Kitabı oku: «Bacteria in Daily Life», sayfa 7
Some German investigators have discovered the interesting fact that the centrifugal method of separating milk not only has a remarkable effect upon its bacterial contents, but also upon tubercle bacilli when present. On examining the so-called "separator slime," it is found to contain not only large quantities of solid matters, but also masses of bacteria which have been thrown out during the operation. This method of treating milk has, curiously, a particular effect upon tubercle bacilli present, for Professor Scheurlen has found that they are nearly all left in the slime. Naturally his observation was not slow in being tested by other investigators; but Professor Bang has quite independently confirmed Scheurlen's discovery, and, still more recently, Moore purposely infected milk with these bacilli, and found that they were deposited in the slime to a most remarkable extent. Coupled, however, with this peculiar behaviour of tubercle bacilli in separated milk is the fact called attention to by Ostertag, that tuberculosis is much more prevalent among swine in Denmark and North Germany, where the centrifugal process in creaming is extensively used, and where, until recently, this slime was given to the animals in its raw, uncooked condition.
Before leaving this subject of separated milk, reference may be made to a danger, which has recently been publicly called attention to, surrounding the use which is made of skim milk. By an arrangement with the farmers who supply the milk, those clients who principally use it for producing butter return the skim milk to them after it has been through the separator, when it is employed for stock-feeding purposes. The milk in large dairies derived from different farmers is mixed, and hence the skim milk which is returned is also mixed. Thus, in the event of the milk from one farm being infected, not only is the whole milk-supply of a particular dairy infected, but, in returning the mixed skim milk likewise infected in its proper proportion to the different farmers, the virus is distributed over several farms. So real is this danger, and such unfortunate results have followed this practice of returning mixed infected skim milk, that since 1894 the Prussian Government has issued special orders for its disinfection by means of heat, in the hope of coping with this difficulty.
The longevity of the tubercle bacillus and its remarkable vitality under all kinds of untoward circumstances have not unnaturally added fresh significance to this frequent discovery of its presence in milk; moreover, laboratory experiments have shown that these germs can live for upwards of one hundred and twenty days in butter, and from sixty to seventy days in cheese. It is not surprising, therefore, to find a Royal Commission appointed in 1890 with the express object of inquiring and reporting upon "What is the effect, if any, of food derived from tuberculous animals on human health?"
In the summary appended to the report we read: "Tuberculous matter in milk is exceptionally active in its operation upon animals fed either with milk or with dairy produce derived from it. No doubt the largest part of the tuberculosis which man obtains through his food is by means of milk containing tuberculous matter."
That the Commissioners were alive to the great importance of this means of spreading disease is further shown by the following significant paragraph: "In regard to milk, we are aware of the preference by English people for drinking cow's milk raw, a practice attended by danger on account of possible contamination by pathogenic organisms."
The Commissioners spared no pains in endeavouring to throw light upon the important question they were appointed to report upon, and five years elapsed before they published the results of their inquiries. A decade ago the opinions expressed by them represented the current opinions of the leading bacteriological authorities in scientific circles at home and abroad, and these opinions were gradually filtering down to the general public, which is so conservative in clinging to traditions and popular delusions, when, like a flash out of the blue, the bacteriological Jove, Professor Robert Koch, hurled his thunderbolt into the arena, and at the British Congress on Consumption, held in London in the summer of 1901, declared his belief that bovine and human tuberculosis were distinct diseases. The significance of such a challenge to current scientific opinion, and its far-reaching influence if proved to be correct, was quickly appreciated by the distinguished audience who had gathered to hear what so great an authority as Dr. Robert Koch had to say on consumption and its distribution. The vital question raised by the original discoverer of the tubercle bacillus is still the subject of discussion, experimental inquiry, and much controversy, and we cannot here attempt to discuss the pros and cons for the acceptance or rejection of this new theory concerning the character of tuberculosis. It would, however, be regrettable in the extreme if the publication of this opinion were to encourage dairy authorities to relax in the slightest the efforts now so tardily being made by them to protect their dairy produce and ensure its safety for food-supply.
Before leaving this branch of the subject reference must be made to some very important researches recently published by Professor Ostertag, of Berlin, on the presence of tubercle bacilli in the milk derived from cows which, whilst reacting to the tuberculin test, exhibit no clinical symptoms of tuberculosis. The importance of this investigation to farmers and all breeders of stock is evident, for it has not infrequently been urged that all the milk from such tuberculin-reacting cows should be discarded for dietetic purposes. Professor Ostertag, at the request of the German Government, has carried out a most elaborate and very extensive series of investigations to determine the question as to whether such milk is dangerous to health. I cannot do better than quote the conclusions appended to the original memoir, in which Professor Ostertag expresses himself as follows: "The milk of cows which only react to tuberculin does not contain tubercle bacilli; calves and pigs can be fed during weeks and months with milk derived from such cows without contracting tuberculosis."
A very important rider, however, is added, in which it is pointed out that inasmuch as no doubt exists as to the highly infectious character of the milk derived from cows the udders of which are tuberculous, and from animals in which the disease is clinically recognisable, the weeding out of all such animals must be regarded as the most important measure for the prevention of the dissemination of tuberculosis through milk.
We must now pass on to a consideration of some of the methods which are available for obtaining germ-free milk, some of which are, however, attended with too great labour and inconvenience to admit of practical application. Thus, wishing to prepare some sterile milk without altering its chemical composition to feed certain microbes with, I had to patiently heat it for from one to two hours on five successive days, watching the while that the temperature remained between 58° and 65° centigrade. The milk was sterile, and I kept it for months, but such a process, of course, is impossible for domestic purposes.
The addition of chemicals to milk is both undesirable and ineffectual; amongst such substances boracic acid, borax, and salicylic acid are employed, but whilst the two former have been found to produce but little effect upon disease germs present in milk, salicylic acid hinders curdling more than other substances, and even if added in the small proportion of twelve grains per quart is said to impart a taste to the milk, and is, moreover, incapable of destroying typhoid bacilli if present.
Authorities are, moreover, not agreed as to the harmlessness of this ingredient, and in France the employment of salicylic acid in the preservation of food is strenuously opposed by doctors, who consider its habitual use injurious to health.
A Departmental Committee of the Local Government Board was appointed in this country to inquire into the use of preservatives in foods. In their report they state that from 42 up to 126 grains of boracic acid were detected in milk offered for sale, and that on one occasion no less than 80 grains of this material were present in a pint of milk sold to their inspector. It is pointed out that as long as preservatives are permitted there is no guarantee against the addition of excessive amounts to milk, and that evidence has been obtained pointing to an injurious effect of boracised milk upon the health of young children. The Committee report that in Denmark the use of preservatives is strictly prohibited, and the prohibition is strongly enforced; neither are preservatives permitted in Belgium.
The application of heat to milk is, in fact, the only advisable and reliable method for rendering it free from germs, but a great deal depends upon the manner in which the heat is applied and the cleanly condition or otherwise of the milk employed.
The difficulties which have to be overcome in producing efficiently sterilised milk are due, in the first place, to the remarkable power of resisting heat which characterises not only some disease germs, but also some of the microbes which are particularly partial to milk; secondly, to the sensitiveness of milk to heat, as exhibited by its alteration in taste and other respects through exposure to high temperatures.
To overcome these difficulties many ingenious pieces of apparatus have been devised, based upon a process originally introduced by Pasteur for preventing certain defects in wine and beer, and which consists in the application of a temperature of about 60 °Centigrade. This process is known as Pasteurisation, after its renowned initiator.
So-called "Pasteurised" milk has become during the last year or so increasingly popular in this country, whilst on the Continent it has been largely dealt in for several years past, and has commercially proved a great success. Indeed, so strong is the prejudice amongst our neighbours across the Channel against using unboiled milk that in Leipzig and other cities in Germany endeavours have been made by charitable and other societies to encourage the use of sterile milk amongst the poorer classes, whilst it has been stated that the introduction of Pasteurised milk among the poor of New York City, through the philanthropic efforts of Mr. Nathan Straus, has done much to reduce the high rate of mortality amongst infants during the hot summer months. In France, i. e. in Paris and Grenoble, in order to reduce if possible the lamentable mortality amongst infants from diarrhœa in the summer months, which was largely attributed to the use of unboiled milk, sterilised milk was distributed to the poor at the cost of the community in general. In Grenoble, according to statistics collected by Berlioz during the years 1894-6, the death-rate of infants under a year old in the months of July, August, and September fed on raw milk reached 69·3 per 1,000, whilst amongst those supplied with sterilised milk it was reduced to 27·9 per 1,000.
Just, however, as all is not gold that glitters, so all sterilised milk so-called is not necessarily free from bacteria. Indeed, according to a recent German authority, "the complete and certain sterilisation of milk is not yet to hand."
Dr. Weber examined the sterilised milk as supplied by various companies in the city of Berlin. As many as 150 bottles were tested from eight different sources, with the result that not one of these eight companies was found to be supplying milk free from bacteria, or, in other words, what it professed to be – sterile. True, the percentage of sterile bottles varied from 5 per cent. in some of the supplies to 86 per cent. in others.
Thus it may be realised how, as has been already pointed out, difficult a matter it is to devise an efficient apparatus for the reliable sterilisation of milk. So far it appears that the best results have been obtained with an apparatus devised by Flaack, a director of the Brunswick Sterilising Milk Company, and known as the Flaack apparatus. Exhaustive examinations made during the course of a whole year in the Hygienic Institute at Würzburg never once showed a failure, all the samples tested being germ-free.
Some supervision is, therefore, necessary in the case of these milk-sterilising companies to ensure that the public is obtaining what it is paying for, as it has been shown by Professor Flügge, a world-renowned authority on the subject of milk and its sterilisation, that the bacteria left over in these so-called sterilised milk samples are by no means invariably a harmless residue, but, on the contrary, may consist of individuals which he has gathered together in a class under the heading of poisonous peptonising bacteria, and which owe this unfortunate designation to the rapidity and energy with which they can engender the putrefaction of albumen. As indicating how essential it is that every detail in the sterilisation of milk should be adequately assessed, I may mention a paper recently published by H. L. Russell and E. G. Hastings, of the Wisconsin Agricultural Experiment Station in the United States, on the importance of Pasteurising milk in closed rather than in open vessels, bacteria having been found more resistant in milk when heated in contact with the air than in closed vessels, this variation being attributed to the formation of a surface pellicle, which readily forms on milk when heated in open vessels to a temperature of about 60 °Centigrade or above. Experiments showed that organisms present in this pellicle or skin were capable of retaining their vitality when exposed to a temperature six degrees higher than that of the milk beneath the membrane in which they were destroyed.
Objections to the use of boiled milk have been frequently made on the grounds of its being more difficult of digestion, and hence less wholesome than the raw article. I may only point out that in this, as in most other matters where opinions may be made or unmade, and in consequence of the facts available being scanty must be more or less arbitrary in character, Dr. Duclaux, the successor to Pasteur as Director of the Pasteur Institute in Paris, has expressed himself as follows in an article on "La digestibilité du lait stérilisé." After reviewing the various special researches which have been made on the subject, he says: —
"Ceci nous amène à une conclusion qu'il faut bien avoir le courage de tirer, c'est que ces études chimiques sur la digestibilité du lait ne sont pas adéquates à la question à résoudre… En attendant, tenons-nous-en à cette conclusion générale, que le lait pasteurisé, chauffé ou stérilisé, est encore du lait, devant la science comme devant la pratique, et que si son emploi présente parfois des inconvénients, ceux-ci sont légers et amplement compensés par les avantages."
BACTERIA AND ICE
The fate of bacteria when frozen excited the curiosity of investigators already in the early years of bacteriology, for in 1871 we find Burdon Sanderson recording the fact that water which he had obtained from the purest ice contained microzymes, or, as we now prefer to call them, micro-organisms.
It is quite possible that at the time this announcement was made it may have been received with some scepticism, for it was undoubtedly difficult to believe that such minute and primitive forms of vegetable life, seemingly so scantily equipped for the struggle for existence, should be able to withstand conditions to which vegetable life in more exalted circles so frequently and lamentably succumbs.
The tormented agriculturist realises only too well what havoc is followed by a return in May to that season
"When icicles hang by the wall,
And Dick the shepherd blows his nail
And Tom bears logs into the hall,
And milk comes frozen home in pail."
Again, with what solicitude those of us who have gardens wait to see what will have survived the iron grip of winter in our favourite flower borders, and how frequently we have to face blanks in the ranks of some of its most cherished occupants! Numerous bacteriologists, however, have now confirmed this fact, the fields of ice and snow have been repeatedly explored for micro-organisms, and it has been shown how even the ice on the summit of Mont Blanc has its complement of bacterial flora, that hailstones as they descend upon the earth contain bacteria, that snow, the emblem of purity, is but a whited sepulchre, and will on demand deliver up its bacterial hosts. Quite apart from its general scientific interest, the bacterial occupation of ice is of importance from a hygienic point of view, and a large number of examinations of ice as supplied for consumption have been made. Thus, Professor Fraenkl and also Dr. Heyroth have submitted the ice-supply of the city of Berlin to an exhaustive bacteriological examination. These investigations showed that the bacterial population of ice as supplied to Berlin is a very variable one, and fluctuates between great extremes, rising to as many as 25,000 bacteria in a cubic centimetre (about twenty drops) of ice-water, and falling to as few as two in the same measure.
There are numerous circumstances which come into play in determining the density of the bacterial population in ice. First, of course, the initial quality of the water from which the ice is derived is a factor of great importance, for the purer the water the fewer will be the bacteria found in the resulting ice.
Again, if the ice field is wind-swept by air bearing an unduly rich complement of bacteria, as may be expected in the vicinity of populous cities, for example, then the ice will reflect in its bacterial contents the undesirable neighbourhood in which it was produced. Water in repose, again, yields purer ice than water in movement during freezing, for during rest opportunity is given for the bacteria present in suspension to subside, the process of sedimentation or deposition of bacteria which takes place under these conditions playing an important part in water-purification; when, however, the water is disturbed by swift currents, or agitated by storms, this process is interrupted, and the bacteria become entangled in the ice and frozen in situ.
The importance attaching to the physical conditions under which ice is produced in enabling an estimate to be formed of the safety or otherwise of the same for consumption may be gathered from the following extract from an American report on the subject: —
"On the whole it is evident that the conditions surrounding water when it freezes are very important factors in determining the purity of the ice formed. If there is a considerable depth of water in portions of a somewhat polluted pond or river, and the ice is formed in these portions in comparatively quiet water with but little matter in suspension, this ice will probably be entirely satisfactory for domestic use. On the other hand, ice formed in shallow portions of such ponds or rivers, even during still weather, or in any portion if there is a considerable movement of the water by currents or wind while it is forming, may be rendered by these conditions entirely unfit for domestic use."
We have learnt that ice contains bacteria, that its bacterial contents are to a certain extent dependent upon the bacterial quality of the water before crystallisation, and that an important factor in determining its purity is afforded by the physical conditions prevailing at the time of freezing.
It will be of interest to ascertain in more detail what effect the process of freezing has upon the number of bacteria present in the water – what is the degree of bacterial purification effected during the conversion of water into ice.
Now Professor Uffreduzzi, in his investigations on the ice-supply of Turin, part of which is derived from a much-polluted portion of the River Dora, found that about 90 per cent. less bacteria were present in the ice than were present in the water from which it was produced. In the making of ice, therefore, a remarkable removal of bacteria may be effected which approaches very nearly the degree of bacterial purification which is achieved during the best-conducted sand-filtration of water.
Uffreduzzi's results have been repeatedly confirmed by other researches. Thus, in regard to ice obtained from the River Merrimac, water which contained originally about 38,600 bacteria per cubic centimetre, on its conversion into ice had only from three to six. Sewage, again, containing about a million and a half bacteria per cubic centimetre after being frozen only contained under 74,000. It should be mentioned that this last figure represented the number of bacteria obtained by thawing the outside of the sewage ice-cake; inside the cake there were more found – about 121,000. The difference in these figures is due to the fact that, whereas the outer layers of ice looked quite clear, towards the centre the ice contained sewage sludge and hence more bacteria had become arrested; but in spite of this the bacterial purification effected is very striking, although not sufficient to render the use of ice from such a polluted source either palatable or desirable.
It is, of course, a well-known fact that water possesses the power of purifying itself during its transformation into ice, and that the process of crystallisation not only prevents a considerable proportion of the matters in suspension from becoming embodied in the ice, but also eliminates a large percentage of the matters in solution, the latter being driven from the water which is being frozen into the water beneath. If, therefore, ice in the act of forming can get rid of matters in solution, it is not difficult to understand how it can eject bacteria, which though so minute are yet bodies of appreciable dimension and in suspension. But that there are limits to this power of excluding bacteria, and that, as in the case of other mechanical processes, an overtaxing of the available resources is at once reflected in the inferiority of the product, is shown by the frozen sewage experiment, in which the ice, having had too large a supply of bacteria in the first instance to deal with, was unable to get rid of more than a certain proportion, and was obliged to retain a very considerable number. Hence great as is the degree of purification achieved by ice in forming, yet it must be recognised that its powers in this direction are limited, and that the fact of water being frozen does not necessarily convert a bad water into immaculate ice.
It is worthy of note that the city of Lawrence, in Massachusetts, obtains the greater portion of its ice from a river which in its raw, unpurified condition was rejected for purposes of water-supply in consequence of the numerous and severe epidemics of typhoid fever which accompanied its use. Since the application of sand-filtration to this water, however, the death-rate from typhoid in this city, instead of being abnormally high, has fallen abnormally low, and this improvement is attributed to the excellent quality of the water supplied to the city, and has taken place despite the use which still continues of ice from the polluted river. The authorities consider the city's immunity from typhoid amply justifies their sanctioning the distribution of this river-ice, the freezing of the water having rendered it sufficiently pure to remove all danger to health from its consumption.
So far we have been considering the effect on bacteria of freezing carried on under more or less natural conditions; but much interesting work of a more detailed character has been carried out with reference to the behaviour of particular varieties of micro-organisms when frozen under more or less artificial conditions.
Thus Dr. Prudden froze various bacteria in water at temperatures ranging from -1 °C. to -10 °C., and he found that different varieties were very differently affected by this treatment; that, for example, a bacillus originally obtained from water, and introduced in such numbers as represented by 800,000 individuals being present in every twenty drops, after four days' freezing had entirely disappeared, not one having survived. On the other hand, similar experiments in which the typhoid bacillus was used resulted in the latter not only enduring a freezing of four days' duration, but emerging triumphant after it had been carried on for more than 103 days!
In these experiments it should be borne in mind that, as the ice was frozen to a solid block or lump, there was no opportunity for the mechanical committal of the bacteria during freezing to the water beneath; all the bacteria present were imprisoned in the ice, and the fact that the typhoid bacteria were not destroyed by being frozen shows that they can withstand exposure to such low temperatures, although, as we have seen, the other variety of bacillus employed was destroyed.
Dr. Prudden, however, discovered an ingenious method by which even typhoid bacilli were compelled to succumb when frozen. In the course of his investigations he found that bacteria which had offered the stoutest resistance under freezing were extremely sensitive to this treatment if the process was carried on intermittently, or, in order words, if the temperature surrounding them was alternately lowered and raised.
In this manner the bacteria may be said to be subjected to a succession of cold shocks, instead of being permitted to remain in a continuously benumbed condition. The vitality of typhoid bacilli was put to the test under these circumstances, the freezing process being carried on over twenty-four hours, during which time, however, it was three times interrupted by the ice being thawed. The effect on the typhoid bacteria was striking in the extreme; from there being about 40,000 present in every twenty drops, representing the number originally put into the water, there were only ninety at the end of the twenty-four hours; and after a further period of three days, during which this treatment was repeated, not a single bacillus could be found. This signal surrender to scientific tactics forms a marked contrast to the stout resistance maintained for over 103 days under the ordinary methods of attack.
But, although the typhoid bacillus appears to submit and meekly succumb to this plan of campaign, yet the conclusion must not be rashly drawn that all descriptions of bacteria will be equally feeble and helpless in these circumstances.
Doctors Percy Frankland and Templeman have shown that the spore form of the anthrax bacillus is able to successfully challenge all such attempts upon its vitality. Thus when put into water and frozen at a temperature of -20 °C., the process being extended over a period of three months and interrupted no fewer than twenty-nine times by thawings, when examined even after this severe series of shocks, it showed no signs of submission and clung to life as tenaciously as ever.
The more sensitive form of anthrax, however, the bacillus, was readily destroyed; for after one freezing its numbers were already so much reduced that it was only with difficulty that even one or two could be found, and after the second freezing every one out of the large number originally present had died.
Renewed interest has been of late revived in the question of the behaviour of bacteria at low temperatures, in consequence of the possibility of obtaining, by means of liquid air and liquid hydrogen, degrees of cold which were undreamt of by the scientific philosophers of fifty years ago. Public interest has also been quickened in such inquiries on account of the important part which low temperatures play in many great commercial developments, their application rendering possible the transport from and to all parts of the world of valuable but perishable foodstuffs, thus encouraging local industries by opening up markets, and bringing prosperity to countries and communities which before were seeking in vain an outlet for their surplus produce.
The application of cold storage for preservation purposes is, however, no novelty; for nature ages ago set us the example, and of this we have been lately reminded afresh by the discovery announced by Dr. Herz of a mammoth in Siberia, which, despite the thousands of years which have elapsed since it was originally overwhelmed and frozen, is described as being in a marvellous state of preservation.
Thus we are told that "most of the hair on the body had been scraped away by ice, but its mane and near foreleg were in perfect preservation and covered with long hair. The hair of the mane was from four to five inches long, and of a yellowish brown colour, while its left leg was covered with black hair. In its stomach was found a quantity of undigested food, and on its tongue was the herbage which it had been eating when it died. This was quite green."
Considering that certainly more than eight thousand years had elapsed since this creature was peacefully consuming what proved to be its last meal, nature's method of cold storage must indeed be regarded as unsurpassable in the excellence of its results.
I believe it was in the year 1884 that the first attempts were made to follow more closely and in greater detail the precise effect upon different bacteria of submitting them to temperatures of such a low degree as -130 °C., obtained by means of solid carbonic acid. These experiments were carried out by Pictet and Young, and are recorded in the Comptes Rendus of the Paris Academy of Sciences.
They differ from those which we have so far been considering, inasmuch as the bacteria were not frozen in water, but in culture-material, or, in other words, like the mammoth, whilst enjoying a midday meal!
One of the micro-organisms experimented with was a bacillus known at that time as the rinderpest bacillus, capable of producing disease in animals when inoculated into them and existing both in the spore and bacillar form. Pictet and Young specially state that the spore form was present in the specimens employed by them, and hence the fact that this micro-organism was alive after being frozen and exposed to this low temperature of -130 °C. for the space of twenty hours is not, perhaps, so surprising when we bear in mind the remarkable feats of endurance exhibited by spores which have with justification obtained for them a prominent place amongst the so-called curiosities of bacteriology. But of more interest than their mere survival in these circumstances is the fact that, on being restored to animation – or, in other words, released from their ice-prison – these bacteria were discovered to have retained all their pathogenic properties, this period of enforced rigidity having in no way affected their disease-producing powers.
