Streptococcus pneumoniae infection differential diagnosis

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Interaction with Haemophilus influenzae

Both H. influenzae and S. pneumoniae can be found in the human upper respiratory system. A study of competition in a laboratory revealed that, in a petrì dish, S. pneumoniae always overpowered H. influenzae by attacking it with a hydrogen peroxide and stripping off surface molecules that H. influenzae needs for survival.

When both bacteria are placed together into a nasal cavity, within 2 weeks, only H. influenzae survives. When both are placed separately into a nasal cavity, each one survives. Upon examining the upper respiratory tissue from mice exposed to both bacteria, an extraordinarily large number of neutrophils immune cells were found. In mice exposed to only one bacteria, the cells were not present

Lab tests show that neutrophils that were exposed to already dead H. influenzae were more aggressive in attacking S. pneumoniae than unexposed neutrophils. Exposure to killed H. influenzae had no effect on live H. influenzae.

Two scenarios may be responsible for this response:

1. When H. influenzae is attacked by S. pneumoniae, it signals the immune system to attack the S. pneumoniae
2. The combination of the two species together sets off an immune system alarm that is not set off by either species individually.

It is unclear why H. influenzae is not affected by the immune system response.