CHAPTER XI.

THE BACTERIOLOGIC EXAMINATION OF THE AIR.

IT has been repeatedly emphasized-and indeed at the present time almost every one knows-that micro-organisms float almost everywhere in the air, and that their presence there is a constant source of danger, not only of contamination in our bacteriologic researches, but also a menace to our health.

Such micro-organisms are neither ubiquitous nor equally disseminated, but are much more numerous where the air is dusty than where it is pure-much more so where men and animals are accustomed to live, than upon the ocean or upon high mountain-tops. The purity of the atmosphere bears a distinct relation to the purity of the soil over which its currents blow.

The micro-organisms that occur in the air are for the most part harmless saprophytes which have been separated from their nutrient birthplace and carried about by the wind. They are almost always taken up from dried. materials, experiment having shown that they arise from the surfaces of liquids in which they grow with much difficulty. They are by no means all bacteria, and a plate of sterile gelatin exposed for a brief time to the air will generally grow moulds and yeasts as well as bacteria.

The bacteria present are occasionally pathogenic, especially in localities where the discharges of diseased animals. have been allowed to collect and dry. For this reason the atmosphere of the wards of hospitals and of rooms in which infectious cases are being treated is much more apt to contain them than the air of the street. However, the dried expectoration of cases of tuberculosis, of in

fluenza, and sometimes of pneumonia, causes the specific bacteria of these diseases to be far from uncommon in street-dust.

Günther points out that the majority of the bacteria which occur in the air are cocci, sarcina being very abundant. Most of them are chromogenic and do not liquefy gelatin. It is unusual to find a considerable variety of bacteria at a time; generally not more than two or three species are found.

It is an easy matter to determine whether bacteria are present in the air or not, all that is necessary being to expose sterile plates or Petri dishes of gelatin to the air for a while, close them, and observe whether or not bacteria grow upon them.

To make a quantitative estimation is, however, much

FIG. 37.-Hesse's apparatus for collecting bacteria from the air.

more difficult. Several methods have been suggested, of which the most important may be considered.

The method suggested by Hesse is simple and good. It consists in making a measured quantity of the air to

be examined pass through a horizontal sterile tube about 70 cm. long and 3.5 cm. wide (Fig. 37), the interior of which is coated with gelatin in the same manner as an Esmarch tube. The tube, having been prepared, is closed at both ends with sterile corks carrying smaller glass tubes closed with cotton. When ready for use the tube at one end is attached to a hand-pump, the cotton is removed from the other end, and the air passed through very slowly, the bacteria having time to precipitate upon the gelatin as they pass. When the required amount has passed the tubes are again plugged, the apparatus stood away for a time, and subsequently, when they have grown, the colonies are counted. The number of colo

nies in the tube will represent pretty accurately the number of bacteria in the amount of air which

passed through the tube.

In such a cylindrical culture it will be noted that if the air is passed through with the proper slowness, the colonies will be much more numerous near the end of entrance than that of exit. The first to fall will probably be those of heaviest specific gravity-i. e. the moulds and yeasts.

A still more exact method is that of Petri, who uses small filters of sand held in place in a wide glass tube by small wire nets (Fig. 38). The sand used is made to pass through a sieve whose openings are of known size, is heated to incandescence, then arranged in the tube so that two of the little filters, held in place by their wire-gauze coverings, are FIG. 38. superimposed. One or both ends of the tube are closed with corks having a narrow glass tube. The apparatus is heated and sterilized in a hot-air sterilizer, and is then ready for use. The method of employment is very simple. By means of a hand-pump 100 liters of air are made to pass through in from ten to twenty minutes. The sand from

Petri's sand filter for airexamination.

the upper filter is then carefully mixed with sterile. melted gelatin and poured into sterile Petri dishes, where the colonies develop and can be counted. Sternberg remarks that the chief objection to the method is the presence in the gelatin of the slightly opaque sand, which interferes with the recognition and counting of the colonies. This objection has, however, been removed by Sedgwick and Miquel, who use a soluble material—granulated or pulverized sugar-instead of the sand. The apparatus used for the sugarexperiments differs a little from the original of Petri, but the principle is the same, and can be modified to suit the experimenter. Petri points out in relation to his method that the filter catches a relatively greater number of bacteria in proportion to moulds than the Hesse apparatus, which depends upon sedimentation.

A particularly useful form of apparatus is a granulated sugar-filter suggested by Sedgwick and Tucker, which has an expansion above the filter, so that as soon as the sugar is dissolved in the melted gelatin it can be rolled out into a lining like that of an Esmarch tube. This cylindrical expansion is divided into squares which make the counting of the colonies very easy (Fig. 39).

FIG. 39.-Sedgwick's expanded

The number of germs in the atmosphere will naturally be very variable. the number may be estimated at from 100 amination.

to 1000 per cubic meter.

Roughly,

tube for air-ex

In reality, the bacteriologic examination of air is of very little value, as so many possibilities of error may occur. Thus, when the air of a room is quiescent there may be very few bacteria in it; let some one walk across the floor and dust at once rises, and the number

of bacteria is considerably increased: if the person be a woman with skirts, more bacteria will probably be raised from the floor than would be disturbed by a man; if the room be swept, the increase is enormous. From these and similar contingencies it becomes very difficult to know just when and how the air is to be examined, and the value of the results is correspondingly lessened.

The most valuable examinations are those which aim at the discovery of some definite organism or organisms regardless of the number per cubic meter.