Enumeración de coliformes en leche natural y pasteurizada

Abstract

Four selective culture media for the coliform group of bacteria have been experimentally and comparatively studied. Two of them were liquid (Mac Conkey broth and brilliant green bile broth) and the two others were solid (desoxycholate lactose agar and violet red bile agar). They all were considered of great use for the determination of the coliform count in milk. The research work was carried out with 30 samples of raw milk and 30 samples of pasteurized milk in order to determine with precision the comparative efficiency of said media to detect and to enumerate the coliform microflora of milk. From the results obtained we may deduce that there are not any significant differences between the media and the technics or methods as to decide on which of them should be utilized. So, after overcoming the quantitative aspect of the enumeration of coliforms, the choice or selection of a count medium should be done on the basis of qualitative or technical conditions such as: clearness and conspicuity to interpret the results, development of the colonies, selective capacity of the medium, time of incubation, easiness to utilize a technique or method as a routine test, etc.
In accordance with the preceeding argument we think the violet red bile agar would be the most adequate or suitable to be used as a routinary method to control the coliform bacteria in raw milk.
This statement is based on their selective qualities, short period of time of incubation (22 hours), easy development and differentiation of the colonies, as well as the complexity of the seeding technics or method (a great number of dilutions and of tubes to be seeded) in liquid media when an analysis of raw milk with a number of coliforms, which may oscillate between large limits, is concerned. Regarding the pasteurized milk there were some significant differences between the media and the method utilized but not among the media. These differences were mainly due to the temperature of the incubation. Therefore, what we have said about raw milk would be true if in pasteurized milk there would not be a new limitative condition for the use of solid media as a method to determine the coliforms index. Such condition is that as there is a little number of bacteria of this type when the pasteurization is right and the milk is kept at low temperature, the number of coliforms per plate would be low, keeping also in mind the convenience of not seeding more than 1 ml. of milk in each Petri dish. Instead, the seeding in tubes would permit to use greater volumes of milk, so insuring satisfactory and correct results. On the other hand, the inconvenience of the number of dilutions and tubes to be used in this case does not exist in the order or grade indicated for raw milk, and then the technique or method is simplified and it may even be recommended as a routine test due to its simplicity. The brilliant green bile broth liquid medium would be, according to the above indicated, the most adequate to be used to control the coliforms in pasteurized milk, with the main advantage against Mac Conkey medium of showing a greater selective capacity