The phenomenon of bacteriophage in milk
The phenomenon of bacteriophage can easily occur not only in milk, but also on other media, such as, for example, in whey. But milk is the best medium in this respect, since it develops streptococcus best.
The specified bacteriophage is characterized by strict specificity. For the first time, the phage isolated by Whitehead and Cox were tested against many strains of lactic streptococci, but it turned out to be active only against that strain of Str. cremoris, from the culture of which was isolated. Scientists recommend to keep the drug of this phage at 4C. In this case, it does not lose activity for several months. Phage is sensitive to high temperatures and is completely destroyed at a temperature of 70 ° C for 30 minutes.
A phage-sensitive culture, when transplanted in the presence of phage without aeration, did not show even signs of lysis, and the resulting serum was inactive with respect to the original strain of streptococcus both in a liquid medium and on the surface of the agar. Whitehead and Cox come to the conclusion that under normal conditions the phage is in such a culture as if in an adsorbed state, and only aeration puts the phage into an active state and contributes to its normal reproduction.
The composition of milk is also of great importance for the manifestation of bacteriophage activity. Milk obtained from various sources has a different effect, other things being equal, on a streptococcus culture containing the phage “in the adsorbed state”. However, with a certain change in the main components of milk, this phenomenon cannot be linked.
Scientists believe that in this case, elusive differences in the components that are in milk in small quantities can play a crucial role.
Scientists note that phage can be introduced into the culture from the outside, which is probably what happens often in practice. Therefore, having even more or less reliable immune strains, we must always consider the possibility of the penetration of phages to which this strain will not be resistant.
It is characteristic that the bacteriophage exerts its action mainly during the fermentation of large masses of milk. In order to weaken the phenomena of this kind, it is recommended to increase the amount of the added starter and avoid strong mixing in which air enters the milk in large amounts.
Scientists have found out all the stages of this phenomenon. At first, streptococci, in the presence of bacteriophage, are mostly developing quite normally. After a while, there is a delay, and then they begin to decrease in number. This moment corresponds to the change in the microscopic preparation; streptococci, which were previously stained normally, begin to produce weakly stained forms, and soon they stop completely staining, and under the microscope one can observe the disintegration of individual cells. This is the moment of lysis, i.e. the dissolution of the cells, to which most of them are then subjected. As a result, the milk is either not fermented at all, or the ripening occurs very late, and the clot is unsatisfactory.
It was established that the cause of this phenomenon was the presence of bacteriophage-sensitive strains in the starter culture. The bacteriophage fell into the leaven, together with the culture infected by it, and from the outside, that is, together with the milk itself.
Such a bacteriophage is characterized by significant heat resistance, and when using a synthetic ferment it is necessary to check each of the cultures involved in the starter for the presence of a bacteriophage.
To do this, the easiest way is to bring a solution of litmus or methylene blue into a test tube with sterilized milk when it is inoculated with a culture of streptococcus. In the absence of a bacteriophage, after a few hours there is (due to the restoration of the indicator to leucobase) an indicator discoloration, which persists until the time of coagulation of milk. In contrast, with the presence of a bacteriophage, soon after the disappearance of the color, it reappears in the milk. This is due to the fact that as a result of the death of streptococci (due to lysis), their restorative effect ceases. On the other hand, due to the access of oxygen, an increase in the redox potential occurs in the milk, and the indicator changes from the reduced form to the oxidized one.
Activation of the bacteriophage occurs with stirring only due to mechanical action. However, there are other indications of the activating effect of oxygen on the bacteriophage. Tours in Bukhara