by Roberto
Elaborate mechanisms that protect bacteria against phage are well known, with CRISPR leading the way. Less familiar but equally fascinating is the fact that, somewhat unexpectedly, phages themselves sometimes offer bacteria protection in harsh environments. Filamentous phages do just that. I did quick search of STC posts on these phages and found just passing mentions, so a brief primer is in order because of their uniqueness. Can't really call them lytic or lysogenic. Rather, infected cells harbor the phage genomes (integrated in their chromosome or as plasmids) and continuously excrete phage particles, not usually leading to lysis. These are known as persistent infections. In a typical plaque assay, it is not lysis that is detected but just the reduced growth rate of the infected cells. As their name implies, the phage particles are filaments of a coat protein enclosing a single stranded DNA genome. While they've been known for some sixty years (with the discovery of phage fd from E. coli), this filamentous phage story starts less than ten years ago with phage Pf4, whose genome is integrated in the chromosome of Pseudomonas aeruginosa strain PAO1.
Fig. 1. Schematic model of the mechanism of Pf4 phage-mediated antibiotic tolerance. Source
The early discovery that Pf4 genes were highly overexpressed in P. aeruginosa biofilms led investigators to study the role the phage played in biofilms. The findings of Secor et al., published in 2015, were striking. They discovered that in biofilms and due to the presence of extracellular matrix polymers, Pf4 particles form highly ordered yet dynamic liquid crystals (tactoids) that surround the cells. Quite interestingly, these tactoids confer on the cells an increased tolerance to antibiotics. Subsequent work provides more detailed explanations of how these tactoids function. A 2020 paper by Tarafder et al., shows that the tactoids form even around inanimate colloidal rods that mimic rod-shaped bacteria. The physical nature of the tactoids that encapsulate individual cells – the liquid crystal nanochambers – create a diffusion barrier that provide protection against antibiotics. They also protect the cells from desiccation. The interactions between tactoids and cells are rather non-specific. A 2023 paper shows that filaments of Pf4 and fd encapsulate E. coli and P. aeruginosa interchangeably. Amazing how just the physical properties of filamentous phage result in a protective shield for the bacteria!
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