Popular Science Monthly/Volume 23/May 1883/Microscopic Life in the Air
ANCIENT Pantheism animated all nature. Gnomes in caverns, naiads in springs, sylphs in the air, represented life, pervading everything. Twenty centuries having passed, science has resuscitated these elementary genii under the form of organic germs; and we are forced to-day to recognize that the reality surpasses all the bold conceptions of the fable. From pole to pole the atmosphere transports myriads of microscopic animals and plants. They are counted by hundreds in each cubic metre of air that we breathe in Paris. Developing themselves in the organic infusions into which they fall, they soon determine their complete decomposition; and they play their parts in virulent diseases and in fermentations. No doubt is permissible on this point after the admirable labors of M. Pasteur; and every day a new workman brings his stone as a contribution to the grand edifice of which this illustrious physiologist has drafted the plan and himself laid the impregnable foundations.
A considerable work has just appeared on this subject. For several years, M. Miquel has pursued interesting researches upon the microbes of the air; and, in addition to the regular publication of his investigations in the "Annuaire" of the Observatory at Montsouris, he has just completed an important memoir, which includes valuable facts respecting these organisms. We propose to show here how this department of science has been developed, and what means of carrying out its objects it possesses. F. A. Pouchet devised the aëroscope that bears his name, for collecting dust from the air. It consists of a small cylinder connected with an aspirator; a disk of glass coated with glycerine, and placed at the bottom of the cylinder, receives the jet of air which is produced by the aspirator. The glycerine retains the corpuscles which are brought in by the current, and it is then easy to observe them. This apparatus has been modified in various ways,
but with it, or something like it, the earlier investigators, MM. Pasteur, F. Pouchet, Maddox, Douglas Cunningham, and others, have explored the atmosphere. It did not take long to discover that the air around us contains remnants of articles that we use existing in the condition of impalpable dust. Wool from our clothing, cotton, silk, starch, are floating in it, associated with fragments of various kinds—butterflies' scales, dried tarsuses of insects, feather-barbs, and skeletons of little worms. Pollens of the coniferæ and of numerous plants are abundant in it during the floral season. Particles of mineral matter are also found there, among them those curious spherules of meteoric air which have been described by M. Gaston Tissandier.
Attention is, however, most strongly fixed upon the number and variety of spores of cryptogams of which the air operates as a vehicle of dissemination. Germs of the common molds, and the reproductive cells of the algae that live on the roofs of houses, on walls, and on damp earth, are nearly always abundant. M. Miquel has tried to determine the laws that govern the appearance of these plants in the atmosphere. He first counted them, by disposing his aëroscopes so that they should register the volume of the air passing through them, and estimating the spores deposited by those volumes, and from this deducing the number of spores contained in a cubic metre of air. Repeating these measurements every day and every hour for several years, and taking care to notice all the anterior or concomitant meteorological
conditions, he succeeded in defining the influence of the seasons, and of temperature, dryness, and moisture in the progress of the phenomenon.
This method, applied at the Montsouris Observatory, shows that the number of mold-spores is weak in January and February, diminishes in March, and rises in April; the increase is very sensible in May, and the maximum is reached in June. The number then diminishes slowly till October, considerably in November, and reaches a minimum in December.
It may be said, generally, for the locality where the experiments were made, that a cubic metre of air contains on the average seven thousand mold-spores in December, January, and February; twelve thousand in May; thirty-five thousand in June; twenty-three thousand in August; fourteen thousand in October, and eight thousand in November. If, instead of considering the means of several years, we compare different periods of the same year, we shall not find the variations so regular. Sometimes the number of germs diminishes while the heat is increasing. In that case the effect of temperature is masked by the preponderance of another factor—the hygrometric condition of the air. This fact is explained by remembering that
the development of molds is dependent upon both heat and moisture. The effect of moisture, however, varies according to the season, and with changes in temperature. Dry weather diminishes the number of germs in summer, and increases it in winter, while moist weather operates in an inverse manner.
Storms in the pleasant season are followed by a growth of cryptogamic vegetation, and purify the atmosphere for only a very short time. Fifteen or eighteen hours after a rain, says M. Miquel, "the spores appear to be five or ten times as numerous as before. On the other hand, mineral dusts and several kinds of microbes continue to be rare till the moisture which has caused them to adhere to the blades of grass and the moist soil of the surface has dried away."
These investigations, while they are profitable in a purely scientific aspect, are also destined to be of service in agriculture and hygiene. As M. Miquel remarks, the use of aëroscopes will be of value for the discovery in the air of the germs of the molds which attack our cereals. Regarding the etiology of certain contagious affections, he continues: "It does not seem to be proved that the various spores introduced into our economy, to the number of 300,000 a day, or 100,000,000 a year, are perfectly innocuous. The appearance of thrush in the mouths of young children and in the respiratory canals of the dying seems to demonstrate also that the molds form a part of the class of parasites which are ready to take possession of our organism whenever it presents a vulnerable point or a point of weak resisting power."
M. Pasteur has long insisted on the utility of these researches. "I believe," he wrote in 1862, "that it would be of great advantage to multiply the studies on this subject, and to compare in the same place at different seasons, and in different places at the same time, the number of corpuscles disseminated in the atmosphere. Our knowledge of the phenomena of morbid contagion, especially during the prevalence of epidemics, would, it appears to me, gain from researches prosecuted in this direction."
Since M. Pasteur has established the parasitic character of zymotic diseases like the hen-cholera, sheep-rot, septicæmia, measles, etc., the micrographic statistics of the air has risen to a considerable importance. It has had, however, to concentrate its efforts chiefly upon a class of rudimentary organisms very different from the green algae and the molds of which we have spoken. This group is the one to which the viruses belong. The plants composing it, and which are designated under the common denomination of bacteria, escape the process of numeration in use for the higher cryptogams. In consequence of their extreme minuteness and refractive power, they are
Fig. 4. Specimens of bacteria. A, Micrococcus in isolated cells or aggregated into balls and strings; B, Bacterium; C. Bacillus; a, bâtonnets (adult bacilli; b, bâtonnets with spores; c, isolated spores; d, germinating spores.
invisible, and unrecognizable in the preparations of the aëroscopes. Their existence in the air was long denied, and the proof that they abound in it only dates from the experiments that were instituted by M. Pasteur for the solution of the question of heterogeny. The methods be devised are the ones that are still used to detect the presence of these plants in the atmosphere. Generally, these methods are founded on the fact that organic liquids become peopled with microphytes, or remain unchanged, according as the air comes to them charged with its normal quantity of germs, or after having been cleared of them by filtration. We can, therefore, examine the bacteria in the atmosphere by causing the air or the rain-water containing them to pass into liquors favorable to their nutrition, but previously free from them. Liquids for the culture of bacteria are easy to procure. Among them are the mineral solutions of Pasteur and of Cohn, infusions of hay and of turnips, neutral urine, broths of chicken or beef, and Liebig's extract. It is, however, very hard to obtain such liquids absolutely pure of every living being. Eminent physiologists have thought that all the germs could be killed by boiling them for a considerable time. Apparently the protoplasm, being an albuminoid substance, would be coagulated at a temperature of between 167º and 176º; but very exact experiments have shown that while the protoplasms of different living beings belong to the same class of substances, they are not identical. Marked differences in this respect have been perceived even in the same beings. Thus, in the bacillus the protoplasm of the developed organism and that of the spore are not of the same quality. The former is in active life, the second in a state of life so low that it appears latent. A spore of this kind, as M. Chamberland has observed, will resist boiling water for hours, while the bâtonnet which is developed from it would perish rapidly in the same water at 122º.
M. Koch has conceived a method of discontinuous heating to sterilize liquids that are coagulable by heat. He raises his liquid to a temperature of not quite 158º to kill the adult bacteria; then having cooled it, to give the spores time to germinate, he raises it again to about 158º; and he believes that he can in this way destroy all the germs. M. Miquel makes a just criticism of this singular theory. We arrange that the spores of the microbes "must germinate in twelve or twenty-four hours, so that we may surely kill them if they go into the trap we set for them. But some of the germs may be obstinate or hardy, and we make a new trial, and for prudence a third and a fourth trial, after which we assume that all the bacteria have been destroyed. Unfortunately for the method of discontinuous heating, there are wary germs the development of which does not begin till after the fifth, the tenth, and even the twentieth day, and which, far from being stimulated in their growth by the successive heatings, at every repetition shut themselves up more closely in their latent seed-life. This method of sterilization can not, then, be depended upon."
A still more subtile cause of error must be guarded against. Cohn's mineral liquid will remain clear for an indefinite period after having been exposed to a heat of from 158° to 167°, without being free from germs; for, if we afterward add a quantity of broth which has been sterilized at 230°, the mixture of the two liquids, which, separate, would have continued perfectly limpid, will shortly swarm with bacilli and other organisms. Cohn's liquid can not be fully deprived of active germs till it has been boiled for four hours. If, continues M. Miquel,
we add sewer-water that has been heated for several hours in an hermetically sealed retort at 176° to Cohn's liquid which has been sterilized at 230°, and place the mixture in a hot bath, nothing will appear in it even after a month. Apparently it is perfectly free from living-germs. But if a few drops of it are placed in a broth, also fully sterilized, the broth will in a day or two appear full of bacilli.
We must, then, unless we would expose ourselves to grave errors, distinguish between apparent and real sterilization. While beef-broth, neutralized with potash and heated to 230°, will remain sterile for an indefinite time, it is a good plan with other compositions and for particular bacteria to attain a temperature as high as 302.
Heat, unfortunately, modifies the composition of organic liquids, and diminishes their nutritive properties. To obviate this, we must find means to sterilize the liquids without heating them. Filtration has been employed, and a number of adaptations of apparatus have been devised by means of which this object is accomplished satisfactorily.
In order to study the germs in the air, we must not only possess a sensitive and wholly pure liquid, but must have means of arranging it for the cultivation of the organisms, under such circumstances that we may be sure it shall contain no germs except those that are derived from the air we introduce for the experiment. MM. Chamberland and Miquel have employed simple apparatus which seem to effect this purpose perfectly.
If the experiments are made with rain-water, to ascertain the number of germs it collects in passing through the air, it may sometimes happen that, when a determined volume of water is evenly distributed in a considerable number of the cultivation-tubes, only a part of the tubes will become troubled. Generally, it may be said that if the water contains as the average one bacterium per cubic centimetre, nearly every tube that receives a centimetre of the water will become turbid; but, if the number of bacteria is only half as many, half the tubes will remain sterile. This rule, though inexact in theory, seems to prevail with an approach to exactness in practice. M. Miquel applied it to the estimation of the bacteria in rain-water, and found that at the beginning of storms the water of precipitation contained a considerable number, amounting sometimes to as many as fifteen per cubic centimetre, and that the number immediately began to diminish; but that, strange to say, "after two or three days of moist and rainy weather the meteoric water frequently contained more bacteria than at the beginning of the rain. As the atmosphere was then in a condition of extreme purity—a fact that was established simultaneously by the statistics of the germs in the air—it seemed to be shown that the bacteria could live and multiply in the very midst of the clouds, or, perhaps, that the clouds might in their course through space charge themselves with a very valuable contingent of germs."
In studying under the microscope the development of these little organisms, in the preparations of which they have taken possession, a very curious evolution of one of the microbes of the air is revealed. The organism is a bacterium, which presents at first sight the characteristics of a very long, filamentous bacillus. M. Miquel affirms that he has seen this organism afterward divide itself into segments of unequal size, in such a way as to form strings of micrococci. The observation deserves consideration, for, if it is confirmed, and the
habit is proved to be general, it will establish a line of union between the different types of the inferior algae, which were believed to be fixed, but may be, after all, only transient genera.
It is important to have the microbes collected in the broth of the tubes sown in different kinds of liquors. Such treatment furnishes the only means of discovering the nature of the organisms, for characteristics deduced from their shape are of no significance. Most frequently they can he distinguished only by the fermentations they produce. Numerous experiments based on this principle will be required for the exact determination of the bacteria in the atmosphere. In the present condition of the science, we have to limit ourselves to the general statistics of the Micrococci, Vibrios, Bacteria, Bacilli, and Cladothrices that live in the air, without undertaking to classify in any precise way all the beings comprehended under each of these denominations.
The observations conducted at the Observatory of Montsouris show that there are on the average eighty bacteria in a cubic metre of air. The highest number was observed in the fall, the lowest in the winter. There were found fifty bacteria in December and January, only thirty-three in February, one hundred and five in May, fifty in June, and one hundred and seventy in October. The diagrams of daily observations
show that the number of spores of these alga? increases with the temperature. Inversely to what takes place in the case of the molds, the number of the schizophytes, small in rainy weather, rises when all the moisture has disappeared from the surface of the soil. The counter-action of moisture is stronger than the direct action of temperature; and this fact accounts for the rarity of the bacteria after the great rains of February, April, and June. Still a long period of dry weather does not appear to be favorable to the development of the plants. The number rises at first during the hot season, but diminishes under the influence of a progressive desiccation toward the second or third week.
The diminution in hygrometric conditions manifested in September and October explains the recrudescence of the bacteria during these months. Some micrographers have suggested that the germs may be transported by the vapor of water; but M. Miquel's experiments invalidate this hypothesis, and indicate that the evaporation of water from the surface of the ground never carries any schizophytes with it. On the other hand, numerous tests have shown that dry dusts, especially those of hospitals, proceeding from substances in a state of putrefaction, sanious pus, and the dejections of the sick, are charged with microbes. Great agglomerations of men furnish the most of them. According to the measurements made in the Rue de Rivoli and Montsouris, the air in the interior of Paris is nine or ten times richer in bacteria than that in the neighborhood of the fortifications. At the observatory, the winds from the north bring many more than the winds from the south.
The vertical distribution of the microbes also shows that they come mostly from the dirt of our streets and houses. While a cubic metre of air at the top of the Pantheon contains only twenty-eight of them, the same quantity of air in the Park of Montsouris contains forty-five, and in the mairie of the fourth arrondissement, four hundred and sixty-two. These numbers are, however, insignificant in comparison with those furnished by the analyses of sewer-waters. We give a few specimens from M. Miquel's analyses, in which is shown the number of microbes found in a litre of water from each of the sources named: Condensed atmospheric vapor, 900; water from the drain of Asnières, 48,000; rain-water, 64,000; water of the Vanne (Montrouge basin), 248,000; water from the Seine (drawn at Bercy), 4,800,000; water from the Seine (drawn at Asnières), 12,800,000; sewer-water (drawn at Clichy), 80,000,000. These figures represent the minima. Left to stagnate, sewer-water putrefies in a very short time, through the multiplication of its germs, and the microbes become a thousand times as numerous as indicated in the summary.
Thus, we see, we are surrounded on every side by myriads of schizophytes. What proportion, among these minute inhabitants of the atmosphere and the waters, have a part in producing contagious maladies and the epidemics by which the populations of our large cities have been decimated at times? We do not know yet. The continuation of the statistical researches that have been begun at Montsouris, and the microscopic analysis of the air and of water, particularly of sewer-water, cultivation, botanical and physiological investigation, and inoculation with the resultant germs, will certainly conduct to the solution of the problem. Then only, having become acquainted with our enemies, shall we be able to destroy them. The precautions that we take against such evasive foes—working as we do in the dark—by using antiseptics, are evidently insufficient. A substance that kills one bacterium may not hinder the development of its neighbor, and our employment of antiseptics is always dependent upon their specific action. There exists no universal remedy against microbes. Science alone can teach us how to contend against them.—Translated for the Popular Science Monthly from the Revue Scientifique.
- See "Popular Science Monthly," July, 1880, article "Atmospheric Dust."