L.6 S. pneumoniae Bacterial Cell Wall

Question 1

What are the names of pathogen name for S. pneumoniae?

Answer 1

Streptococcus pneumoniae (also known as ‘pneumococcus’ or ‘pneumo’)

Question 2

How is S. pneumoniae classified?

Answer 2

Gram-positive bacteria

Question 3

What diseases are caused by S. pneumoniae?

Answer 3

Pneumonia, Meningitis, Sepsis, Otitis Media (ear infection)

Question 4

What are the symptoms of pneumonia caused by S. pneumoniae?

Answer 4

Shaking chill, fever, cough, discomfort, heavy breathing. Symptoms can be very subtle, but the onset of severe illness is abrupt, requiring immediate oxygen administration.

Question 5

How many deaths worldwide are caused by S. pneumoniae annually, and which populations are most affected?

Answer 5

At least 1.2 million deaths worldwide every year, mainly in patients >65, <5 years old, or immunocompromised individuals.

Question 6

Why is the number of deaths caused by S. pneumoniae underreported?

Answer 6

Pathologists often ignore polyps in the intestine and bowel as everyone past a certain age has them. Pneumonia often hits old people due to weaker immune systems but isn’t often what finishes people off; they are finished off by issues such as heart attacks, so the fact they have pneumonia wasn’t recorded.

Question 7

What are 3 major virulence factors of S. pneumoniae (Opal and Pall, 2009?

Answer 7

1. Polysaccharide Capsule: Prevents mucosal clearance, and sterically inhibits complement and immunoglobulin-binding to host receptors (Opal and Pall, 2009).
2. Pneumolysin: Cytolytic, TLR4 ligand, induces ciliostasis (cilia unable to move properly, leading to build-up of mucus for S. pneumo to eat) (Opal and Pall, 2009).
3. Choline Binding Protein: Binds to factor H blocking C3b fixation preventing opsonization (Opal and Pall, 2009).

+Many others

Question 8

How is S. pneumoniae transmitted?

Answer 8

Direct contact with respiratory secretions containing the organism, such as coughing or sneezing. It is not classed as airborne as humans have to be very close together for transmission.

Question 9

What are the treatment and prevention strategies for S. pneumoniae?

Answer 9

1. Antibiotics: Usually penicillin or related compounds.
2. Vaccination: Against capsule types.

Question 10

What is notable about the shape of S. pneumoniae when it drops on a surface?

Answer 10

The pointed ends cause it to fall on its side, which is important as its virulence factors are found on its side.

Question 11

What is the ‘Golden Rule’ for understanding a pathogen like S. pneumoniae?

Answer 11

The pathogen has no malice and it is not ‘trying’ to kill you; it is a consequence of their survival strategy.

Question 12

Where does S. pneumoniae grow in the human body?

Answer 12

In the nasopharynx (back of nose/top of throat).

Question 13

What is the composition of the cells in the nasopharynx where S. pneumoniae grows?

Answer 13

60% squamous (‘flat’) epithelial cells and 40% ciliated columnar cells which move mucus.

Question 14

What role do lymphocytes and seromucous glands play in the nasopharynx?

Answer 14

Lymphocytes buried in the submucosa along with seromucous glands that produce mucus, so the innate immune system patrols here regularly.

Question 15

What are the 2 carbon/nitrogen sources for S. pneumoniae?

Answer 15

1. Eats the mucus directly (a complex mixture of glycoproteins).
2. Glycoproteins on the surface of epithelial cells.

Question 16

What adaptations does the genome of S. pneumoniae have for its diet?

Answer 16

The genome is full of sugar enzymes and transporters.

Question 17

What is S. pneumo often deficient in and why?

Answer 17

Phosphate and Nitrogen due to high sugar diet (for carbon and nitrogen- sugars have high carbon but not all have nitrogen)

Question 18

Why is nitrogen and phosphate important for S. pneumoniae?

Answer 18

As it mainly lives off sugars (high in carbon, not always containing nitrogen), S. pneumoniae is very nitrogen and phosphate starved, meaning it is a high priority for it to get access to these two elements.

Question 19

How does S. pneumoniae cause meningitis?

Answer 19

There is a hole in the bottom of the skull between the nasopharynx and brain cavity. Soft material and lymphocytes carry S. pneumoniae across if migrating inwards.

Question 20

How does S. pneumoniae cause sepsis?

Answer 20

The nasopharynx is covered in blood vessels. Sepsis usually occurs after a lung infection (localized pneumonia) as the lungs are full of vasculature, allowing the bacteria to exit the lungs into the bloodstream.

Question 21

How does S. pneumoniae cause otitis media (middle ear infection)?

Answer 21

The inner ear has an empty air pocket that is open to air. A dramatic change in pressure opens the Eustachian tubes, allowing air to equalize pressure. These tubes open into the nasopharynx, enabling S. pneumoniae to move from the nasopharynx to the middle ear, causing infection. It is the number one cause of infection-based hearing loss in childhood.

Question 22

How does S. pneumoniae cause pneumonia?

Answer 22

Pneumonia is caused by the bacteria being inhaled into the lungs.

Question 23

Why might S. pneumoniae become invasive?

Answer 23

Becoming invasive could be a survival strategy, though in the long term it leads to mutually assured destruction.

Question 24

What is the reservoir for S. pneumoniae?

Answer 24

> 10% of adults and up to 60% of infants are colonized at any one time.

> Carriage lasts only weeks/months.

Question 25

Why does S. pneumoniae often have to be an aggressive infector?

Answer 25

S. pneumoniae does not have a stable niche, as a different 10% of adults are colonized often. It only lasts weeks to months in humans due to the immune system, so it has to be an aggressive infector (van der Poll and Opal, 2019).

Question 26

Where does S. pneumoniae sit on the continuum between mutualism and parasitism?

Answer 26

S. pneumoniae sits between commensalism and parasitism. If something happens during commensal colonization, it causes infection.

Question 27

Where do most bacteria sit on the continuum between mutualism and parasitism?

Answer 27

Most bacteria sit between mutualism and commensalism

Question 28

What factors allow S. pneumoniae to become parasitic?

Answer 28

Co-infection or immune system suppression while already colonized with S. pneumoniae allows it to become parasitic. Examples include HIV patients, young age (undeveloped immune system), old age (immune system perishes over life), and viral infection.

Question 29

How can we prevent invasive infection by S. pneumoniae?

Answer 29

By blocking colonization, we can prevent invasive infection (role of vaccine)

Question 30

What does the S. pneumoniae vaccine do to help prevent disease?

Answer 30

The vaccine primes the immune system to clear carriage, removing the risk of invasive disease.

Question 31

What is the target of the S. pneumoniae vaccine?

Answer 31

The capsule. There are 92 known capsule types (probably 100).

Question 32

What was the first significant S. pneumoniae vaccine, and when was it deployed?

Answer 32

The 7-valent vaccine in 2006 (targeted 7 types of the capsule)

Question 33

What happened after the 7-valent vaccine was deployed in 2006?

Answer 33

It worked well for some time, but after 2-3 years, it left the niche free for other variants to colonize; invasive disease returned to pre-vaccine levels.

Question 35

What was the second significant S. pneumoniae vaccine, and when was it produced?

Answer 35

The 13-valent vaccine in 2010.

Question 36

What happened after the 13-valent vaccine was produced in 2010?

Answer 36

The same thing happened as with the 7-valent vaccine: the niche was left free for other variants, and invasive disease returned to pre-vaccine levels.

Question 37

What was the third significant S. pneumoniae vaccine, and when was it introduced?

Answer 37

The 23-valent vaccine in 2014.

Question 38

What is the challenge with continuously adding more capsules to the S. pneumoniae vaccine?

Answer 38

The immunogenicity keeps decreasing after vaccination because 23 is a lot of bacteria to be administered at once.

Question 39

What additional challenge exists with different serotypes of S. pneumoniae and vaccination?

Answer 39

Different places have different serotypes of S. pneumoniae, making it difficult to cover all variants with the vaccine.

Question 40

What is the overall trend with adding more capsules to the S. pneumoniae vaccine?

Answer 40

The vaccine gives diminishing returns each time more capsules are added.

Question 41

What are the primary treatment approaches for S. pneumoniae?

Answer 41

Penicillin-related drugs, such as ampicillin, cephalosporins, and other β-lactams.

Question 42

What are the British Thoracic Society (BTS) recommendations for treating S. pneumoniae?

Answer 42

500 mg of ampicillin every 6 hours.

Question 43

What are the American Thoracic Society (ATS) recommendations for treating S. pneumoniae?

Answer 43

Penicillin G 6-10 million units per day.

Question 44

What are the European Respiratory Society (ERS) recommendations for treating S. pneumoniae?

Answer 44

Penicillin or third-generation cephalosporin with or without macrolide, as antibiotic resistance is more prevalent in Europe.

Question 45

Why is the bacterial cell wall a good antibiotic target for S. pneumoniae?

Answer 45

Bacterial cells are under very high osmotic pressure because the contents of the cell are much higher than outside, so water wants to travel inside. This pressure is usually balanced by tensile forces in the cell wall. If the cell wall is compromised, it causes cell lysis.

Question 46

What are the challenges faced by the bacterial cell when building its cell wall?

Answer 46

> To build the cell wall from the inside out while maintaining its shape and without compromising its integrity—all at the maximum possible rate; otherwise, it will be outcompeted.

> It is like building a space station in space; any slight hole will be catastrophic

Question 47

How do antibiotics like penicillin work against S. pneumoniae?

Answer 47

They interfere with the synthesis of the bacterial cell wall, leading to cell lysis due to the high internal osmotic pressure.

Question 48

What happens to a bacterium cell when its cell wall is compromised?

Answer 48

It causes cell lysis.

Question 49

What is peptidoglycan composed of?

Answer 49

Peptidoglycan is composed of repeating disaccharide units cross-linked together by short peptides.

Question 50

What is the function of peptidoglycan in bacterial cells?

Answer 50

Peptidoglycan is the material resisting the osmotic pressure in bacterial cells.

Question 51

What is the precurser of the peptidoglycan biosynthesis pathway?

Answer 51

Precursor Synthesis (Lipid II): Lipid-linked precursor, lipid II, has a peptide stem with 2 terminal D-alanine amino aids.

Question 52

What enzyme is responsible for polymerizing glycan strands in peptidoglycan biosynthesis?

Answer 52

Transglycosylase (TG)- generates repeating disaacharide units

Question 53

What is the role of transglycosylase (TG) in peptidoglycan biosynthesis?

Answer 53

Transglycosylase polymerizes glycan strands by taking lipid II and polymerizing it into glycan strands (making long chains of repeating disaacharides)

Question 54

What enzyme cross-links the peptidoglycan strands?

Answer 54

Transpeptidases (TP).

Question 55

How do transpeptidases (TP) function in peptidoglycan biosynthesis?

Answer 55

They cross-link the peptidoglycan. Lipid II glycan strands are cross-linked, and D-alanine is used as the leaving group for this reaction (transfer of amine bond from one peptide to another, forming a cross-link) where transpeptidase cleaves the terminal D-ala amino acid, allowing the remaining D-ala to form a peptide bond with the adjacent glycan strand peptide stem

Question 56

How does penicillin act in the peptidoglycan biosynthesis pathway?

Answer 56

Penicillin specifically inactivates transpeptidase enzyme activity by acting as a chemical mimic of the D-ala D-ala leaving group, blocking transpeptidation through irreversible competitive inhibition.

Question 57

What happens when penicillin binds to enzymes with transpeptidase activity?

Answer 57

Transpeptidase will recognize the similar structure of penicillin and try to transpeptidate it, leading to penicillin covalently linking to TP, leaving it useless. The only way to restore this is by making a new TP, but by this time, the bacterium has already died as transpeptidase cannot cleave the terminal D-ala peptide so the other D-ala amino acid cannot form a peptide link between glycan strands at the peptide stem

Question 58

What is the consequence of penicillin acting through irreversible competitive inhibition?

Answer 58

As a new transpeptidase needs to be made, Peptidoglycan synthesis still partially carries on through the transglycosylation pathway, resulting in broken peptidoglycan (non-cross-linked peptides) in the structure. Due to high osmotic pressure, after some of this is made, it causes lysis.

Question 59

What are Penicillin Binding Proteins (PBPs)?

Answer 59

The enzymes which build/remodel the cell wall were identified biochemically (assay) as they are all covalently modified through penicillin binding.

Question 60

How were PBPs identified?

Answer 60

PBPs were identified by running penicillin binding proteins on gel, named 1,2,3,4; as the resolution of the gel got better and better, the bands split into different enzymes.

Question 61

How many PBP proteins does S. pneumoniae have?

Answer 61

S. pneumoniae has 6 PBP proteins.

Question 62

What are the 6 PBPs in S. Pneumo?

Answer 62

1. PBP2B
2. PBP1A
3. PBP1B
4. PBP3
5. PBP2X
6. PBP2A