

What is a clinical picture of scaled skin syndrome?

Answer 15

Widespread blistering and loss of the epidermis in neonates.

Question 16

What is the type of food that causes food poisoning with staph aureus? And what is the incubation period?

Answer 16

Diary milk products, 1-6 hours

Question 17

What is the clinical picture of food poisoning with staph aureus?

Answer 17

Nausea, vomiting, and slight diarrhea, but no fever.

Question 18

Where does toxic shock syndrome occur?

Answer 18

Occurs in young females on using tampons

Question 19

What is the clinical picture of toxic shock syndrome?

Answer 19

Fever, vomiting, diarrhea, desquamation of skin and

hypotension.

Question 20

What are the samples used for the diagnosis of staphylococcus aureus?

Answer 20

Differ according to the site of infection (swabs, pus, sputum, C.S.F, blood, …).

Question 21

Direct film of Staph aureus

Answer 21

Stained with Gram stain for characteristic morphology.

Question 22

Culture of staph aureus

Answer 22

• 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.

Question 23

How are colonies of staph aureus identified?

Answer 23

• Film stained by Gram for characteristic morphology.
• Biochemical reactions:
  ✓ All staphylococci are catalase positive.
  ✓ S. aureus is coagulase and DNAase positive and ferment mannitol.

Question 24

What are the methods of diagnosis of Staph aureus?

Answer 24

• Direct film
• Culture
• Identification of colonies
• Antimicrobial susceptibility: to select the effective drug.

Question 25

What is the Treatment of staph aureus?

Answer 25

Penicillinase-resistant β-lactams.

Question 26

What are coagulase-negative staphylococci (CONS)?

Answer 26

• S. epidermidis

- S. saprophyticus

Question 27

What is the habitat of S. epidermis?

Answer 27

Normal flora of the skin, nonpathogenic.

Question 28

What are the diseases caused by S. epidermidis?

Answer 28

It may cause infections in immunocompromised individuals & infections on prosthetic implants such as heart valves and hip joints.

Question 29

What are the diseases caused by S. saprophyticus?

Answer 29

Causes UTI in young females.

Question 30

What are the differences between Staphylococcal species?

Answer 30

S. Aureus is positive to “AEH CDM” protein A, exotoxin production, hemolysis production, coagulase, DNase, and mannitol fermentation.

While S. Epidermedis and S. saprophyticus are negative to all of them

Question 31

How to differentiate between S. Epidermedis and S. saprophyticus?

Answer 31

S. saprophyticus tests positive to resistance to novobiocin.