chapter 12.2 case study, streptococcus pneumoniae capsule determines disease severity in experimental pneumococcal meningitis

aim

determine if capsule serotype directly affect streptococcus pneumoniae disease severity

hypothesis

Changing capsule serotype will affect disease severity in an infant meningitis rat model

background

Streptococcus pneumoniae: encapsulated bacterium that causes meningitis and pneumonia

• Their strains have many distinct variations or mutations in the capsule

• Serotypes: the different versions of strains. Diff in diff geographies, canada vs US 

• There are 90+ different serotypes

Stereotyping: lab technique to classify microorganisms based on their surface antigens (proteins, carbohydrates, lipids). This is how they distinguish different strains/ subtypes of a pathogen. Classifying microbes based on which version of antigen they express↓↓↓

• Can serotype bacteria based on surface antigens (proteins)

  • K antigen: capsule sugars. Group that finds k, it is found by detecting capsule sugars

  • O antigen: LPS (gram -ve). Gram +ve does NOT contain O antigens

  • H antigen: flagella

• Diff versions of the antigens have diff numbers

serotype 6B: collected from patient

serptype 7F: modified capsule version of 6B

step 1:

line graph:

1. rats infected with serotypes 6B or 7F

2. count survived rats over time

box plot

• rats infected with serotypes 6B or 7F

• clinical score is calculated at 21h

line graph

incr survival with serotype 7f than 6b

box plot

6b interquartile 2 to 4.5

• Lower score, worse infection

7b 4+

• More animals survive

Which capsule serotype is more virulent? Which causes the worse disease

6b serotype is more virulent. Lower = more harmful

1. Serotype: distinct variation within a species of bacteria, classified base on surface antigens

Virulent: severity/ harmfulness of a pathogen in causing a disease

step 2: look at capsule thickness before and after serotypes

When looking at serotype 6B, how does the capsule look after infection compared to before infection?

• Capsule becomes thicker, it is seen as darker as a result of infection

When looking at serotype 7F, how does the capsule look after infection compared to before infection?

• Gets thinner

Why is this happening

conclusion

• Some capsule serotypes exhibit more virulence than others, with 6B being more virulent than 7F

• Capsule thickness during infection correlated with virulence (6B had thick capsule and was more virulent in rats) (7F capsule became thinner/ denser during infection and was less virulent in rats)

recall// the hypothesis was that changing capsule serotype will affect disease severity in an infant meningitis rat model. Was this accurate?