Leonardo Mancini
Surface versus volume synthesis governs growth-dependent efficacy of a β-lactam antibiotic
Keywords: AMR, Cell wall, Microscopy, Mathematical modelling, Microfluidics.
Many antibiotics kill bacteria by disrupting essential enzymes, leading to errors and damage that accumulate over time. Non-growing and slow growing cells accumulate less damage, surviving antibiotics and paving the way to antibiotic resistance and infection relapses. Understanding the mechanisms that allow these cells to evade damage could significantly improve treatment approaches, something much needed in the rapidly unfolding antimicrobial resistance crisis.
In this work we examined the antibiotic mecillinam which, similar to penicillin, targets the main load-bearing structure of bacteria: the cell wall. When bacteria lose the integrity of their cell wall, they burst. Like other cell wall targeting antibiotics, mecillinam is inefficient against slow growers and we sought to find out why. We focused on Escherichia coli, a rod-shaped bacterium that lives in the gut, where it can cause disease. Unlike other antibiotics of the same class, mecillinam causes a very specific type of damage that leads to enlarged, spherical cells.
We observed that whether or not they are growing slowly, cells susceptible to mecillinam abandon their rod-shape and grow spherical, but the ones that grow slowly remain smaller and manage to continue dividing and proliferating without exploding.
Spherical cells are especially intriguing because they can contain a larger volume for a given surface area. For bacteria with impaired cell wall synthesis, growing spherical could be an adaptive response, allowing them to survive by minimizing surface growth needs. Larger spheres require less surface area per unit volume, allowing fast volume growth with minimal surface increase, which favours survival. However, there is a physical limit to cell size before division becomes unviable.
Using a simple mathematical model, we calculated the maximum diameter that cells can attain before division starts failing. We called this the fatness threshold. Controlling the speed at which bacteria grow by controlling the nutrients in their growth medium, we could demonstrate that only cells that grow faster than a certain rate surpass the fatness threshold and lyse.
But is it really all about cell diameter or are there other growth rate-dependent factors at play that we do not know about? To find out we needed a way to control cell diameter during mecillinam treatment independently from growth rate and medium. We achieved this by growing bacteria inside microchannels that allowed them to grow indefinitely in length but restricted their width. When the channels were narrower than the fatness threshold, bacteria survived mecillinam treatment no matter the growth rate or the antibiotic concentration. When the channels were larger, the cells swelled and exploded in the same way we had seen them do outside the channels, confirming that geometry was key to survival.
The work reveals a novel mechanism by which slow-growing cells survive antibiotics, emphasizing that fundamental physical parameters, like in this case the surface-to-volume ratio, can play a key role in bacterial survival. The findings enriches our understanding of bacterial adaptation and survival strategies, showing that conceptual and quantitative models can inform antibiotic strategies, potentially leading to more effective treatments.
How the work contributes to bridge fields of biology, physics and/or mathematics:
Our research bridges biology and physics by showing how bacterial survival under antibiotic treatment is influenced by physical principles like surface-to-volume ratio. The work demonstrates that the conceptual and methodological approach offered by physics can be both suitable and efficacious for addressing biological questions. Indeed, as showcased here, certain mechanisms of biology might actually only be accessible when the cell is considered at the system level, and not just as the sum of its molecular parts. Our findings, therefore, show that interdisciplinary approaches can be key to understand the vast complexity of biology.
Department of Physics, University of Cambridge
Prof. Pietro Cicuta Lab
