Practical bacteriology, microbiology and serum therapy (medical and veterinary) : a text book for laboratory use / by A. Besson ; translated and adapted from the fifth French edition by H.J. Hutchens.

  • Besson, Albert, 1868-
Date:
1913
Catalogue details

Licence: In copyright

Credit: Practical bacteriology, microbiology and serum therapy (medical and veterinary) : a text book for laboratory use / by A. Besson ; translated and adapted from the fifth French edition by H.J. Hutchens. Source: Wellcome Collection.

    896/928 (page 864)
    a smooth uniform layer on the lower surface of the tube not deep enough to reach the orifice ot the small glass tube nor the opening in the india-rubber at the other end. [It will be found more satisfactory to warm the tube before pouring the gelatin into it and then holding it horizontally to rotate the tube until the gelatin has set so that the medium forms a thin coating over the whole of the interior.] The apparatus is now ready for use. When about to carry out an experiment, remove the outer piece of india-rubber, attach the outer end of the small glass tube to an aspirator and draw 10-15 litres of air slowly through the tube. The air enters the hole in the india-rubber capsule and passes over the surface of the gelatin on which it is intended that the suspended dust should be deposited. When the required volume of air has been drawn through, the outer india-rubber capsule is replaced and the tube incubated in the cool incubator (20° C.). Colonies begin to appear on the gelatin in the course of a day or two and if the technique was satisfactory should be more numerous at the end at which the air entered. The colonies can be counted and any which it may be desired to investigate further, picked off. If, for example, 15 litres of air have been aspirated and 6 colonies of bacteria and 10 moulds are subsequently counted on the gelatin the air will have contained approximately xV x 1000 = 400 aerobic bacteria per cubic metre. 13 x 1000 = 606 moulds per cubic metre. In practice, however, it happens that many organisms stick to the glass wall of the tube and so do not enter into the computation [this source of error is avoided if the medium be coated over the whole surface]: further, if the experiment be con- tinued for any length of time the gelatin will become dry and fail to act as a satis- factory culture medium • and lastly, the current of air must pass very slowly other- wise the organisms suspended in it will be carried through the tube without being deposited. [Two other objections may be raised, namely the difficulty of reaching the colonies should it be desirable to sub-cultivate them and the fact that some organisms rapidly liquefy the gelatin and render the experiment useless.] 2. Methods employed at the present day. The methods just described have now been superseded by others which depend upon removing the organisms contained in the air either by bubbling the latter through a viscous fluid or by filtering it through a powder. By adopting either of these methods all the organisms suspended in a given volume of air can be collected in a small space, being either disseminated in the liquid or mixed with the powder as the case may be. It will then only be necessary to proceed on the lines already laid down in the sections dealing with the isolation of organisms and with the examination of water. It is always well to sow both agar and gelatin plates since the latter generally liquefy in a short space of time. In carrying out these experiments with air some form of aspirator is necessary. For choice, a water aspirator would be used such as is to be found in chemical laboratories. With the aid of this apparatus the volume of air aspirated can be very accurately measured. An ordinary water exhaust pump can also be used. In this case it will of course be necessary to interpose between the liquid through which the air is to bubble and the pump a gaso- meter which will record the volume of air aspirated. [A still more simple and quite satisfactory method consists in using a large glass barrel fitted with a tap below and stoppered above with an india-rubber plug through which a narrow piece of glass tubing is passed, such as is used in operating theatres for storing antiseptics. This vessel can be graduated once for all by pouring in measured volumes of water and marking the level on the glass with a carburundum pencil. If it is required to aspirate, say, 10 litres of air, the vessel is filled with water up to the 10 litre mark, the apparatus is then attached to the small glass tube above and by regulating the flow of water from the tap below the air can be aspirated at? any speed which is considered desirable. For the examination of air in places where it is inconvenient to