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Reposted by Lucas Le Nagard

STCmicrobeblog @stcmicrobeblog.bsky.social · 27/03/2025

oldies but goodies from 𝕏 - now for #MicroSky on 𝕏 by 𝗠𝗮𝗴𝘁𝗲𝗿𝗶𝗮 𝗨𝗣𝗩/𝗘𝗛𝗨 @MagteriaUPV on Sep 10, 2020 Here is our contribution to #InternationalMicroorganismDay A group of the magnetotactic bacteria MSR-1 🦠 filled with nanomagnets (magnetosomes) 🧲 @femsmicro.bsky.social

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Lucas Le Nagard @lucasln.bsky.social · 12/03/2025

What are those three white blobs, and why do we spend hours at our microscopes recording movies like this? 🧐 Take a look at this short article on the 'bead assay' - a simple yet powerful technique that helps us, and many others, uncover the secrets of the bacterial flagellar motor!

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Lucas Le Nagard @lucasln.bsky.social · 12/03/2025

Thanks for publishing this, those experiments really are a lot of fun!

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Lucas Le Nagard @lucasln.bsky.social · 09/10/2024

This raises some interesting questions: what sets the value of this limiting torque and how can we use this in our models of the flagellar motor? In which regime (saturated? Linear?) do cells typically swim in their typical ecological niche? (8/9)

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Lucas Le Nagard @lucasln.bsky.social · 09/10/2024

As expected, the saturation at high PMF/torque became visible when we increased the viscosity of the medium 3.5 fold to mimic biological fluids: the swimming load was now equivalent to that of a 1 um bead and the swimming speed no longer increased with PMF at the highest PMF. (7/9)

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Lucas Le Nagard @lucasln.bsky.social · 09/10/2024

This load is close to the threshold above which the PMF-speed relationship saturates at high PMF/high torque, meaning that cells swimming in low-viscosity buffer may experience this saturation, but only when fully energized. (6/9)

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Lucas Le Nagard @lucasln.bsky.social · 09/10/2024

The first direct result is that our protocols allow us to estimate experimentally the mechanical load under which the flagellar motors operate in free-swimming E. coli, which is hard to compute theoretically. We find the load to be equivalent to that of a 0.6 um diameter bead.

Graph in which the vertical axis is the normalized speed, varying between 0 and 1, and the horizontal axis is the concentration of butanol in the medium, varying between 0% and 1%. Butanol is used to decrease the PMF. Three datasets are presented: one shows the speeds of 0.5 um beads attached to individual motors. The second is the same for 0.75 um beads. The third is the normalized swimming speed recorded via differential dynamic microscopy. All speeds are normalized by the speed measured at 0% butanol. The three curves display speeds that decrease as the concentration of butanol increases. The 0.5 um curve decreases the fastest, and the 0.75 um curve the slowest. The swimming curve falls in between the two bead assay curves.

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Lucas Le Nagard @lucasln.bsky.social · 09/10/2024

We then wondered if this matters only to us flagellar motor aficionados who like to stick beads to bacteria, or if this has implications for swimming bacteria too. Short answer: it has! For this, we measured the average swimming speed of E. coli while changing PMF. (4/9)

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Lucas Le Nagard @lucasln.bsky.social · 09/10/2024

To confirm our findings, we studied cells that had both a small bead and a large bead. This way, both motors were driven by the exact same PMF. Again, small bead (low torque) motors remained sensitive to PMF in all conditions, while large bead speeds saturated at high PMF. (3/9)

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Lucas Le Nagard @lucasln.bsky.social · 09/10/2024

We attached beads of various sizes to FM of single cells, and measured their rotation speed while modifying the PMF. Motors attached to larger beads were much less sensitive to those changes, suggesting that the well-established PMF-speed linearity saturates at high torque. (2/9)

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Lucas Le Nagard @lucasln.bsky.social · 09/10/2024

New preprint! We find the relationship between the flagellar motor (FM) speed and the proton motive force (PMF) to be nonlinear in E. coli, limited by the maximum torque that a motor can supply. Surprisingly, this max. torque is reachable physiologically. (1/9) biorxiv.org/cgi/content/...

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