Bacteria Chapter 12: Capsule as Virulence Factor

Question 1

What do polysaccharide capsules do?

Answer 1

• prevents phagocytosis — attachment and engulfment by neutrophils or macrophages
• prevent other host cellular immune responses
• prevent deposition of antibodies and complements on bacterial surface — avoids complement-mediated killing

Question 2

What bacteria uses capsule to avoid phagocytosis by host macrophages?

Answer 2

Streptococcus pneumoniae

Question 3

What are S. pneumoniae characteristics?

Answer 3

• non-motile
• non-sporulating
• Gram-positive

Question 4

What is S. pneumoniae sensitive to?

Answer 4

• heat
• cold
• drying

Question 5

How is S. pneumoniae transmitted?

Answer 5

close person-to-person contact with respiratory droplets

Question 6

What are the major virulence factors of S. pneumoniae?

Answer 6

• polysaccharide capsule
• adhesins
• pneumolysin

Question 7

What is pneumolysin?

Answer 7

toxin that binds to cholesterol in host cell membrane and disrupts them by forming pores

Question 8

What are some diseases caused by S. pneumoniae?

Answer 8

• otitis media (ear infection)
• pneumonia (inflammation of lungs)
• bacteremia (bacteria in blood)
• meningitis (inflammation of membrane surrounding brain and spinal column)

Question 9

S. pneumoniae causes more deaths than any other vaccine-preventable disease

Answer 9

-

Question 10

What does S. pneumoniae do as a transient member of microbiota?

Answer 10

colonizes nasopharynx of 40% of healthy adults and children with no adverse effects

Question 11

What does S. pneumoniae do as an opportunistic pathogen?

Answer 11

common bacterial complication of influenza and measles infections

Question 12

Why are smokers more at risk of S. pneumoniae infection?

Answer 12

cigarette smoke can injure airways and damage cilia

Question 13

Why are alcoholics at increased risk of S. pneumoniae infection?

Answer 13

alcohol acts as sedative and can diminish reflexes that trigger coughing and sneezing

Question 14

What are the minimum criteria for identification of S. pneumoniae?

Answer 14

(observe sputum, blood, or CSF)

• Gram-positive staining
• hemolytic activity
• bile sensitivity

Question 15

What are the different types of hemolysis that S. pneumoniae may cause?

Answer 15

• alpha hemolysis
• beta hemolysis
• gamma hemolysis

Question 16

What is alpha hemolysis?

Answer 16

in aerobic conditions:

partial or greening hemolysis from reduction in RBC hemoglobin

Question 17

What is beta hemolysis?

Answer 17

in anaerobic conditions:

complete lysis of RBCs surrounding colony

Question 18

What is gamma hemolysis?

Answer 18

lack of hemolysis of RBCs around the colony

Question 19

What problems may arise when diagnosing S. pneumoniae?

Answer 19

• bacteria can be difficult to grow, therefore a negative result does not necessarily rule out pneumonococcal pneumonia
• positive culture from sputum sample may be the result of contamination from bacteria in the mouth of a colonized patient

Question 20

What is used for treatment of S. pneumoniae?

Answer 20

• penicillin & penicillin-derivatives used to work well, but penicillin-resistant strains are becoming more common
• erythromycin or tetracycline are alternative treatments, but there’s also resistance to these

Question 21

What does the current vaccine for S. pneumoniae contain?

Answer 21

purified, capsular polysaccharide antigens of 23 types of S. pneumoniae

• cross-reactivity occurs for several of the capsular types

Question 22

What is cross-reactivity?

Answer 22

antibody against one serotype will bind to another serotype because of a shared epitope

Question 23

Is the current S. pneumoniae vaccine given to everyone?

Answer 23

no — only high-risk individuals

Question 24

Does the current vaccine elicit a strong immune response? Why?

Answer 24

no — polysaccharides are less immunogenicity than proteins

especially true in high-risk groups

Question 25

What does the conjugate vaccine for S. pneumoniae consist of?

Answer 25

polysaccharide antigens covalently bonded to proteins (to make the vaccine more immunogenic) covers only 7 capsular types

Question 26

Compare the mechanism of immunogenecity of polysaccharide vaccines (PPV) and polysaccharide-protein conjugate vaccines (PCV).

Answer 26

PPV: T cell independent B cell activation - B cell → plasma cell →IgM - not effective in children < 2 years old - no memory effect PCV: T cell dependent B cell activation - APC + T cell interaction + B cell interaction → plasma cell → IgG - B cells also become memory B cells - effective in children < 2 years old - has memory effect

Question 27

Is the capsule of S. pneumoniae toxic?

Answer 27

non-toxic and does not induce inflammatory response

Question 28

How can different capsular types be identified?

Answer 28

serological testing - sample of bacterial colony is mixed with a specific monoclonal antibody - positive test is indicated by capsule swelling, which can be viewed under a phase-contrast microscope

Question 29

What is the essential determinant of the virulence of S. pneumoniae? Why?

Answer 29

capsule - encapsulated = pathogenic - mutant without capsule = non-pathogenic

Question 30

What is the primary reason S. pneumoniae can invade and grow?

Answer 30

their resistance to host phagocytic responses - antibodies and/or complement cannot bind to bacterial cell surface - capsule interferes with phagocytosis by preventing C3b opsonization of bacteria and formation of C3 convertase that would normally result in MAC

Question 31

Can S. pneumoniae be part of the microbiota of healthy people?

Answer 31

yes

Question 32

The vaccine against S. pneumoniae consists of only 23 of the 90 possible capsule types, yet it induces humoral immunity to more than the 23 capsules types present in the vaccine. How would you explain this observation?

Answer 32

- many of the polysaccharides may share a common structure - antibody produced to one specific polysaccharide may cross-react with other polysaccharides that have similar molecular structure - binding of these cross-reacting antibodies to bacteria with capsule types not present in the vaccine may not be precise, but is adequate to opsonize bacteria so that phagocytic cells can engulf it