Question

Bacterial Quorum Sensing, Pathogenicity Islands, and Secretion Systems

1. Briefly describe how bacterial quorum sensing may play a role in pathogenicity by:

a) Promoting initial colonization of a new host;

b) Enabling the bacterium to persist within that host; and

c) Allowing some of the bacteria to spread to a new location within a host or to a new host.

2. Briefly describe how the ability to produce a type 3 secretion system might play a role in a pathogen colonizing the body and causing an infection.

Answer 1

There are two questions here. According to the Chegg Guidelines, I am answering the first question with all the subparts.

Quorum sensing refers to a mechanism employed by many species to detect the population density in a given region. It is most widely studied in bacteria. This is essential so as to form a robust mechanism to survive in harsh conditions and obtain the maximum output. Shown below is a brief overview of the mechanism.

Quorum Sensing Molecule Population increase Bacterium Biofilm Formation Recolonization

As can be seen, initially when the population density is low, the bacteria release out certain molecules. The exact nature and chemical composition of the molecule depends on the species under investigation. For simplicity, let us call it Quorum Sensing Molecule (QSM). Each bacteria releases only a certain low amount of QSM in the external environment. They also have a sensor to detect the presence of QSM in the external environment. When the population density is low, the QSM concentration is low. By diffusion, the QSM spreads of into the solution and hence the bacteria has very low chances of recepting the QSM.

This promotes them to divide and rapidly multiply. As the density of bacteria increases, the density of QSM in the external environment also increases. This increases the reception of the QSM by the bacteria. QSM reception triggers a multitude of pathways in the bacteria. This induces the bacteria to slow their multiplication rate and form colonies. It also induces them to reduce the expression of QSM. Thus QSM forms a feedback loop.

a) Coming to the scenario of colonization of a new host, the initial concentration of bacteria is low. This allows bacteria to multiply rapidly thereby populating the host with the bacteria. These bacteria will release a lot of QSM thereby triggering the bacteria to colonize into the host tissue.

b) When the colony density increases beyond a threshold, the QSM density starts falling off and the bacteria form a metabolically active phase called biofilm with a protective covering around the colonies to prevent any damage by the host immune system. This allows the bacteria to persist within the host and spread the infection.

c) The decrease in QSM, in turn, triggers the bacteria along the periphery to divide and multiply. The bacteria produced as a result are free-floating and can drift off. On the other hand, the bacteria within the biofilm are adhered to the surface. The free-floating bacteria have the potential to move to a different tissue within the same host or infest a new host. Within the new host, the quorum sensing can start again leading to the colonization in this new host.