DeltaDeltaG Mutant Scanning of FhuA Loops L3/L10 with T5 pb5 Distinguishes Fitness-Constrained vs Free Resistance Mutations, with Phase-Variation Rate Included as a Competing Pathway

Bacteria and the viruses that infect them (called bacteriophages, or 'phages') are locked in an evolutionary arms race. Bacteria can mutate a surface protein to dodge a phage, but those same proteins often serve critical functions — like importing iron, a nutrient bacteria desperately need. So not every possible escape mutation is a viable one; some would save the bacterium from the virus but starve it to death. This hypothesis proposes a clever way to map that trade-off using a lab technique called isothermal titration calorimetry, or ITC — essentially a device that measures the heat released when two molecules bind together. The idea is to take a bacterial surface protein called FhuA — which the T5 phage uses as its docking port — and systematically mutate the bits the phage grabs onto. For each mutant, you'd measure two things: how badly the mutation disrupts the phage's grip (using ITC to quantify binding energy), and how well the bacterium can still import iron through that same protein. Plotting these two values creates a map with four regions. Mutations that block the phage AND preserve iron import are the dangerous ones — the bacteria can 'escape for free.' Mutations that block the phage but cripple iron import are evolutionary dead ends. The hypothesis adds another wrinkle: bacteria can also dodge phages through a completely different trick called phase variation, where they essentially switch certain genes on or off at random. Whether bacteria use point mutations or phase variation to escape might depend entirely on whether iron is scarce or abundant in the environment. This matters because phage therapy — using viruses to kill drug-resistant bacteria — is being explored as a weapon against superbugs. One of its biggest problems is that bacteria evolve resistance, sometimes within days. If you could predict in advance which escape routes are genuinely available to a bacterium and which are thermodynamic dead ends, you could design phage therapies that corner the bacteria evolutionarily, leaving them with no good options.

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