Breaking down the microbiology world one bite at a time

Microbial hitchhiking.

Written by:

Charlotte van de Velde

We all probably have experienced this before: the nice smell outside after a light summer rain. This smell called petrichor is mainly composed of the chemical geosmin, produced by bacteria in the genus Streptomyces. These streptomycetes are not only known for the production of this compound: they are probably more famous for their production of most antibiotics used in the clinics today.

In the soil, their natural habitat, streptomycetes can help improve plant host health and plant productivity, and provide sustainable solutions to crop yields. They grow like filamentous fungi, as they produce hyphae which form a mycelial network and spores (read more about fungal hyphae growth here). Streptomycetes are non-motile and can distribute their spores over long distances through the wind or attached to insects and nematodes. However, until now it was still unclear how they relocate themselves over small distances (a few centimeters) to get to their preferred microenvironment, such as plant root systems.

*Streptomyces* life cycle. Created with Biorender.

Recently, a study by the group of Ariane Briegel looked at how Streptomyces spores could be transported over such small distances. They used microscopy, motility assays and genetics to demonstrate that the spores hop on motile soil-dwelling bacteria and use them as a taxi! More specifically, they looked at how Bacillus subtilis (Bs) transports Streptomyces coelicolor (Sc – more about S. coelicolor below) spores by direct attachment to the flagella.

They performed several motility experiments, where they put one or both bacteria on a plate and looked at their growth patterns. They noticed that Sc spores only moved in combination with the growth of Bs, and that they moved in the same direction as Bs growth. See A and B in the figure below for photos. A third interesting observation was that Bs moves the spores closer to plant tissue (figure below, C), and not in a random pattern around the plate as is shown in A.

A) left: Sc spores (white dots) moved from the middle with help from Bs’ motility. Middle: Sc without Bs, so no movement to outside seen. Right: Bs growth without Sc. B) Sc colonies seem to move in the direction of Bs growth (asterix = Bs inoculation point). C) Left: Sc spores moved from the middle with help from Bs’ motility to form colonies preferably next to plant roots. Middle: Sc alone does not form colonies near the plant roots. Right: Only growth of Bs. Figure from the original article

Bs has two modes in which it can transport itself by using its flagella: they swim in liquid environments and swarm on solid surfaces. The researchers showed that Sc spores are only transported when Bs was swarming and that they were not transported when Bs was swimming. In addition, Bs can move by ‘sliding’ over surfaces. They used an immobilized Bs (with flagella but cannot move the flagella) vs a Bs that has no flagella to show that spore dispersal was facilitated by the sliding movement of bacteria in presence of the flagella.

*Spores can be mobilized by wildtype ‘swarming’ Bs (WT) and ‘sliding’ Bs(ΔmotAB) with flagella, but not by ‘swimming’ Bs (LC) or by Bs that has no flagella* (Δhag)*. Figure adapted from original article*.

With electron- and fluorescence microscopy the authors confirmed that there is indeed an interaction between the flagella and the spores, and not between the Bs cell itself and the Sc spores.

A: Spores (dyed) are found at the end of the bacteria, where the flagella normally localize. B: The spores do not directly attach to the bacteria. C: Flagella co-localize with Sc spores. D: Dyed and sheared flagella interact with the spores directly. Figure from original article

Not only Streptomyces coelicolor spores use bacteria as a taxi to travel to a new place: the researchers looked at several other streptomycetes and saw that their spores could also be moved by a bacterial taxi. But how exactly do they catch this flagellum-ride?

Most Streptomyces spores have a thick protein coat made from rodlin proteins. Interestingly, one of the other streptomycete strains they tested, Streptomyces avermitilis, does not have these surface proteins and was not able to move as far as the other streptomycete spores. To confirm that this rodlin coat was needed for hitchhiking, they tested a mutant Sc that was also lacking this rodlin coat. And bingo, it was not able to hitchhike anymore. So these rodlin proteins seem to be vital for the attachment of the spores to the flagella.

C) Scanning electron microscope image of Sc spore with rodlet layer. D and E) Cryo-electron microscope images, showing interaction of flagella with rodlet layer. F) reconstruction of E with green = spore, yellow = flagella, purple = rodlet layer. Image adapted from original article.

Not only does Bs act as a taxi for the spores: the authors also showed that other soil-dwelling bacteria with flagella can transport spores, such as Pseudomonas fluorescens. So hitchhiking is a widespread mechanism that allows streptomyces spores to disperse at a centimeter scale. The motile bacteria used by Sc are known to associate with plant roots, therefore the hitchhiking mechanism could provide a transportation route to a beneficial environment. Why are plant roots beneficial for bacteria? Generally, the area around plant roots is rich in plant root exudates, and bacteria can use these metabolites for their growth. In return, the bacteria, and in particular Streptomyces, produce antibiotics that can protect the plant from potential phytopathogens.

This discovery could have implications for industrial initiatives that aim to improve soil conditions for Streptomyces root colonization. In addition, insight into hitchhiking of spores could clarify unknown infection mechanisms by other spore-forming organisms, such as the human pathogen Aspergillus fumigatus, or plant pathogen Aspergillus niger, which are known to also interact with motile bacteria.