microbial ecology motility bacteria fungi chemotaxis

Hit the Road Jack

Picture swimming in a pool of honey. Not the easiest of tasks right? This is how microbes feel moving through water!

image alt text

Fig 1 How does it feel to move like bacteria? (

Inspiration for this post was the classic Ray Charles’ “Hit the Road Jack.” To me, this song just makes you want to groove, whether snapping your fingers or maybe trying out the fly.

So, before you start reading, put on the tune and give it a shot! As the melody takes over, notice the ease of your movement. Glide across the room and appreciate how quickly you travel the floor. Now I want you to picture yourself in pool trying to do the same dance in water. The ease of movement lessens, right? Finally, what about dancing in a pool of honey? Well, at this point you’d be lucky to tap your foot quickly… Welcome to the life of a bacterial cell trying to move.

I make this comparison based on Reynold’s Number, which is the ratio of the liquid’s inertial force (the liquid’s resistance to an object moving) to the liquid’s viscous force (the friction between the liquid and the object). It can be calculated as:

Reynold’s number = (liquid density) x (size of object) x (speed object)(viscosity of liquid)

To give some perspective using the imagery we’ve been working with: if you are swimming in water, the Reynolds Number is 1000. If you are swimming in honey, the Reynolds number is 0.001. As for that bacterial cell trying to swim in water? Well, that Reynolds number can be as low as 0.00001[1]. Looking back at the equation, we can attribute this to the small size and speed of bacteria. So, even though we don’t experience swimming in water as being as viscous as honey, bacteria definitely do!

A bacterial cell’s ability to move from place to place relies heavily on whether or not there is enough moisture surrounding it. In soils, this can be difficult to achieve. So how do bacteria travel around in the environment? There are many ways, but for today we are going to focus on one very neat method bacteria have evolved to travel in dry soil: fungal highways.

image alt text Figure 2 Fungal hyphae.

This kind of highway makes LA’s set up look like a drive in a local park. A fungal network can take up 20,000 km per cubic meter of soil [2], making it very dense and very vast. Fungal spreading is also not limited by how much water is in the soil. Therefore, picture them like bridges between patches of moist and dry spots along the forest floor. Finally, the branch-like extensions of fungi, known as hyphae, have liquid films surrounding them, making just enough moisture for their bacterial neighbors to hitch a ride.

Okay, that’s great that bacteria can hop on board the crazy fungal train, but why would bacteria want to live in the fast lane in the first place? Well, I’ll paint the picture this way: When you heard the ice cream truck around the corner as a kid, did you sit on the curb and hope that you happened to be on its route, or did you make a 200 yard dash towards the sounds of sweetness? My guess is you did the latter. Resources in soil don’t evenly spread to all the bacteria –even when it rains. They usually end up in patches [3]. If you are a cell, you want to get to the food to grow and thrive – but you aren’t the only one! Fungi spread to do the same thing – find food and keep growing. So why not use this to your advantage as a bacterial cell? Take a ride along the liquid film and plop yourself in a feast.

Finally, I mentioned that the fungal highways are vast and complicated – how do bacteria know which way to go to get to the resources they need? This involves chemotaxis – which is the movement of an organism towards a chemical signal. Think of it like you running towards the sound of the ice cream truck. You don’t see the ice cream yet, but the louder song, the closer you know you are. For bacteria, there are chemoattractants (chemical signals) that the bacteria can detect for all manner of nutrients [4]. As it gets closer to the source, the level of the chemoattractants become more concentrated along the liquid films of the fungal hyphae [5].

What is the take home of all this? Well other than the fact I hope you actually did do the fly in your living room listening to “Hit the Road Jack”, researchers have looked at this relationship between bacteria and their fungal speedways and asked: how can we use this knowledge to potentially solve problems such as soil pollution [3]? Bacteria have quite the extensive palate, meaning some can eat contaminants that would otherwise persist in the soil for years and hurting our environment. For example, if certain fungus and bacteria combinations degradate aromatic compounds, could they be implemented as bioremediation of a an oil spill? Take a second and think about it. What other ways could fungal highways be useful to us? Leave comments and ideas – they could be the next inspiration for a scientist!


[1] Cohen, Netta, and Jordan H. Boyle. “Swimming at low Reynolds number: a beginners guide to undulatory locomotion.” Contemporary Physics 51, no. 2 (2010): 103-123.

[2] Venieraki, A., P. Ch Tsalgatidou, D. G. Georgakopoulos, M. Dimou, and P. Katinakis. “Swarming motility in plant-associated bacteria.” Hellenic Plant Protection Journal 9, no. 1 (2016): 16-27.

[3] Banitz, Thomas, Karin Johst, Lukas Y. Wick, Susan Schamfuß, Hauke Harms, and Karin Frank. “Highways versus pipelines: contributions of two fungal transport mechanisms to efficient bioremediation.” Environmental microbiology reports 5, no. 2 (2013): 211-218.

[4] Mitchell, James G., and Kazuhiro Kogure. “Bacterial motility: links to the environment and a driving force for microbial physics.” FEMS microbiology ecology 55, no. 1 (2006): 3-16.

[5] Furuno, Shoko, Katrin Päzolt, Cornelia Rabe, Thomas R. Neu, Hauke Harms, and Lukas Y. Wick. “Fungal mycelia allow chemotactic dispersal of polycyclic aromatic hydrocarbon‐degrading bacteria in water‐unsaturated systems.”Environmental microbiology 12, no. 6 (2010): 1391-1398.

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