What is phage?

By: Dave Potter | Technical Specialist & President, Dairy Connection Inc.

What is phage?

You may have heard of the term “phage” in either cheese making books or from discussion with other friends and acquaintances in the cheese business. But what exactly is phage, and where does it come from?

The commonly abbreviated “phage” is short for bacteriophage, which is a virus that infects bacteria and can kill them. The bacteriophage virus is about 1/1000 the size of bacteria. Viruses are a part of nature, and bacteriophage is one of the many types of viruses we are exposed to every day. They exist everywhere. They are in the air, on plants, animals, liquids — anywhere they can attach and survive. The good news is that dairy starter culture bacteriophage cannot infect or harm people. The bad news is that they can and do affect dairy starter cultures. For this reason, it is important for cheese and yogurt producers to understand how to recognize, manage and prevent phage.

How does phage take over?

Bacteriophage needs a host to infect in order to maintain its viability and growth. It cannot multiply on its own. It may survive in a dormant state for years and not reproduce until a host culture comes along. One unique thing about bacteriophage is that it needs to have a single host culture, or bacteria, in which to infect. Once they infect the host, the phage virus takes over the mechanisms inside the culture cell wall and change the function of the culture, ultimately killing it. After that infection occurs, the bacteriophage begins realigning the DNA within the cell, instructing it to no longer function as a dairy starter culture, but rather to function as a phage factory. Following is a diagram showing the phage reproductive cycle.

The bacteriophage will select a specific host and site to attach itself and penetrate the cell wall. Once that occurs, it injects its own DNA sequencing in through the cell wall and basically takes over the control of the starter culture functions. The invaded culture now starts to reproduce more bacteriophage. The culture bursts and the new phage is released into the environment, similar to an inflated balloon bursting that contains a small amount of water inside. This is not a big deal unless you actually need that particular culture to produce lactic acid and texture for yogurt or cheese — then there are problems.

Why phage matters to dairy-product producers

The basic bacteria used in cheese and fermented dairy products usually have one main goal in life which is to grow and produce lactic acid. They do this in part by converting milk sugar (lactose) into lactic acid. A typical growth rate for a dairy starter culture can range from 20-60 minutes to reproduce and double in numbers. We usually start with over 1 billion bacteria per gallon of milk. However, bacteriophage can corrupt a host starter culture, causing it to produce 50 to 200 new phage viruses in the same time it takes a starter culture to increase from one cell to two cells and so on. Based on that rate, one can calculate that a phage can and will quickly overcome a starter culture, killing it before the acidification process is complete. This results in no acid or flavor production in the cheese or no coagulation in yogurt, buttermilk or sour cream production. Typically you will know if you have a phage infection when acid production begins to slow down and eventually stops. If multiple vats are being made, the acid production in the first vat may be okay and then in successive vats it begins to slow down until it stops. In the case of fermented milk products, the acid production stops altogether, causing a lack of coagulation, or, the pH may drop down as normal, but the texture of the fermented milk is very grainy with excessive whey-off.

What do you do if you suspect a phage attack?

In most cases, it is too late to salvage the fermentation. There is no way to reintroduce culture into cheese or to effectively add it to fermented products without causing structural damage to the milk protein. A good way to confirm a suspected phage attack is to take a sample of the whey or milk and send it to the technical staff at Dairy Connection. We will work with the primary culture manufacturer to perform the necessary tests on the samples to detect which specific culture is being affected by a suspected bacteriophage virus. A positive phage test usually results in the need to use a similar rotation culture that will replace the culture that is infected. Rotating different strains within a culture series provides a break in the cycle, as any specific phage is only able to infect one particular culture strain.

Although it may be difficult to eliminate or prevent altogether, bacteriophage can be controlled through proper cleaning, sanitation and regular rotation of cultures. It is much easier to prevent phage than it is to rectify the problem once a bacteriophage takes hold in your production area.

Many dairy cultures are available in a series, with rotations to help prevent phage. In addition, the use of a surface sanitizer such as a mild chlorine solution of 50-100 parts per million of available chlorine in water will reduce the likelihood of a phage attack in a clean environment. It is important that all cracks in floors or equipment are repaired, and to guard against standing water in your production area. Other tips to help prevent phage include sanitizing all drains, regularly take equipment/valves apart for sanitization, and ensuring that the area underneath cheese vats is clean (this is an often overlooked area and a haven for phage). In addition, it is important that filters for air handling units are inspected and replaced every 3-6 months.