The Invisible War: Phages
You can see mold. You can smell kahm yeast. But the most lethal predator in your sourdough starter is entirely invisible.
They are Bacteriophages (or simply "Phages").
They are viruses that hunt bacteria. And in the dense microbial city of a sourdough culture, they are the apex predators.
While bakers worry about temperature and hydration, a microscopic war is raging in the jar. Phages are constantly infecting, hijacking, and exploding your Lactic Acid Bacteria.
This sounds like a disaster. But biologically, it is a necessity.
What is a Phage?
A phage is not alive in the traditional sense. It is a genetic syringe—a strand of DNA or RNA encased in a protein shell.
It has one goal: to find a specific bacterium (e.g., Fructilactobacillus sanfranciscensis), attach to its surface, and inject its own DNA inside.
Once inside, the phage hijacks the bacterium's machinery, forcing it to print thousands of new phage copies. Eventually, the bacterium bursts open (lysis), dying and releasing the new phages to hunt again.
Why the Rise Stops:
When the bacteria explode, they stop metabolising flour. They stop producing the acids that give structure to the dough and the subtle gases that contribute to the rise. If a phage wipes out a significant portion of your bacterial population, your starter loses its engine. It becomes liquid, flat, and silent because the workers are dead.
Troubleshooting: Is It a Phage Attack?
Have you ever had a robust, active starter that suddenly—overnight—stopped rising? You didn't change the flour. You didn't change the temperature. It just... quit.
This is often a Phage Attack. Here is how to diagnose it:
Symptom 1: Sudden Collapse. The starter was doubling yesterday, but today it is flat and lifeless despite feeding.
Symptom 2: Change in Consistency. The starter becomes thin, watery, or "soupy" very quickly, even without rising. This is due to cell lysis (bacteria exploding).
Symptom 3: Loss of Aroma. The rich, sour smell disappears, replaced by a flat, floury, or slightly metallic scent.
Context: You haven't changed flour brands, water sources, or temperatures.
If all these are true, you are likely witnessing an "ecological crash."
In commercial yeast production, a phage outbreak is a catastrophe that requires shutting down the factory. But in a wild sourdough culture, it is a mechanism of evolution [2].
The Red Queen Race
In your jar, the bacteria and the phages are locked in an "arms race" [3].
The Phage evolves a key to enter the Bacterium.
The Bacterium evolves a new lock (CRISPR immunity) to block the Phage.
The Phage evolves a new key.
This constant pressure forces diversity. A starter that is attacked by phages must maintain a diverse population of bacteria to survive. If one strain is wiped out, another (resistant) strain takes its place.
This explains the "boom and bust" cycle. A starter might be incredibly active (boom) as a dominant bacterial strain thrives, then suddenly crash (bust) as a phage catches up to it. The recovery happens when a new, resistant bacterial strain rises to take its place.
Research suggests that phages effectively "farm" the bacteria, preventing any single strain from becoming too dominant and lazy. They keep the ecosystem dynamic and resilient [1].
The Recovery Protocol: How to Save Your Starter
You cannot see phages, and you cannot kill them without killing the starter (they are tougher than bacteria). But you can outmaneuver them.
Step 1: The "Backslop" Rule (Prevention)
Always keep a "backup" of your starter in the fridge (a "mother"). Phage attacks are often spontaneous and localised. If your bench starter collapses, your fridge mother is likely safe.
Step 2: Diversify Your Flour (Intervention)
Phages are often specific to the bacteria that eat specific grains.
Action: If your white flour starter crashes, feed it with 50% Whole Rye or Whole Wheat for 3 days. This introduces new bacterial strains that the current phages cannot infect, allowing the population to rebuild.
Step 3: The 1% Inoculation (Recovery)
If the population is decimated, do not overfeed.
Action: Use a very small amount of the "crashed" starter (5g) to inoculate a fresh mixture of flour (50g) and water (50g). This low inoculation rate reduces the phage load while giving the surviving, resistant bacteria a fresh buffet to colonise.
Step 4: Persistence
Keep feeding. The "crash" is rarely total. A small population of resistant bacteria always survives. By continuing to refresh, you allow these resistant warriors to rebuild the population.
Summary
A sourdough starter is not a pet; it is a battlefield.
The invisible war between bacteria and viruses is what makes your culture strong. It drives diversity, ensures resilience, and guarantees that the starter you have today is genetically tougher than the one you had yesterday.
References
Koskella, B., & Meaden, S. (2013). Understanding bacteriophage specificity in natural microbial communities. Viruses.
Paillet, T., & Dugat-Bony, E. (2021). Bacteriophage ecology of fermented foods: anything new under the sun? Current Opinion in Food Science.
Samson, J. E., & Moineau, S. (2013). Bacteriophages in food fermentations: new frontiers in a continuous arms race. Annual Review of Food Science and Technology.
