The marvels of single celled organisms is that they are able to integrate all kinds of stimuli and make one grand decision that affects how they proceed. Bacteria do in one cell what we as humans do with billions. However, do bacteria contain the ability to think as a group or community?
The answer is absolutely. It is called quorum sensing. The pioneer for this research is Bonnie Bassler from Princeton University. Listening to her tell her story of the curiosity she felt when observing how and why a certain group of bacteria emitted light, or bio-luminescence is great. (Watch here). Through her investigation with a insignificant bacterium, Vibrio harveyi, she opened up a whole new field of microbiology.
Many bacteria synthesize signaling molecules that serve as messages to other bacteria saying, “I am here”. Since bacteria don’t have senses that we are familiar with like sight and hearing, these signaling molecules tell other bacteria who is around. When there aren’t a lot of bacteria sending out the signal, no big decisions are made. However, when enough bacteria are around to tell all other village members the approximate population, all village members act together to make a committed decision. In the case of V. harveyi it is the production of a light emitting molecule, but for other bacterial species it may be activation of pathogenicity. From the perspective of the bacterium, you don’t want to decide alone to make a big commitment like invading another organism. By taking a bacterial census through quorum sensing, these bacteria make a educated decision only when their population is high enough to make an impact. For some species, this critical number may be less than ten. However, in some cases, the population needs to be in the millions.
I think bacteria can teach us a very important lesson via quorum sensing: don’t go it alone. It takes a village.
Time again for My Tiny Highlight (MyTH) of a particular bacterium. This week is the infamous Pseudomonas aeruginosa. This trooper can be found almost anywhere on earth due to its ability to use all kinds of material as food including diesel and jet fuel. Most of us know it as a common infection you get while in the hospital because it is so darn hard to kill. If you know someone that suffers from cystic fibrosis, this bacterium can be fatal as it is commonly found in the lungs of these patients causing pneumonia. I personally learned about this bacterium in school when talking about hot tubs due to its resistance to disinfectants.
P. aeruginosa is one of the most famous microbes for forming biofilms. A biofilm is a group of cells that attach to each other and to some surface usually by secretion of sticky sugars from the cells. A common sticky sugar, or exopolysaccharide (EPS), excreted by P. aeruginosa is alginate, an uncommon sugar for bacteria since it is most commonly found associated with brown algae. Its existence in Pseudomonas has led to significant knowledge about the synthesis of alginate (alginate biosynthesis; another ring in the bacterial circus). This knowledge is leading to novel medical applications for alginate.
P. aeruginosa is also known for its role in a multicellular behavior which seems strange for a single-celled organism but is becoming more and more common. Quorum sensing was first described over 40 years ago. I beg you to check out Bonnie Bassler‘s TED talk about this. QS is how single cell bacteria are able to send signals into the environment to communicate with members of their own species and with other types of bacteria as a way to assess their population.
Another feature of P. aeruginosa is their modes of transportation, i.e. motility. P. aeruginosa has two major modes of getting around their environment, swimming motility with flagella or “twitching motility” on a surface using Type IV pili. We have discussed flagellar motility before (Chemotaxis). Type IV pili (TFP) is different. Pili are somewhat similar to flagella because they protrude out of the cell and are used to move around. Unlike flagella, pili don’t rotate but rather extend out before retracting back to the cell pulling the cell in the direction of the pili tip. The coordination of pili extension and retraction is guided by a chemotaxis-like pathway. Many bacteria have evolved to use chemotaxis proteins that have adapted to new roles within the cell.
I will end this MyTH with a special shout out to one of my favorite researchers, Carrie Harwood. Caroline Harwood is a professor at the University of Washington in Seattle. Carrie’s daughter suffers from cystic fibrosis and this led to Carrie’s curiosity in P. aeruginosa. She is a pioneer and a role model for many female scientists including Becky Parales. For her tireless career, Carrie was inducted into the National Academy of Science a few years back. A recognition well deserved.