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.

There is a point of no return. A moment when a neglected starter stops being a culture of beneficial microbes and becomes a Petri dish for pathogens. This transition is chemical. It is driven by a single variable: pH.