Bacteriophages: The microscopic enemy of the dairy industry

The majority of people relate viruses with diseases that affect plants and animals only, what we don't know is that being the smallest living entity in the universe, viruses are capable of infecting other microorganisms like bacteria. Having viruses affecting bacteria has proven to be beneficial to the human race especially in recent years where there was a rise in antibiotics resistance and they are used as antibiotics alternatives. These viruses are known as bacteriophages and sometimes termed as phages in short form, these entities are of variety and infect every kind of bacteria that has so far been discovered by scientists while maintaining their specificity (meaning every bacteria has got a certain variety of phages that can specifically infect its strain). Bacteria being well known for their diseases doesn't mean all of them cause one, and in fact, we have a lot of bacteria that are beneficial to our lives either direct or indirect. Just to mention a few some bacteria help in nitrogen fixation in plants, decomposition, act as normal flora on our skins and stomachs and some are even used in food processing.
Lactic acid bacteria (LAB) are among the most beneficial microorganisms in the food industry. These microorganisms produce lactic acid from sugar fermentation. For this reason, they have been used for thousands of years in the production of fermented milk products, like yogurt and cheese, among others. Just like other bacteria, they are also prone to attacks, phages penetrate their genetic materials and command the bacteria to act as a piece of machinery for making viral copies before the cell burst and die off to release new particles.
Dairy products. photo by Change food

Effect of Lactic Acid Bacteria on milk during processing

The LABs trigger a microbial process where they transform the –sugar in milk- into lactic acid. That acid modifies the structure of the milk proteins making them curdle and reproducing the same effect on the product’s texture. Besides, it provides the milk with that characteristic slightly bitter taste. It was proved that these microorganisms provide beneficial effects on the health of those who consume them. Furthermore, it supplements the health with the bacterium present in the intestinal flora promoting the good performance of the digestive system. For this reason, they are very important for the dairy industry considering the context of growing interest in the market of probiotic products.

Effect of phage on the action of Lactic Acid Bacteria (LAB) during milk processing

Phage activity causes decreased starter (Starter is a set of LAB bacteria that are readily available to initiate the fermentation process) activity in cheese and fermented milk manufacturing, resulting in fermentation in which acid production is markedly reduced or, in extreme cases, totally blocked. This failure in fermentation develops low-quality products that are not safe from a microbiological point of view because they can contain contaminants and/or pathogens that do not have to compete with the lactic bacteria that protect the product (probiotic way).

Where is the source of phage contamination in the dairy industry?

Raw milk is considered to be the principal source of phages, either as free virions or as prophages present in wild strains of lactic acid bacteria (LAB), and constitutes the primary phage entranceway to the industrial environment. Sometimes equipment, people, and the environment can be the source of contamination although this kind of contamination is much easier to handle just by modifying sanitation procedures and decontaminating surfaces.

Methods used to inactivate Dairy Bacteriophages contaminants

Phages are difficult to eliminate because they rapidly disseminate in dairy plants. The following are commonly used methods in eliminating phage contaminants in dairy processing plants.

Heat Treatments

Most dairy processing plants use heat to kill a number of microbes from milk, among others phages are heavily affected especially when subjected to the high heat of 70°C for a long time. The conditions recommended by the International Dairy Federation (IDF; 90°C for 15 min) to guarantee complete phage inactivation. For cheese production, low-temperature long time (LTLT, 63°C for 30 min) or high-temperature short time (HTST, 72°C for 15 s) pasteurization were traditionally applied. Ultra-high-temperature (UHT) processing, also called ultra-pasteurization (more than 135°C for 1–2 s), is a sterilization treatment as it produces spores destruction. However, this high heating can cause Maillard browning and negatively affect the taste and smell of dairy products. The milk used for yogurt production is generally treated at 80°C for 30 min or at 95°C for 10 min. Regardless of the high efficiency of this heat treatment to inactivate microorganisms, it is not always effective against dairy bacteriophages.

Chemical treatment (Biocides)

A biocide should fulfill several criteria to be usable in the food industry, for example possessing a fast antimicrobial activity, ease of application, low cost, lack of negative impact on the final product, and degradation into harmless final products. However, the inactivation of bacteriophages was taken into consideration only recently as a criterion of biocides selection, which is reflected by an increasing number of studies directed to quantify their effectiveness in this sense. Ethyl alcohol (ethanol), isopropyl alcohol (isopropanol), sodium hypochlorite, and peracetic acid are among the most common biocides.

High-Pressure Treatments

High-pressure processing is one of the most promising because it combines maximal retention of the chemical and physicochemical product properties with efficient germ reduction. The most studied and applied pressure-based processes are high hydrostatic pressure (HHP) and high-pressure homogenization (HPH). HPH is a high hydrodynamic process during which the fluid is forced to pass through a small orifice and then subjected to an ultra-rapid decompression. The sudden fall in the local pressure of the fluid at constant temperature leads to the nucleation and growth of vapor bubbles (or cavities) within the body of liquid, the collapse of which could transmit several localized forces to surfaces or particles, including the microbial cell.

Photocatalysis

Photocatalysis is a process in which light energy is used to drive pairs of chemical reactions. This method is well utilized if phage particles are aerosolized in the air. The capability of bacteriophages to remain in the air for long periods makes bioaerosols one of the most important dispersion routes of virions. 

The first negative effect of bacteriophages on dairy fermentation was reported in the mid-30s of the 20th century. Regardless of sanitary precautions, starter strain rotations, and the constant development of new phage-resistant bacterial strains, phages remain one of the main and economically most serious sources of fermentation failures. Due to their natural presence in the milk environment, bacteriophages cause problems in industrial dairy fermentations worldwide. Their short latent period, relatively large burst size, and/or resistance to pasteurization make them difficult to eliminate. Phage-induced bacterial cell lysis leads to failed or slow fermentation, decrease in acid production, and reduction of milk product quality (e.g. nutritive value, taste, texture, etc.), which in effect cause profound economical losses

REFERENCES
  • Role of Bacteriophages in the Implementation of a Sustainable Dairy Chain by Gutierez et al, (2019)
  • Review: efficiency of physical and chemical treatments on the inactivation of dairy bacteriophages by Guglielmotti et al, (2012).
  • Marcó, M. B., Suárez, V. B., Quiberoni, A., & Pujato, S. A. (2019). Inactivation of Dairy Bacteriophages by Thermal and Chemical Treatments. Viruses, 11(5), 480. https://doi.org/10.3390/v11050480
  • Lactic Acid Bacteria Resistance to Bacteriophage and Prevention Techniques to Lower Phage Contamination in Dairy Fermentation by Szczepankowska et al, (2013).

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