Question

Match each strategy for microbial invasion of the CNS with its description. ___________intercellular entry ___________transcellular entry ___________leukocyte-facilitated entry ___________nonhematogenous entry A. pathogen gains entry by infecting peripheral white blood cells B. pathogen bypasses the blood-brain barrier by travel along the olfactory or trigeminal cranial nerves C. pathogen passes through the cells of the blood-brain barrier D. pathogen passes between the cells of the blood-brain barrier

Short answer

Intercellular entry - D, Transcellular entry - C, Leukocyte-facilitated entry - A, Nonhematogenous entry - B

Step-by-step solution

Understanding the Modes of Microbial Invasion

Identify the descriptions given for each strategy by understanding the nature of the blood-brain barrier (BBB) and the typical pathways microbes might use to breach it.

Matching Intercellular Entry

Intercellular entry refers to the passage between cells. Match this strategy with the description that mentions passage between cells.

Matching Transcellular Entry

Transcellular entry refers to the passage through cells. Match this strategy with the description that describes pathogen passage through the cells of the blood-brain barrier.

Matching Leukocyte-Facilitated Entry

Leukocyte-facilitated entry refers to the infection and transportation of pathogens by white blood cells. Match this strategy with the description that involves infection of peripheral white blood cells.

Matching Nonhematogenous Entry

Nonhematogenous entry refers to the direct invasion of pathogens without traveling through the bloodstream. Match this strategy with the description that mentions travel along the cranial nerves.

Key concepts

Blood-Brain Barrier

The blood-brain barrier (BBB) acts as a highly selective semipermeable border that separates the circulating blood from the brain and extracellular fluid in the central nervous system (CNS). This barrier is crucial in maintaining the CNS's stable environment, which is essential for proper neural function.

The BBB is composed of tightly packed endothelial cells with tight junctions, a basement membrane, astrocyte end-feet, and pericytes. Due to its selective permeability, the BBB restricts the passage of substances, including many pathogens, from the bloodstream into the brain. However, some microorganisms have evolved strategies to circumvent the BBB and gain access to the CNS, which can result in infections such as meningitis or encephalitis.

Intercellular Entry

Intercellular entry is a strategy by which pathogens invade the CNS by passing between the cells that constitute the blood-brain barrier. Microorganisms utilize this route when they exploit the tiny spaces that can exist between the adjacent endothelial cells.

During inflammation or infection, the BBB can become less effective, and the tight junctions between these cells can loosen to allow immune cells to pass. Pathogens may also release toxins or use other mechanisms to disrupt these junctions, creating pathways to enter the brain tissue intercellularly. This mode of entry is an indirect breach of the BBB and is often characteristic of bacterial infections that induce inflammation, allowing the bacteria to infiltrate while the barrier’s integrity is compromised.

Transcellular Entry

In contrast to intercellular entry, transcellular entry involves a pathogen crossing the blood-brain barrier by penetrating through the endothelial cells themselves. This process entails the pathogen being taken up by a cell on one side of the BBB and then being transported across the cell body to be released on the opposite side.

Pathogens that use this method may have mechanisms to manipulate host cell transport machinery, allowing for their internalization and passage through the cellular barrier. Viruses often employ this method as they can hijack the host cell's internal machinery to cross the BBB, gain entry into the CNS, and establish an infection.

Leukocyte-Facilitated Entry

Some pathogens may gain entry to the CNS via leukocyte-facilitated entry. This strategy involves infecting or 'hitchhiking' on peripheral white blood cells (WBCs) that are naturally able to traverse the BBB.

Once the pathogen infects a WBC, it can use the cell's own mechanisms to cross the barrier as if it were 'undercover'. This type of entry is insidious because it exploits the immune system's regular function – WBCs are normally granted passage through the BBB to surveil for signs of infection or damage. Pathogens like the HIV virus use this method to invade the CNS and can remain hidden within WBCs to evade the immune response.

Nonhematogenous Entry

Moving away from the bloodstream, nonhematogenous entry refers to the invasion of pathogens into the CNS through structures that are directly connected to the brain, such as the cranial nerves. This entry mode does not involve crossing the traditional blood-brain barrier.

Pathogens may travel along peripheral nerves like the olfactory (smell) or trigeminal (facial sensation) nerves and gain access to the CNS that way. This tactic bypasses the circulatory route completely, exploiting a more 'trojan horse' style of infiltration. An example of this is the herpes simplex virus, which after initial infection remains dormant in sensory nerve cells and can later reactivate to enter the CNS directly.