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← Theoretical 1

Q23 — Gut Microbiome — Bacterial Crossing of the Blood-Brain Barrier

Theoretical 1 Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license
Nervous System Physiology Immune Physiology

The gut microbiome comprises a vast ecosystem of commensal bacteria, archaea, fungi, and viruses. The gut modulates CNS (central nervous system) functions via microbiota-derived metabolites, neuroactive substances, and gut hormones. Direct microbial invasion beyond the blood-brain barrier (BBB) is also possible: bacteria can traverse the endothelium via the transcellular pathway through brain endothelial cells, the paracellular pathway via disruption of intercellular junctions, or through infected phagocytes.

Figure 1. Diagram of paracellular and transcellular bacterial transport pathways across the blood-brain barrier. Figure 1. Paracellular and transcellular transport pathways across the blood-brain barrier.

Figure 2. Mechanisms used by four common meningitis-causing bacteria (N. meningitidis, E. coli, Group B Streptococcus, S. pneumoniae) to cross the blood-brain barrier. Figure 2. Mechanisms for four common meningitis-causing bacteria to cross the blood-brain barrier: N. meningitidis, E. coli, Group B Streptococcus (GBS), and S. pneumoniae.

Using the information and figure, determine whether the following statements are true or false:

A. Group B Streptococcus (GBS) bacteria can penetrate by endothelial layer breakdown when serine-rich bacterial protein (Srr2) causes rupture of intercellular connections.
B. All CNS pathogens have to cross an additional layer of BBB formed by the astrocyte endfeet.
C. E. coli bacteria can cross paracellularly, causing disruption of intercellular connections.
D. Bacteria S. pneumoniae can either enter transcellularly through endothelial monolayer disruptions caused by the bacterial production of pneumolysin and H2O2, or paracellularly via several interactions.
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Question reproduced from IBO 2022, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)