Bulk starter collapses: phage or residue?
An independent dairy produces 5,000 L of set yogurt per day in two 1,000 L tanks, using a bulk starter of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus grown in 10% reconstituted skim milk. Over six days the time to reach pH 4.6 has risen from 4.5 h to 9–10 h, and the coagulum is weak and grainy. Bulk starter viable counts fell from 2×10^9 CFU/mL to 3×10^4 CFU/mL, while the count in the milk powder reconstitution tank is normal. Microscopy of the bulk starter shows Gram-positive cocci and a few long rods but no obvious contaminants; the plant does not use antibiotics and the Delvotest on incoming raw milk is negative. The plant recycles whey from the previous day's batch into the starter room drain and uses a single starter strain pair all year. Plates of the bulk starter on M17 agar show plaques when a lawn of the previous week's culture is overlaid.
1. State the most probable cause and explain how you would exclude the main alternatives using the evidence given.
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The most probable cause is bacteriophage infection of the bulk starter, most likely of the S. thermophilus component.
The positive evidence is the five-log drop in bulk starter counts within days (10^9 to 10^4 CFU/mL) while the reconstitution tank stays normal, which localises the problem to starter propagation rather than to the incoming milk; the doubling of the coagulation time; and, decisively, plaques on an overlay lawn of the previous week's culture. Phage are host-specific, so the rods survive and are still visible on microscopy while the cocci are lysed; because the two species interact synergistically, loss of the acid-producing cocci is enough to collapse the whole fermentation.
Excluding alternatives:
- Antibiotic or disinfectant residues: Delvotest is negative, and residues carried in the milk would depress the reconstitution tank and both species together, not one component of the starter. They would also not produce plaques.
- Plasmid loss or a weak/attenuated starter: this develops gradually, gives intermediate counts (10^7–10^8), not 10^4, and does not generate plaques on a sensitive lawn.
- Chemical inhibition from cleaning residues in the starter vessel: would give a dose-related, vessel-specific effect without plaques and would recover after cleaning, not worsen over six days.
- Lysogenic induction of the strains themselves: possible as a source of the phage, but the plaque assay proves that free, infectious phage are present and amplifying; the epidemiological pattern (single strain pair used all year, whey recycled near the starter) explains where they come from.
2. Design a control programme, ranked by expected impact, and explain why simply increasing the inoculum ten-fold is likely to make the fermentation worse rather than better.
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Ranked controls:
- Break the amplification cycle now: stop using the current bulk starter, discard it, and sanitise the starter room and all transfer lines (peracetic acid or hypochlorite, then rinse). Every day of continued use multiplies phage load.
- Segregate whey completely: whey is the single richest phage source in a dairy. Never recycle it near the starter room, use separate hoses, drains, personnel and footbaths, and keep the starter area under positive pressure with filtered air.
- Rotate strains: build a defined rotation of three to four strain pairs with demonstrably different phage sensitivity and do not reuse a pair for three to four months. A single pair used all year selects a phage population adapted to it.
- Phage-inhibitory medium and process: use starter medium containing phosphate/chelating agents that sequester Ca2+ needed for phage adsorption, heat-treat the milk (e.g., 95 °C, 30 min), use closed transfer, and protect the bulk starter vessel.
- Monitoring: run a plaque assay on each bulk starter batch and on whey every day; act on the first plaques, not on the first failed tank.
Why more inoculum fails: phage replication is density-dependent. A ten-fold higher cell density supplies ten-fold more hosts, so adsorption and the ~40-minute lytic cycle with a burst of ~100 progeny proceed faster and reach a higher titre. At high multiplicity, infection also occurs from outside (lysis from without), so the extra cells are killed without producing acid. The result is a shorter lag followed by a sharper collapse, not a rescue.