ii6 



SCIENCE. 



[Vol. XIV. No. 341 



lime-kilns, sand-gatherers, rolling-mills for structural and roofing 

 iron in sheets and beams, for tinners and roofers, and the thousand 

 other trades engaged in construction, not only of the 14,000,000 

 new homes, but of the markets, stores, warehouses, post-offices, 

 court-houses, city-halls, jails, penitentiaries, etc., necessary in the 

 administration of an additional population equal to all that exists 

 now on the northern continent ! What will be the work of provid- 

 ing, and delivering at every house, three meals a day, and every 

 day, for each inhabitant thereof ? M. C. MEIGS. 



BACTERIA IN IMILK AND ITS PRODUCTS. 



During the past year, investigations on the bacteria of milk 

 have been carried on in the laboratory of the Agricultural Experi- 

 ment Station, Mansfield, Conn., under the direction of H. W. Conn, 

 professor of biology in Wesleyan University. The following is a 

 brief summary of some of the more interesting results of this 

 work. 



The term " bacteria " is used to comprise a class of organisms 

 found abundantly in the air, water, and soil, and in plants and 

 animals. As commonly employed, the term includes a large 

 variety of organisms, which naturalists divide into the three classes, 

 bacteria, yeasts, and moulds. The term "microbe" has been 

 recently introduced to cover this same ground, and is for many 

 reasons preferable. The plants included under this head are ex- 

 ceedingly numerous, and the part they play in nature is of great 

 importance. They multiply with the greatest rapidity, a single in- 

 dividual in the course of a few days being able to give rise to mil- 

 lions. While they are thus growing and multiplying, they produce 

 great changes in the medium in which they grow. All fermenta- 

 tion (such as raising of bread, fermenting of beer, cider, etc.), 

 putrefaction and decay (such as rotting of potatoes, decay of wood, 

 etc.), are produced by the organisms here included. They are of 

 immense value as well as injury. Through their agency, dead 

 animal and vegetable matter is decomposed, and prepared to be 

 incorporated with the soil and to be used as food by plants. It is 

 doubtful if vegetable life could be long continued without their 

 aid. On the other hand, they cause disease in plants, and disease 

 in animals ; many of the most dangerous diseases, as cholera, 

 typhoid- fever, consumption, hcg cholera, bovine tuberculosis, 

 chicken cholera, etc., being produced by these disease germs. 

 These organisms are extremely minute and simple. They are 

 commonly not more than one two-thousandth of an inch in length. 

 In shape they show three chief varieties, which may be compared 

 to a lead-pencil, a ball, and a corkscrew. To-day they are uni- 

 versally regarded as plants, in spite of the fact that many of them 

 are endowed with motion. 



Methods of Experiment. 



The method of experiment has been that common in modern 

 bacteriological research. For culture solutions the ordinary beef 

 peptone solution, stiffened by gelatine, or more commonly by agar- 

 agar, has been used. For most of the experiments with cream, 

 " ripened cream " has served as a starting-point. In some cases 

 sweet cream has been ripened in the laboratory, and examined each 

 day, but more commonly specimens of ripened cream have been 

 obtained from the dairy of a butter-maker and directly studied. 

 Plate cultures have been made from the cream, usuallv with agar- 

 agar, since the organisms found grow in this medium most readily. 

 From the various colonies found in the agar plates, needle cultures 

 have been made in gelatine. Subsequent purification of the or- 

 ganisms has been made in the ordinary way, by transferring from 

 tube to plate, and plate to tube, until the bacteria were separated 

 from each other in pure cultures. 



For further experiment, milk has been sterilized in test-tubes. 

 This can be done at a temperature of about 70° C, but it has been 

 found more convenient to put the tubes for a few minutes in a 

 steam sterilizer. Sterilization upon three successive days is com- 

 monly sufficient, but in a few cases milk was found to change 

 even after such treatment. The sterilization of cream has been 

 accomplished in the same way. There is more difficulty in this, 

 however, for the cream is apt to form a thick layer on the surface, 

 with a thin watery layer below ; and this occurs even in cream that 



is thoroughly sterilized. In the experiments upon the action of the 

 different bacteria upon milk, the inoculations have been made, and 

 the tubes allowed to remain at the temperature of the laboratory 

 for a day or two. If no change occurs, they are then placed in a 

 thermostat at a temperature between 30° and 35° C, and allowed 

 to stay there till they have produced their effect upon the milk. 



Accompanying all of the experiments upon milk and cream, a 

 series of experiments have been carried on with the same organ- 

 isms upon three different solutions. One was the ordinary beef 

 peptone solution without gelatine ; the second, the same solution, 

 to which a small amount of milk-sugar had been added ; and the 

 third, the beef peptone solution, with the addition of glucose in- 

 stead of milk-sugar. 



Inasmuch as the object has been to determine the general effect 

 upon milk and its products of the various bacteria present in the 

 air, it has been necessary to work with all the numerous species 

 that have been found in ripened cream. This has necessitated . 

 a very large number of experiments, continuing through eight 

 months. The account of these experiments, which, to be in any 

 way useful, will require a large number of pages of detailed de- 

 scription of individual species of bacteria, as well as their action 

 and effects, is reserved for the next annual report of the station. 

 At present it is designed to give only a brief summary of the most 

 important facts concerning the relation of bacteria to milk and its 

 products. For this reason the following remarks include results 

 of the work done at the station, and of other investigators as well, 

 and some conclusions derived from them. 



Bacteria in Milk, Cream, and Butter. 



Milk is a medium in which bacteria grow with the greatest readi- 

 ness. Experiments have thus far given indication of some thirty 

 or forty species of bacteria that are floating in the air in this vicin- 

 ity, every one of which is found in cream, and grows with the 

 greatest facility in milk. Probably none of those which were 

 studied produce disease, and hence are called non-pathogenic. 

 The researches of others have shown that many of the disease 

 (pathogenic) germs also find in milk a favorable medium for growth. 

 According to experiment, cream seems to be even a better medium 

 for the growth of bacteria than milk ; for it will keep longer with- 

 out putrefying, and thus allow some of the slower-growing species 

 to develop. Butter is not a good mediuin for the growth of bac- 

 teria, apparently because they require for their development a 

 certain amount of albuminous material, of which good butter, being 

 mostly fat, contains only a minute amount. Bacteria have, how- 

 ever, always been found present even in the sweetest of butter, but 

 usually in small numbers. When for any reason they become very 

 numerous, the butter becomes tainted. 



If milk, cream, or butter is kept free from bacteria, the ordinary 

 changes do not take place in them. For example : the bacteria in 

 milk can be readily killed by heating the milk to a boiling or even 

 lower temperature for a few moments upon three successive days ; 

 and then, bacteria being excluded, the milk is found to keep sweet in- 

 definitely. Killing the bacteria by heat is known as sterilizing. If 

 a lot of milk is thus sterilized, and then a few of any particular 

 species of bacteria are put into it, the effect which this species pro- 

 duces upon the milk can very easily be determined. It is in this 

 way that the experiments have been made. 



Milk and cream under ordinary conditions cannot be kept free 

 from bacteria. Milk drawn from a healthy cow is free from them, 

 but they may get into it when the milk is in contact with the air 

 during milking. A single experiment will indicate the difiiculty 

 of keeping them out of milk. Eight test-tubes were washed per- 

 fectly clean, and plugged with a mass of cotton. They were then 

 heated very hot until all living matter_in them was killed. These 

 were taken into a niilking-yard, and, after the teats of the cow and 

 the hands of the milker had been carefully washed, the cotton 

 plug was taken out and milk drawn directly from the cow into the 

 tubes, and the cotton plug replaced. Of these eight tubes, seven 

 soured in a few days, and many bacteria were found in them. 

 The other remained sweet for a long time, but eventually it also 

 changed. From this experiment it is seen that in the few seconds 

 in which it was exposed to the air the milk was coVitaminated with 

 bacteria. A very common source of contamination of milk is from 



