Bacteriophages and the Industrial Problem Behind Starter Rotation

What a phage actually is

A bacteriophage is a virus that infects bacteria — typically a protein capsid containing DNA, with a tail and fibres that dock onto specific receptors on the bacterial surface. Once attached, the phage injects its DNA. The bacterial cell machinery is hijacked to make more phages, which then burst the cell (the lytic cycle) and go find more hosts. A single phage particle in milk can, under the right conditions, reproduce to billions within a few hours — enough to collapse a starter population mid-vat.

The signs of a phage attack in a vat

• Slow or stalled acidification — the pH curve flattens instead of dropping.
• Cheese sets normally but never develops characteristic flavour because starters died early.
• Recurrent problems in specific vats, days, or parts of a facility — phages localise.
• Successful diagnosis often needs a lab phage assay; by-feel diagnosis is unreliable.

Where dairy phages come from

Dairy phages are endemic in dairy environments. They arrive on raw milk, in the air, on equipment, and — frequently — recycled from previous vats via whey. Whey is a phage concentrator: if a small phage population grew during the make, the whey may be carrying millions of particles per millilitre. Whey handling and disposal practices are a major control point in modern dairies for exactly this reason.

Rotation: the classical defence

If you use the same starter every day, you are breeding phages specific to it. Rotating between different starter blends on different days — blends with different phage-susceptibility profiles — denies a growing phage population a continuous host. Traditional dairies run schedules of three to five rotating starters; sophisticated operations rotate weekly. Rotation is cheap, proven, and complementary to everything else — it's still the single most useful phage-management practice for small and mid-size producers.

CRISPR: discovered in a yoghurt lab, now everywhere

Bacteria evolved their own anti-phage system hundreds of millions of years ago. When a phage attacks, specialised proteins capture small chunks of its DNA and store them in the bacterial genome in a region called a CRISPR array — a kind of molecular memory of past infections. On future attacks, the bacterium can transcribe those stored sequences into RNA guides that direct an enzyme (Cas9 and relatives) to cut invading phage DNA on sight. This system was characterised in detail by studying Streptococcus thermophilus starter cultures at Danisco in the 2000s — and the tool the broader biology world later built from it, CRISPR gene editing, is arguably the most important biotechnology of the 21st century. Cheesemaking culture research, quietly, is where a Nobel-prize technology came from.

Phage-resistant starters: how they're made

Modern phage-resistant strains are selected, not typically genetically modified. Culture houses expose a starter strain to a high phage load; most cells die; the survivors — which happen to have acquired a new CRISPR spacer, or lost a phage-receptor protein, or developed another resistance trait — are purified and tested. The result is a naturally-derived, phage-resistant version of the same strain. Some CRISPR-edited resistance work exists in the literature but is not yet the dominant commercial approach, partly due to regulatory caution around 'edited' food-grade microbes.