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HMS Immunology and Microbiology > Gram Positive Cocci > Flashcards

Gram Positive Cocci Flashcards

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Biology 101 flashcards
1
Q

Prevention of S. pyogenes infection

A

As of yet, there is no vaccine.

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2
Q

Coagulase Test

A

Organism is mixed with a small amount of plasma. A positive result is indicated by coagulation of the plasma, indicating that the organism produces coagulase

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3
Q

__% of people who are sick with Staph. aureus are sick with the strain they are colonized with.

A

80% of people who are sick with Staph. aureus are sick with the strain they are colonized with.

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4
Q

Why is capsule particularly imporant for a foreign organism to survive in the bloodstream?

A
  1. Complement is everywhere in the bloodstream
  2. Evasion of spleen phagocytes
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5
Q

Staphylococci

A

Arranged in clusters. Express catalase.

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6
Q

Simple test for staphylococci

A

Add one drop of H2O2 and analyze for catalase activity (O2 production appears as bubbling)

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7
Q

Factors that may predispose to pneumonia

A

Aspiration that can occur with loss of consciousness, cigarette smoking, alcohol consumption, viral infections, or excess fluid in the lungs

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8
Q

Hemolytic classification of streptococci

A
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9
Q

Classification of Streptococci and Enterococci

A

Hemolytic pattern, Group-specific antigens (Lancefield serotype classification), biochemical or molecular tests (to determine species).

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10
Q

Diseases caused by S. pyogenes

A
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11
Q

MRSA

A

Methicillin-resistant Streptococcus Aureus

Increasingly common. Require treatment with vancomycin.

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12
Q

Your patient comes in with symptoms of diarrhea and vomitting. He tells you that he recently had old meat which he thought may have gone bad, but said this couldn’t have been the cause since he cooked it thoroughly to be sure it was okay. What happened?

A

Cooking doesn’t do the job. Staphylococcal enterotoxins are heat stable, not all of them will be destroyed by cooking.

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13
Q

Three major staphylococci species

A

Staphylococcus aureus (cat +, coag +)

Staphylococcus epidermidis (cat +, coag -)

Staphylococcus saprophyticus (cat +, coag -)

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14
Q

Treatment of S. pneomoniae infection

A

The first line is vaccination. If infection does occur:

Penicillin and 3rd generation cephalosporins are usually effective, but resistance is growing. This resistance is β-lactamase independent, so β-lactamase inhibitors are no good. All S. pneumoniae are sensitive to vancomycin

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15
Q

Staphylococci are arranged in ____

A

Staphylococci are arranged in clusters

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16
Q

Staphylococcus saprophyticus almost exclusively cause ______

A

Staphylococcus saprophyticus almost exclusively cause urinary tract infections.

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17
Q

Staphylococcus damage mechanism

A
  • protein A, cell wall component that binds Fc region of IgG.
  • Catalse counteracts killing by leukocyte-derived ROS
  • Coagulases convert fibrinogen to fibrin, clotting to avoid phagocytosis
  • More than 90% are encapsulated
  • Secrete pore-forming toxins, including hemolysins and Panton-Valentine leukocidin, which kill neutrophils
  • Most strains also make lipases, proteases, deoxyribonucleases
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18
Q

Clinically significant streptococci

A
  • Streptococcus pyogenes*
  • Streptococcus pneumoniae*
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19
Q

Most streptococci form ____ chains, with the exception of ____.

A

Most streptococci form long chains, with the exception of streptococcus pneumoniae.

Streptococcus pneumoniae is arranged as pairs or short chains.

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20
Q

Transmission of S. pneumoniae

A

Respiratory droplets. Highly efficient. Thankfully it is usually quite innocuous

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21
Q

Toxic shock syndrome symptoms and treatment

A

a sunburn-like rash (erythroderma), bright red eyes (conjunctivitis), vomiting and diarrhea, and significantly elevated creatinine (acute kidney failure)

Treat with clindamycin and IVIG

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22
Q

Why is it particularly important to give antibiotics for strep throat, even if it is not a particularly severe case?

A

Antibiotics significantly reduce the risk of acute rheumatic fever 1-4 weeks post-infection. Unfortunately, this does not reduce the risk of acute poststreptococcal glomerulonephritis.

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23
Q

S. pyogenes-induced necrotizing fasciitis and myositis

A

1–3 per 100,000 in the United States, very rare

S. pyogenes reaches fascia between the skin and muscle. Enables rapid spread of S. pyogenes along this plane. Streptokinase enables cell to travel through clotts so it cannot be cut off, and its degrative enzymes and streptolysins S and O​ destroy whatever is in its path.

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24
Q

Most common S. pneumoniae infection manifestations

A

Pneumonia

Meningitis

Otitis media (middle ear infection)

Sinusitis

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25
Q
A

Staphylococcus

26
Q

S. pyogenes

A

Found in the nasopharynx and skin of ~20% school-aged children, lasting weeks or months at a time. Usually asymptomatic.

Person-to-person spread by respiratory droplets or skin-to-skin contact.

27
Q

Diagnosis of S. pneumoniae infection

A

Pneumococcal infection is often suspected for pneumonia.

Sputum (pneumonia) or CSF (meningitis) must first be obtained. If Gram stain yields neutrophils and abundant lancet-shaped (oval) Gram-positive diplococci, then S. pneuoniae is likely. Culture may be performed to confirm. For CSF, PCR may be performed.

28
Q

Based on symptoms alone, streptococcal pharyngitis cannot be reliably distinguished from ____.

A

Based on symptoms alone, streptococcal pharyngitis cannot be reliably distinguished from viral pharyngitis

29
Q

Mechanisms that typically prevent lung infection

A

The tortuous pathway that air and inhaled particles must follow to reach the lungs, the epiglottis that protects the airway from aspiration, the cough reflex, the mucociliary escalator, and alveolar macrophages that phagocytose bacteria and foreign particles, mucus IgA, the mucosal associated lymphoid tissues that provide adaptive immune defense.

30
Q

Prevention of S. pneumoniae infection

A

Occurence of S. pneumoniae-induced pneumonia and meningitis have both decreased due to the success of the S. pneumoniae vaccines.

Conjugate vaccine: protects against 13 of the most common childhood disease-associated capsular serotypes. Intended to prevent meningitis in children.

Polysaccharide vaccine: Capsule from 23 common disease causing capsular serotypes. Intended to prevent pneumonia and recommended for older adults at specific risk for infection.

31
Q

Prevention of Streptococcus aureus infection

A

Currently, there is no S. aureus vaccine, and attempts to develop vaccines are ongoing

32
Q

Streptococcal pharyngitis

A

strep throat

Acute (rapid) onset of sore throat, headache and fever in a school-age child. There swollen, tender cervical lymph nodes, and the oropharynx is often red with a grayish-white exudate on the tonsils.

Risk of acute rheumatic fever if not treated.

33
Q

Diagnosis of staphylococcus infection

A

Specimens from the site of infection should be collected for Gram-stain and culture.

34
Q

Gram Positive Cocci are separated into two major groups, ____ and ____, based on the arrangement of the cocci.

A

Gram Positive Cocci are separated into two major groups, staphylococci and streptococci, based on the arrangement of the cocci.

35
Q

Really the only important virulence factor of S. pneumoniae

A

The capsule!! Particularly immunongenic.

Non-capsular S. pneumoniae is not virulent

36
Q

Staphylococcal scalded skin syndrome

A

Mediated by exfoliative toxins A and B (serine proteases that cause separation of the layers of the epidermis at the desmosomes).

Life-threatening. Mainly affects neonates. Characterized by extensive sloughing of the skin.

37
Q

Rapid strep tests

A

Performed to differentiate a pharyngitis of streptococcal origin from one of viral origin.

Throat swab and then culture the organism on blood agar plates to assay for hemoylsins. Very specific, so a positive test immediately diagnostic for S. pyogenes, however not entirely sensitive, so a negative does not rule S. pyogenes out.

38
Q

Source of Staphylococcus infection

A

Colonizes the nostrils in 30% of individuals. Over 90% of people are colonized at some point in their life. Spreads by contact of fomites.

Infections are opportunistic and usually occur when one contracts the S. aureus from another individual’s microbiota.

39
Q

Resistance to ______ has never been detected in GAS

A

Resistance to penicillin has never been detected in GAS.

Thank god.

This makes penicillin the absolute 100% safest go-to treatment for nonallergic patients. For individuals allergic to penicillin, macrolide antibiotics are appropriate; however, erythromycin-resistant GAS have been found.

40
Q

S. pyogenes usually remains localized to _____.

A

S. pyogenes usually remains localized to the area of initial infection.

As it cannot penetrate intact skin.

41
Q

Acute poststreptococcal glomerulonephritis

A

Occurs 1-4 weeks after infection. May follow either a pharyngitis or skin/soft tissue infection. Treatment with antibiotics does not affect the occurrence of this complication.

Immune complexes of streptococcal proteins and antibodies are deposited in the glomerular basement membrane and lead to complement fixation with subsequent renal injury. Have blood and elevated protein in their urine, and they may go on to renal failure and require dialysis.

42
Q

M protein

A
43
Q

Staphylococcus aureus-mediated disease

A

Pyogenic (pus-forming) infections:

Most lead to abscess formation (pus pockets). Can spread to subcutaneous or submucosal tissue and cause diffuse tissue inflammation (“cellulitis”). May infect nearly any site in body: skin, lungs, heart, bone, etc, etc.

Exotoxin infections:

Many S. aureus secrete exotoxins that cause symptoms at sites distant to infection. May cause Toxic shock syndrome, staphylococcal scalded skin syndrome, or staphylococcal food poisoning.

44
Q
A

Streptococcus pneumoniae

45
Q

In addition to hemolysins and Panton-Valentine leukocidin preventing neutrophil-mediated clearance, they also. . .

A

. . .cause release of neutrophil granule contents, including lysosomal enzymes, into the tissue. This may act as another mechanism of damaging the area of infection, or areas distant which are reached by the toxins.

46
Q

Development of an abscess

A

Complicated process involving both host and bacterial participation. Starts as acute inflammatory reaction.

47
Q

Staphylococcus aureus

A

Gram +, Catalase +, Coagulase +, clustering cocci. Colonies appear golden, hence aureus. Most virulent and clinically significant member of Staphylococcus genus.

Identified via coagulase test.

48
Q

How S. pyogenes adheres to the host nasopharynx

A

Uses adhesins and its hyaluronic acid (HA) capsule.

The HA capsule binds to CD44 (a homing cell adhesion molecule that participates in leukocyte rolling) found on the surface of pharyngeal epithelial cells and skin keratinocytes.

49
Q

Streptococcus pneumoniae-mediated damage

A
  • pneumolysin (an exotoxin, attacks membranes of host cells, damaging tissue and killing phagocytes, activates complement at distant sites to deplete total serum complement and inflammatory cells)
  • polysaccharide capsule (the primary virulence factor for S. pneumoniae)
50
Q

Treatment of staphylococcus infection

A

β-lactams, glycopeptides (vancomycin), antifolates, protein synthesis inhibitors (lincosamides, macrolides, aminoglycosides).

Usually responds well to smei-synthetic penicillins, like methicillin or the cephalosporines.

51
Q

S. pneumoniae is the most common cause of _____

A

S. pneumoniae is the most common cause of community acquired pneumonia (although many other organisms also cause this infection) AND bacterial meningitis (especially in adults, but also in children)

52
Q

Streptococci (and the similar enterococci) are arranged in _____

A

Streptococci (and the similar enterococci) are arranged in chains

53
Q

Many streptococcal species cannot be assigned to any serotype group because ____.

A

Many streptococcal species cannot be assigned to any serotype group because there are no antisera that react to their cell wall antigens

54
Q
A

Streptococcus

55
Q

In a case of acute rheumatic fever and acute poststreptococcal glomerulonephritis, diagnosis requires evidence of ______.

A

In a case of acute rheumatic fever and acute poststreptococcal glomerulonephritis, diagnosis requires evidence of a preceding S. pyogenes infection

Because the organisms may no longer be present, antibodies to GAS antigens can be used to establish a previous infection. The most common are the antistreptolysin O titer, antiDNase B titer, and Streptozyme® screen

56
Q

S. pyogenes-mediated damage

A
  • Adhesins
  • hyaluronic acid capsule (binds CD44 and mimicks human ECM, effectively camoflouging the bacterium as matrix and preventing phagocytosis)
  • M protein (binds fibrinogen, aiding adhesion and preventing alternative complement activation and phagocytosis)
  • Induces a very strong inflammatory repsonse, forming immune complexes
  • Expresses several proteases, DNAses, and hyaluronidase
  • Streptokinase (converts plasminogen to plasmin, enabeling travel through fibrin clots)
  • streptolysins S and O (hemolysins that lyse the membranes of various host cells)
57
Q

Lancefield group A streptococci

A

S. pyogenes

58
Q

Acute rheumatic fever

A

A significant cause of valvular heart disease in the preantibiotic era and continues to be in low-income and lowermiddle income countries.

Preceded 1-4 weeks by pharyngitis. Damage to heart valves persists even if acute inflammation ceases. May lead to scarring and valve stenosis. Turbulent valve flow also predisposes to future infections.

Possibly a result of cross-reactivity between similar proteins in S. pyogenes and tissues in the heart, synovium, and brian.

59
Q

Toxic shock syndrome

A

Mediated by toxic shock syndrome toxin-1 and staphylococcal enterotoxins serotypes B and C. All are superantigens which cross-link one chain (β-chain) of the TCR with Class II MHC on macrophages.

This causes massive and nonspecific cytokine release, thus inducing TNF-α-mediated shock.

60
Q

The reservoir of S. pneumoniae is _______

A

The reservoir of S. pneumoniae is humans colonized by the organism

It usually exists as a commensal organism.

61
Q

Staphylococcal food poisoning

A

Staphylococcal enterotoxin serotypes A through E and I cause food poisoning (think vowels). They cause intensive intestinal peristalsis, apparently by working directly on the vomiting control center of the brain.

They are heat stable, resist proteases, and are not necessarily destroyed by cooking

HMS Immunology and Microbiology (27 decks)

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