Hence it has come to be recognised that one of two manipulations must precede such microscopic examination. These simple processes are known by the terms of sedimentation and centrifugalisation. Sedimentation means merely
A Centrifuge
Used in the Examination of Milk placing the milk in conical glasses in a cool place for twenty-four hours. The introduction of improved forms of the centrifuge has brought the second method of securing a sediment into preference. Five cubic centimetres of the milk are introduced into the graduated bottle, which is then placed in the centrifuge, and whirled for one or two minutes. Thus a deposit of particulate matter is ensured. Cover-glass specimens of the sediment or deposit are then prepared and stained in the ordinary way.
In testing for tubercle something more is generally necessary. To the 50 cc. of the milk set aside for sedimentation 10 cc. of liquefied, colourless carbolic acid are added. The mixture is shaken and poured into the conical glass. After standing for twenty-four hours a little of the sediment is taken by means of a pipette and examined by ordinary methods, though after "fixing" the films with heat they are some times passed through equal parts of alcohol and ether. The stain is of course that usually adopted in tubercle, namely, the Ziehl-Neelsen. Scheurlen suggested a method for demonstrating the tubercle bacillus in milk by steeping the cover glasses first in alcohol and then ether, after which they were stained with Ziehl-Neelsen.
2. Plate Culture. The milk is to be diluted a thousand or more times with sterile water, and ordinary plate cultures made in Petri dishes or flat-bottomed conical flasks. The colonies should be counted as late as possible; but even then the isolation of pathogenic germs is uncertain. As regards further procedure, the ordinary methods of sub-culturing adopted in water examination must be strictly followed, and the special tests for Bacillus typhosus and B. coli applied. As we have already seen, the quantitative estimation of organisms in milk is not of the same value as in water.
3. Inoculation. To test the capacity of the milk for causing disease, before or after centrifugalisation, preferably the latter, a certain quantity of the sediment may be inoculated into guinea-pigs. In suspected tubercle 2 cc. may be taken; in diphtheria a little less will suffice. The inoculation should be either intraperitoneal or subcutaneous. Many authorities hold that this test is the only safe one to protect the public from milk containing germs of disease.
BACTERIA IN OTHER FOODS
Shell-fish have recently claimed the attention of bacteriologists, owing to the outbreak of typhoid and other epidemics apparently traceable to oysters.
It is four or five years since Professor Conn startled the medical world by tracing an epidemic of typhoid fever to the consumption of some uncooked oysters.[71] Almost at the same time Sir William Broadbent published in the British Medical Journal a series of cases occurring in his practice which illustrated the same channel of infection. Since then a number of similar items of evidence to the same effect have cropped up. Hence there is little wonder that a number of investigators concentrated their attention upon this matter. Professors Herdman and Boyce, of Liverpool, Dr. Cartwright Wood, Dr. Klein, and Dr. Timbrell Bulstrode are some of the chief contributors to the elucidation of this problem.
The mode of infection of oysters by pathogenic bacteria is briefly as follows: The sewage of certain coast towns is passed untreated out to sea. At or near the outfall, oyster-beds are laid down for the purpose of fattening oysters. Thus they become contaminated with saprophytic and pathogenic germs contained in the sewage. It will be at once apparent that several preliminary questions require attention before any deductions can be drawn as to whether or not oysters convey virulent disease to consumers. To the solution of these Dr. Cartwright Wood was one of the first to address himself.