Читать книгу The Fundamentals of Bacteriology - Charles Bradfield Morrey - Страница 24

Oxygen is one of the constituents of protoplasm and is therefore necessary for all organisms. This does not mean that all organisms must obtain their supply from free oxygen, however, as animals and plants generally do. This fact is well illustrated by the differences among bacteria in this respect. Some bacteria require free oxygen for their growth and are therefore called aërobic bacteria or aërobes (sometimes strict aërobes, though the adjective is unnecessary). Others cannot grow in the presence of free oxygen and are therefore named anaërobic bacteria or anaërobes (strict is unnecessary). There are still other kinds which may grow either in the presence of free oxygen or in its absence, hence the term facultative anaërobes (usually) is applied to them. The distinction between facultative aërobe and facultative anaërobe might be made. The former means those which grow best in the absence of free oxygen, though capable of growing in its presence, while the latter term means those which grow best in the presence of free oxygen, but are capable of growing in its absence. The amount of oxygen in the atmosphere in which an organism grows may be conveniently expressed in terms of the oxygen pressure, i.e., in millimeters of mercury. It is evident that the maximum, minimum and optimum oxygen pressures for anaërobic bacteria are the same, namely, 0 mm. Hg. This is true only for natural conditions, since a number of anaërobic organisms have been gradually accustomed to increasing amounts of O, so that by this process of training they finally grew in ordinary air, that is, at an oxygen pressure of about 150 mm. Hg. (Normal air pressure is 760 mm. Hg. and oxygen makes up one-fifth of the air.) The minimum O pressure for facultative anaërobes is also 0 mm. Hg. Some experiments have been made to determine the limits for aërobes, but on a few organisms only, so that no general conclusions can be drawn from them. To illustrate: Bacillus subtilis (a common “hay bacillus”) will grow at 10 mm. Hg. pressure but not at 5 mm. Hg. It will also grow in compressed oxygen at a pressure of three atmospheres (2280 mm. Hg.), but not at four atmospheres (3040 mm. Hg.), though it is not destroyed.

Parodko has determined the oxygen limits for five common organisms as follows:

	Maximum.	Minimum
In atmospheres.	Mm. Hg.	Vol. per cent.	Mm. Hg.
Bacterium fluorescens	1.94 to 2.51	1474 to 1908	0.00016 = 0.0012
Sarcina lutea	2.51 to 3.18	1908 to 2417	0.00015 = 0.0011
Proteus vulgaris	3.63 to 4.35	2749 to 3306	0	0
Bacterium coli	4.09 to 4.84	3108 to 3478	0	0
Erythrobacillus prodigiosus	5.45 to 6.32	3152 to 4800	0	0

These few instances do not disclose any general principles which may be applied either for the growth or for the distinction of aërobes or facultative anaërobes.

It has been shown that compressed oxygen will kill some bacteria but this method of destroying them has little or no practical value. Oxygen in the form of ozone, O₃, is rapidly destructive to bacteria, and this fact is applied practically in the purification of water supplies for certain cities where the ozone is generated by electricity obtained cheaply from water power. The same is true of oxygen in the “nascent state” as illustrated by the use of hypochlorites for the same purpose.

It was stated (p. 74) that certain thermophil bacteria in the soil have an optimum temperature for growth in the air which is much higher than is ever reached in their natural habitat and that they grow at a moderate temperature under anaërobic conditions. It has been shown that if these organisms are grown with aërobes or facultative anaërobes they thrive at ordinary room temperature. These latter organisms by using up the oxygen apparently keep the tension low, and this explains how such organisms grow in the soil.6