Staphylococcus

Question 1

What are 3 general characteristics of Staphylococci?

Answer 1

1. Gram (+) cocci, arranged as grape-like clusters
2. Facultative anaerobes
3. Catalase (+)

Question 2

What test can be used to differentiate Staph from Strep?

Answer 2

Catalase test
Staph are catalase (+)
Strep are catalase (-)

Question 3

What are some general features of Staph in regards to infection? (Where are they commonly found? Do they survive well? Is antibiotic resistance a problem? What types of infections are they a common cause of?)

Answer 3

1. Pyogenic (pus forming) bacteria
2. Commonly found on the skin and mucus membranes of humans
3. Survive for long periods of time on fomites under harsh conditions
4. Common cause of community acquired (CA) and hospital acquired (HA) or nosocomial infections
5. Antibiotic resistance is a problem

Question 4

Staph can be separated into 2 classes based on what test/presence of…? What can those classes be further subdivided into?

Answer 4

1. Staph aureus: coagulase (+) and contains protein A, beta-hemolytic
2. Coagulase negative Staph (CoNS)
   CoNS can be classified based on their novobiocin susceptibility
   Novobiocin-susceptible: S. epidermidis, haemolyticus, hominis, lugdunesis, schleiferi
   Novobiocin-resistant: S. saprophyticus, xylosus

Question 5

Which CoNS is most similar to S. aureus with regards to its virulence?

Answer 5

S. lugdunesis

Question 6

Which CoNS is a large cause of UTI’s in sexually active women?

Answer 6

S. saprophyticus

Question 7

Which CoNS is a large cause of endocarditis?

Answer 7

S. epidermidis (think about IV drug users, common on the right side of the heart)

Question 8

Which CoNS are prevalent in nosocomial infections?

Answer 8

S. epidermidis and haemolyticus

Question 9

What is the most pathogenic species of Staph?

Answer 9

S. aureus

Question 10

What types of conditions/diseases might a S. aureus infection produce? 6 things

Answer 10

1. Skin/soft tissue => abscess
2. Osteomyelitis
3. Septic arthritis
4. Respirator infections (ie aspiration pneumonia)
5. Infective endocarditis (IVDU)
6. Bacteremia

Question 11

What type of Staph may lead to necrotizing pneumonia?

Answer 11

CA-methicillin resistant staph aureus (MRSA)

Question 12

Approximately 90% of S. aureus are resistant to what…? What is the best way to treat them?

Answer 12

Pencillin/Methicillin resistant because they produce a beta-lacatamase.
Best treated using a cephalosporin (ie. nafcillin)

Question 13

How might S. aureus cause problems without causing an infection?

Answer 13

Secretes enterotoxin A => severe vomiting and diarrhea with immediate onset (Food poisoning)
Toxic Shock Syndrome: foreign body (tampon or gauze pads) allow bacteria to grow to very high numbers which can then excrete a superantigen into the blood

Question 14

What are the 7 virulence factors associated with S. aureus? Give examples and function when possible

Answer 14

1. Fibronectin Binding Protein/Collagen Adhesin
2. Protein A (binds Fc part of Ig)
3. Cytolytic Exotoxins (hemolysins) (alpha, beta, delta, gamma, and Panton-Valentine leukocidin => nosocomial adolescent infections => necrotizing pneumonia)
4. Slime production (biofilm)
5. Superantigen Exotoxins
6. Enzymes (Coagulase, catalase, hyaluronidase, fibrinolysin)
7. Clumping factors A and B (fibrinogen binding)
   * Certain types can go intracellular in endothelial cells and macrophages

Question 15

What are the 2 primary methods by which S. aureus spreads?

Answer 15

1. Direct contact

2. Aerosol/Large droplet (associated with respiratory infections)

Question 16

There are 4 types of toxins that are produced by S. aureus. Name them with specific examples and describe how they behave and mediate disease.

Answer 16

1. Cytolytic Exotoxins: alpha, beta, delta, gamma, and Panton-Valentine leukocidin => nosocomial adolescent infections => necrotizing pneumonia
2. Exofoliative Toxins (ETA and ETB): act as superantigens, act as a serine protease that can split desmoglein-1, mediates staphylococcal scalded skin syndrome (SSSS)
3. Enterotoxins (A-X): act as superantigens, heat stable and resistant to low pH and gastric enzymes. Enterotoxin A associated with food poisoning, enterotoxin B and C associated with 50% of non-menstruation associated TSS
4. Toxic Shock Syndrome Toxin (TSST-1): act as a superantigen, responsible for 90% of menstruation associated TSS; also associated with the other 50% of non-menstruation associated TSS, is able to penetrate muscosal barriers

Question 17

Name 3 types of skin infections that are caused by S. aureus

Answer 17

1. Folliculitis/Styes: superficial infections of the hair follicles
2. Furuncles: Deep tissue infection (abscess)
3. Carbuncle: coalescent of furuncles that becomes even deeper in tissues with possible systemic effects such as fever and chills as the organism seeds into the blood

Question 18

What 3 types of cytokines produced in TSS cause the symptoms?

Answer 18

1. IL-1 => Fever
2. TNFa/b => capillary leakage (hypotension)
3. IFNy and IL-2 => rash

Question 19

What is the main carrier or reservoir for Staphylococcal food poisoning? What causes it?

Answer 19

No reservoir for this disease, just has human carriers

Caused by ingestion of pre-formed staph enterotoxins

Question 20

What is the antibiotic of choice for the treatment of MRSA? What is the next choice if the first one doesn’t work?

Answer 20

Vancomycin

If it is vancomycin resistant S. aureus (VRSA) then treat using linezolid, synercid, and daptomycin

Question 21

S. aureus that has become methicillin resistant as well as resistant to other penicillins has done so by what mechanism(s)?

Answer 21

1. Penicillin-binding protein 2a (PBP2a) is a transpeptidase that has been evolved to feature an alteration to the penicillin binding site that deceases affinity for both penicillin and cephalosporins.
   PBP2a is encoded by different cassettes on the chromosome by the SCCmec gene.
2. Beta-Lactamase: present in 90% of strains, hydrolyzes classic penicillins, is encoded on a plasmid

Question 22

There are 2 classes of PBP2a (SCCmec) genes? How do they differ and how are they classified?

Answer 22

1. Hospital acquired strain: Larger SCCmec types I-III (34-67kb) which have greater antibiotic resistance than CA strains
2. Community acquired strain: Shorter SCCmec type IV (~24kb) which has resistance to methicillin, beta-lactam antibiotics, but not other classes. So less resistant than HA strains

Question 23

What are 3 major sites of infection for CoNS?

Answer 23

1. Prosthetic heart valves (can also infect native heart valves but infection is more often associated with S. aureus)
2. Artificial heart valves => endocarditis
3. Catheters and shunts (ie foreign bodies) and account for ~50% of those types of infections

Question 24

Where is S. epidermidis and saprophyticus found? What are their main virulence factors?

Answer 24

Both are part of the normal flora

1. S. epidermidis: most abundant species on skin (can contaminate blood cultures or IV lines). Virulence via slime/biofilm formation
2. S. saprophyticus: Genitourinary muscosa flora. Virulence via epithelial adhesions

Question 25

What clinical diseases are most often implicated in S. lugdunesis and S. schleiferi infections?

Answer 25

Endocarditis of artificial valves (but can also be associated with native-valve endocarditis) and osteomyelitis