Gram positive bacteria

Question 1

How to gram pos and gram neg bac differ?

Answer 1

Gram pos = 1 membrane = fuzzy part is (thicker) peptidoglycan layer = retains stain = purple

Gram neg = 2 membranes and thin cell wall between the 2 = perioplasmic space = do not retain stain = pink/red stain

Question 2

Staphylococcus - coagulase features?

Which is the most important coagulase?

Answer 2

• Coagulase pos or neg
• = enzyme produced by bac that clots blood plasma - fibrin clot formation around bac may protect from phagocytosis

Most important gram pos coagulase = S.aureus
Coagulase neg in opportunistic infecs

Question 3

Name staphylococcus’ normal habitat

Answer 3

Nose and skin

Question 4

Staph/aureus - how is it spread? What is MRSA resistant to?

Answer 4

Spread by aerosol and touch - people are carriers and shedders
MRSA is resistant to;
- beta-lactams
- gentamicin, erythromycin, tetracycline

Question 5

Staph.aureus virulence factors?

Answer 5

Pore forming toxins
- alpha-haemolysis and panton-valetine leucocidin (PVL) = causes haemorrhagic pneumonia

Protease
- exfoliatin (scalded skin syndrome)

Toxic shock syndrome toxin = stimulates cytokine release

Protein A = surface protein which binds Ig’s in wrong orientation

Question 6

Staph.aureus associated conditions?

Answer 6

Pyogenic - wound infec, abscesses, osteomyelitis, pneumonia, endocarditis

Toxin mediated - toxic shock syndrome, food poisoning, scalded skin syndrome

Coagulase negative - infected implants, endocarditis, septicaemia

Question 7

Coagulase-negative staphylococci - features of S.epidermidis and S.saprophyticus?

Answer 7

S.epidermidis = infecs in debilitated, prostheses, catheters
Ability to form persistent biofilms 
S.saprophyticus - acute cystitis 
- haemagglutinin for adhesion
- urease - kidney stones

Question 8

Types of streptococcus?

Answer 8

Alpha - partial, green

• e.g. S.intermedius
• H2O2 reacts with Hb

Beta - complete lysis

• e.g. S.pyogenes
• Haemolysins O and S

Non or gamma - no lysis
- e.g. some S.mutans

Question 9

How are streptococci classified?

Answer 9

Haemolysis, lancefielding typing, biochemical properties

Question 10

What is lancefield grouping (sero-grouping)?

Answer 10

Method of grouping catalase negative, coagulase neg bacteria based on bacterial carbohydrate cell surface antigens
Carbohydrate cell surface antigens - Lancefield A-H and K-V
Antiserum added to bac - clumping (white dots) = positive
Group A - S.pyogenes - important pathogen
Group B - S.agalactiae - neonatal infecs

Question 11

Examples of infecs caused by S.pyogenes?

Answer 11

Wound infec = cellulitis
Tonsilitis and pharyngitis 
Otitis media
Impetigo 
Scarlet fever

Question 12

Complications of S.pyogenes infections?

Answer 12

Rheumatic fever;

• Inflam disease of heart, joints, skin, brain - often follows strep. throat infec
• Antibody cross reactivity

Glomerulonephritis;
- Inflam disease sometimes following S.pyogenes infec

Assessment of risk estimated from anti-SLO titre

Question 13

What does Anti-SLO titre stand for ASLO? SLO?

Answer 13

ASLO - antibody against SLO
SLO - beta haemolysis
Streptococcal antibodies present in serum = rxn occurs
If ASO titre over 200 = agglutination of latex particles (black)

Question 14

What are S.pyogenes virulence factors?

Answer 14

Surface factors;

• Capsule - hyaluronic acid (protection)
• M protein = surface protein (encourages complement degradation)

Exported factors;
Enzymes;
- Hyaluronidase - spreading
- Streptokinase - breaks down clots
- C5a peptidase - reduces chemotaxis
Toxins;
- Streptolysins O and S - binds cholesterol
Erythrogenic toxin - SPeA - exaggerated response

Question 15

Where is S.pneumoniae a normal commensal?

Answer 15

Oro-pharynx - 30% of population

Question 16

What does S.pneumoniae cause?

Answer 16

Pneuomia, otitis media, sinusitis, meningitis

Question 17

Predisposing factors of S.pneumoniae?

Answer 17

Impaired mucus trapping
Hypogammaglobulinaemia
Asplenia

Question 18

What are the virulence factors of S.pneumoniae?

Answer 18

Capsule;

• Polysaccharide, antiphagocytic
• Polyvalent vaccine - 23 types = for >2yr olds at risk of infec

Inflam wall constituents;
- Teichoic acid bound to peptidoglycan

Cytotoxin;
- Pneumolysin

Question 19

What is viridans streptococci? Types? What does it cause?

Answer 19

Oral streptococci
Alpha or non-haemolytic
Cause caries and abscesses, deep organ abscesses
Important in endocarditis

Question 20

Name the viridans streptococci important in infective endocarditis?

Answer 20

S.sanguinis, S.oralis

Question 21

Which viridans streptococci is the most virulent?

Answer 21

Milleri group - S.intermedius, S.anginosus, S.constellatus

Question 22

Signs and symptoms of diptheria?

Answer 22

Thick, dirty white/grey/black/green coating in mouth, bull neck

Question 23

How does C.diptheriae spread? C.diptheriae can grow in the presence of..?

Answer 23

Spreads via droplets

Grows in the presence of potassium tellurite = selective method for isolating this bac from pts throats)

Question 24

What does C.diptheriae toxin do?

Answer 24

Inhibits PS
Toxin recognition (Elek plate)

Question 25

How to prevent diptheria?

Answer 25

Vaccination - toxoid (inactivated toxin)

DTP vaccine = inactivated diptheria toxin

Question 26

How to test for s.epidermidis?

Answer 26

Coag neg
DNase neg
Gamma haemolytic
Blood agar - white colonies

Question 27

How to test for s.saprophyticus?

Answer 27

Coag neg
gamma haemolytic
CLED - pale yellow colonies

Question 28

How to test for S.pneumoniae?

Answer 28

Alpha haemolytic

No lancefield antigens

Question 29

How to test for viridans streptococci?

Answer 29

Alpha or non-haemolytic

Question 30

How to test for streptococcus pyogenes?

Answer 30

Blood agar
Beta haemolytic
Lancefield A

Question 31

How to test for staph.aureaus?

Answer 31

Coag positive
NDAse positive - clear zone around colony
Aerobic
Blood agar - golden colonies
Beta haemolytic