Discovery of bacteriophages for lactococci
Prior to the early 1930's most cheese was made from undefined starter cultures; species and strain composition were generally unknown and if known initially would change with each subculture.
Dr Hugh Whitehead and his colleagues at the New Zealand Dairy Research Institute realised that if the dairy industry in that country was to produce close-textured cheese, free from taste and body defects and manufactured within a consistent time period that it would be necessary to use standardised starter cultures. They also realised that they needed to prevent problems arising from the growth of 'wild' lactic acid bacteria and spoilage organisms in the raw milk and introduced pasteurisation of milk for cheese manufacture.
Whitehead and his colleagues isolated lactic streptococci, known known as lactococci, from the undefined, mixed strain cultures and identified a number of single strains that could be used on their own to produce quality Cheddar cheese.
While the use of single strains of lactococci was initially very successful, instances in which the starter completely stopped producing acid with the resultant complete failure of the cheesemaking process were soon encountered. The New Zealand cheesemaker termed this the 'pack up' phenomenon for obvious reasons. The term 'dead vats' was used to describe the non-acidifying curds in the vats.
Viruses obligately parasitic on bacteria were discovered independently by Frederick Twort in 1915 and Felix d'Herelle in 1917. However, it was d'Herelle who first used the term bacteriophages ("eaters of bacteria") to describe viruses for bacteria.
Whitehead and Cox (1935)* discovered lactococcal bacteriophages as the causal agent of the complete cessation of acid production experienced using single strain starters.
With hindsight it is obvious that the use of single strain cultures had created ideal conditions for phage proliferation, and lysis of starter cells, resulting in catastrophic fermentation failure. Additionally, heat treatment of the cheese -milk had eliminated the natural lactic flora that previously would have produced acid and could have been used as 'a backup' source of acidification. However, the use of single strain starters also facilitated the discovery of phage and the subsequent development of control strategies.
In cheesemaking, lysis of starter cells can result in problems which range from slow acid production to completely lost vats. Lysis of starters during the early stages of cheesemaking may result in cheese of high pH, high lactose content, high redox potential and low lactic acid and lactate content.
These factors may lead to major product quality problems including those resulting from the growth of pathogens e.g. Staphylococcus aureus, especially if raw milk has been used.
Cheddar cheese at the start of pressing contains about 0.5% (w/w) lactose. Failure of starters to utilise residual lactose (due to phage-induced cell lysis) during pressing and in the first few weeks of maturation may result in sweet cheese and/or gas production by heterofermentive non-starter bacteria.
Because acid production by the starter influences syneresis, variations in starter activity may result in variations in the moisture content of the final cheese.
How to cite this article
Mullan, W.M.A. (2005) .
[On-line]. Available from: https://www.dairyscience.info/index.php/discovery-of-bacteriophages-for-lactococci.html . Accessed: 19 July, 2018.
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