Thread by @QuentinLclrc

#Phage are often only seen as bacterial predators, but they’re also major drivers of bacterial evolution through generalised transduction.

In this new preprint, we combine lab work & mathematical modelling to reveal the dynamics of this dual nature.

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biorxiv.org/content/10.110…

Generalised transduction happens when a replicating phage mistakenly packages non-phage DNA. The resulting transducing phage can inject this DNA in another bacteria, as shown in the Figure.

This is notably a powerful way to transfer plasmids, major vectors of #AMR genes.

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We co-cultured two single resistant strains of S. aureus (B_E and B_T) with 80alpha phage (P_L).

Here, horizontal gene transfer can only happen by generalised transduction, so if we detect double resistant bacteria (B_ET), we know that transduction happened.

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Results: we see double-resistant bacteria as early as 7h (red).

We also note that phage-bacteria dynamics vary depending on starting concentration of phage. For 10^3 and 10^4 phage, we see equilibriums after 8h, but for 10^5, the phage are still increasing after 16h…

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Here's where the modelling steps in. We developed & fitted models to the data to reveal the invisible underlying dynamics of phage predation & generalised transduction.
The Figure here shows the model diagram, including organisms above, but also transducing phage (P_E & P_T)

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The models suggest that these dynamics are due to a dependence of phage replication on bacterial growth rate. In other words, phage burst size decreases as bacteria reach stationary phase.

(I’ll let you read the paper to learn about density vs frequency-dependent models…)

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What about transduction then? Well, we can't easily count transducing phage in vitro, but remember that they're included in the model...

So we can use this to estimate parameters related to transduction - that's the strength of interdisciplinary work.

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We estimate that 1 per 10^8 new phage generated was a transducing phage carrying an #AMR gene, similar to previous estimates. That's often dismissed as too rare to be relevant.
Yet we've shown here that it's enough to consistently lead to double-resistant bacteria in just 7h

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Of course, new double-resistant bacteria (DRP) can also be generated by growth of existing DRP, rather than transduction

Again, using the model, we show that new DRP are always predominantly generated by transduction, further highlighting the importance of this process

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Implications for #phage predation: we’re adding to the growing body of evidence that phage replication depends on bacterial growth phase, with slower replication at stationary phase.

This should be considered for future experimental designs, but also phage therapy.

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Implications for #AMR: the contribution of transduction to the increasing global burden of AMR is currently underestimated, and hence opportunities for AMR control are potentially being missed.

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Massive thanks to coauthors @JacobWildfire @gupt_a_rya @jodi_lindsay @gmknght !

This is the 2nd paper from my PhD work, so it’s a big emotional moment! I look forward to hearing other people’s thoughts & reviewer comments on this 😁

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