Micrograph of Pseudomona fluorescens showing vesicles in the matrix surrounding the cell and also forming at its cell membrane. Photo by Hannah McMillan.
What are vesicles?
Vesicles are tiny sacs containing chemicals that are produced by all cells in every type of environment. Any molecule or cell material that is too large to diffuse through the cell membrane needs to be "packaged" into vesicles in order to be imported into the cell or exported out of it. The "envelope" of these molecular packages is made of cell membrane material (phospholipid bilayer) and can fuse with the cell membrane.
Some examples of special types of vesicles are vacuoles (which contain mainly water and are found in plant cells) and lysosomes (which contain enzymes that help cellular eating and breaking down damaged organelles).
Vesicles that are secreted by the cell are called Extracellular Vesicles (or EVs). In micrograph to the left, you can see an EV being produced by a bacterium and getting ready to pinch off from the cell. They have a wide range of roles and are involved in cellular processes such as communication, digestion, waste disposal, cell defense, reproduction and interaction with the environment.
How big are vesicles?
Vesicles vary in size, but they are all found within the "nanoscale" (see the image below). A nanometer (nm) is one billionth of a meter (there are 1,000,000,000 nanometers in a meter, or 109), it is a tiny measure, much smaller than a human hair, much smaller than a cell, even smaller than a virus (as you can see in the picture below, a nanometer is about the size of a glucose molecule). Vesicles can be as small as 20nm, so enough to fit about 20 glucose molecules, or as big as 1000nm (the size of some bacteria!). The vesicles this project focuses on (labeled as "EVs" on the image below are microbial vesicles (around 20nm to 200nm in diameter), usually much smaller than the eukaryotic vesicles produced by plant and animal cells.
Nanoscale showing relative sizes of common molecules, cells and objects. Image by Marta Toran.
How do microbes use vesicles?
As in the animal and plant kingdoms, vesicles have a wide variety of uses among the microscopic kingdoms of bacteria, viruses and fungi. The main role of vesicles in general is as carriers of chemical packages (eg. proteins, lipids, nucleic acids and other large molecules) from one the parent cell where they originate, to another location. Some vesicles are "addressed" to specific cells (using chemical tags that act as address labels on a package), and others are released into the surrounding environment and might have an effect on cells or organisms that happen to be around. Microbes use EVs in many different ways including sticking to surfaces, communicating, and invading other cells. Let's look at some specific examples:
Bacteria use vesicles to protect themselves against host immune responses. For example, an immune response could be macrophages in a human trying to fight off a bacterial infection such as staph. The bacteria (Staphylococcus aureus) produces toxin filled vesicles as "chemical bombs" to try to destroy the white blood cells fighting it, or as shields to protect itself from them.
<Illustration by Hannah McMillan.
Vesicles are also used by microbes to steal nutrients from host cells or other bacteria. This is the case of the bacterium that causes salmonella (Salmonella enterica) which steals fatty acids and glucose from host animal cells. Another example, Legionella pneumophila (the cause of Legionnaire's disease), steals amino acids from its host cells.
Illustration by Hannah McMillan. >
Some microorganisms produce vesicles to help them deal with environmental stress. For example, sources of environmental stress faced by bacteria could be extreme heat, lower availability of water or nutrients, oxygen depletion, drastic fluctuation in the pH of their surroundings or presence of antibiotics. Increased vesicle production has been observed for example in the bottom or innermost layers of biomats, where bacteria have to adjust to less access to oxygen and nutrients than those in the outermost layers.
<Illustration by Hannah McMillan.
Why study vesicles in the environment?
Vesicles have been widely studied in animal systems, specially within mammals. In 2013, the Nobel Prize in Physiology or Medicine was awarded to Americans Rothman, Scheckman and Südhof for their work on how vesicles are assembled and their functions in yeasts and humans. Vesicle research has led to better understanding of cellular processes that play a role in diseases such as Alzheimer's, diabetes and various cancers, which have been linked to faulty vesicle production, as well as development of targeted cell therapies and vaccines. Research into how vesicles move around the environment however, is still fairly new and there is still much to be learned in this area.
A plant uses vesicles as a shield to protect itself against a pathogen. Illustration by Hannah McMillan.
This project focuses primarily on how vesicles move in soil. The soil rhizosphere ecosystem is the area soil surrounding plant roots. This zone contains communities of microorganisms such as bacteria, fungi and a variety invertebrates interacting with each other and with the plant. It is a complex system and vesicles seem to have an important role in mediating these interactions. A better understanding of vesicle production and fate (what happens to them after they are produced, such as the effect do they have on other cells) in soil, could lead to new solutions for agriculture and environment management. Being able to harness vesicle-mediated interkingdom communication could result in development of naturally derived pesticides, seed treatments to reduce disease, irrigation additives that could boost plant immune systems, etc. Artificial vesicles could potentially also be engineered to deliver specific nutrients, act as biomarkers to indicate crop disease or water quality, and even help to manage pollution by sequestering heavy metals and other soil contaminants.
What is the Vesicle Project?
The "Vesicle Project" is a scientific research project involving a collaboration between scientists of different disciplines at Duke University (Durham, NC) and Appalachian State University (Boone, NC). These scientists are environmental engineers, biochemists, molecular biologists, ecologists and environmental scientists working together to understand how vesicles are used in communication between species of different life domains, in particular microorganisms in soil. For more information about the team members, visit the Science Team page.
The full title of the project is Convergence RAISE: Harnessing Extracellular Vesicle mediated Interkingdom Communication.
This project was funded through the support of the National Science Foundation (NSF). Any opinions, findings or conclusions are those of the authors and do not necessarily reflect the views of the NSF.