Another experiment illustrates perhaps even more strikingly the effect of cleanly operations in milking upon the initial bacterial contents of milk. The preliminary precautionary measures were carried out by an ordinary workman, and are in no sense so refined as to be beyond the reach of ordinary daily practice. "The milk was received in steamed pails, the udder of the animal, before milking, was thoroughly carded, and then moistened with water, so as to prevent dislodgment of dirt. Care was taken that the barn air was free from dust, and in milking the first few streams of milk were rejected. The milk from a cow treated in this way contained 330 bacteria per cubic centimetre, while that of the mixed herd, taken under the usual conditions, contained 15,500 in the same volume. The experiment was repeated under winter conditions, at which time the mixed milk showed 7,600 bacteria per cubic centimetre, while the carefully secured milk only had 210 in the same volume. In each of these instances the milk secured with greater care remained sweet over twenty-four hours longer than the ordinary milk."
An organism which has exceptional opportunities for finding its way into cows' milk is the Bacillus coli communis, normally present in the fæces of all animals. This microbe is a very undesirable adjunct to milk, and may greatly interfere with the souring process, by multiplying extensively, and so producing a change in the milk which renders it impossible for the particular souring bacteria to carry on their work, resulting in their collapse and ultimate extinction. But this is not the only injurious effect which these Coli bacilli can produce in milk, for there is a growing conviction that their presence is responsible for many intestinal disturbances with which young children are specially troubled. Quite recently determinations of the bacterial contents of cow-dung have been made, and it has been ascertained that a single gramme,[6] freshly collected, of this material may contain as many as 375,000,000 bacteria, of which the majority were found to be the above undesirable organism, the B. coli communis.
Milk may also contain bacteria characterised by their remarkable resistance to heat, which is due to their possessing what is known as the hardy spore in addition to the ordinary rod form. The numbers in which they are present in milk varies with different samples; but they may be taken as a sort of index as to the care observed in milking, for they are always present in great quantity in uncleanly-collected milk. Careful studies have been made of this class of milk bacteria by Professor Flügge and others, and it has been found that when added to milk upon which puppies were subsequently fed the latter succumbed under symptoms of violent diarrhœa.
The danger of even a few bacteria gaining access to milk is serious, on account of the fabulous rapidity with which they multiply when they find themselves in such congenial surroundings. Professor Freudenreich has made very exhaustive investigations to show how milk microbes may multiply in the time which elapses between milking and the receipt of the milk by the consumer. The following example will convey some notion of what bacterial propagation under these circumstances is capable of.
The sample of milk in question was found to possess on reaching the laboratory, two and a half hours after milking, a little over 9,000 bacteria in a cubic centimetre. The sample was divided into three portions, which were kept at different temperatures, and after definite intervals of time they were examined. The following table shows at a glance the results obtained:—
| Number of Bacteria in about Twenty Drops of Milk. | ||||
|---|---|---|---|---|
| When Examined. | Temperature. | |||
| 15° C. | 25° C. | 35° C. | ||
| After 3 hours | 10,000 | 18,000 | 30,000 | |
| After 6 hours | 25,000 | 172,000 | 12,000,000 | |
| After 9 hours | 46,000 | 1,000,000 | 35,280,000 | |
| After 24 hours | 5,700,000 | 577,500,000 | 50,000,000 | |
Thus, after being kept in the laboratory for three hours the original 9,000 bacteria had in one case doubled, and in another more than trebled themselves. It will be seen that the temperature most favourable to the multiplication of these bacteria was 25 degrees Centigrade.
If a sample of milk containing originally such a comparatively small number of bacteria—for a figure under 10,000 per cubic centimetre sinks into utter insignificance when we read of samples containing 2,500,000—if such relatively bacterially pure samples may support such prodigious numbers of these Lilliputians, what the microbial population of less satisfactory samples may amount to well-nigh baffles our powers of calculation. Professor Russell writes: "If we compare the bacterial flora of milk with that of sewage, a fluid that is popularly, and rightly, supposed to be teeming with germ life, it will almost always be observed that milk when it is consumed is richer in bacteria by far than the sewage of our large cities. Sedgwick, in his Report to the Massachusetts Board of Health for 1890, found that the sewage of the city of Lawrence contained at the lowest 100,000 germs, whilst the maximum number was less than 4,000,000 per cubic centimetre.[7] This range in numbers is much less than is usually found in the milk-supply of our large cities."
Numerous researches have been carried out during the last half-dozen years to try and localise the origin of some of the principal dairy troubles, with a view to their possible extinction, or at least control. In the course of these investigations quite a number of the bacteria found in milk have been successfully hunted down, and their offences brought home to them.
Thus, from so-called "bitter" milk a bacillus has been isolated by Professor Weigmann, and found responsible for this particular change. Another microbe was discovered in bitter cream whose office apparently consisted in rendering milk strongly acid and extremely bitter. Again, that objectionable condition of milk known as slimy, ropy, or stringy, is brought about by certain bacteria which render it viscous; whilst another crop of microbes are occupied in conferring upon it the power of sticking to everything that touches it, making it capable of being drawn out into threads from several inches to several feet in length.