3. Higher resistance than the parent organism to such lethal agents as heat, desiccation, starvation, time, etc., this resistance being due to

(a) Low water contents of plasma of the spore.

(b) Low heat-conducting power } of the spore (c) Low permeability } membrane.

This resistance varies somewhat with the particular species--e. g., some spores may resist boiling for a few minutes--but practically all are killed if the boiling is continued for ten minutes.

~Germination.~--When transplanted to suitable media and placed under favourable conditions, the spores germinate, usually within twenty-four to thirty-six hours, and successively undergo the following changes which may be followed in hanging-drop cultures on a warm stage:

1. Swell up slowly and enlarge, through the absorption of water.

2. Lose their refrangibility.

3. At this stage one of three processes (but the particular process is always constant for the same species) may be observed:

(a) The spore grows out into the new bacillus without discarding the spore membrane (which in this case now becomes the cell membrane); _e.

g._, B. leptosporus.

(b) It loses its spore membrane by solution; e. g., B. anthracis.

(c) It loses its spore membrane by rupture.

In this process the rupture may be either polar (at one pole only _e.

g._, B. butyricus), or bipolar (e. g., B. sessile), or equatorial; (e. g., B. subtilis).

In those cases where the spore membrane is discarded the cell membrane of the new bacillus may either be formed from--

(a) The inner layer of the spore membrane, which has undergone a preliminary splitting into parietal and visceral layers; e. g., B.

butyricus.

(b) The outer layers of the cell protoplasm, which become differentiated for that purpose; e. g., B. megatherium.

The new bacillus now increases in size, elongates, and takes on a vegetative growth--i. e., undergoes fission--the bacilli resulting from which may in their turn give rise to spores.

[Ill.u.s.tration: FIG. 92. Simple.]

[Ill.u.s.tration: FIG. 93. Solution.]

[Ill.u.s.tration: FIG. 94. Polar.]

[Ill.u.s.tration: FIG. 95. Bipolar.]

[Ill.u.s.tration: FIG. 96. Equatorial.]

~Food Stuffs.~--1. _Organic Foods._--

(a) The pure parasites (e. g., B. leprae) will not live outside the living body.

(b) Both saprophytic and facultative parasitic bacteria agree in requiring non-concentrated food.

(c) The facultative parasites need highly organised foods; e. g., proteids or other sources of nitrogen and carbon, and salts.

(d) The saprophytic bacteria are more easily cultivated; e. g.,

1. Some bacteria will grow in almost pure distilled water.

2. Some bacteria will grow in pure solutions of the carbohydrates.

3. _Water_ is absolutely essential to the _growth_ of bacteria.

Food of a definite reaction is needed for the growth of bacteria. As a general rule growth is most active in media which react slightly acid to phenolphthalein--that is, neutral or faintly alkaline to litmus. Mould growth, on the other hand, is most vigourous in media that are strongly acid to phenolphthalein.

~Environment.~--The influence of physical agents upon bacterial life and growth is strongly marked.

1. _Atmosphere._--The presence of _oxygen_ is necessary for the growth of some bacteria, and death follows when the supply is cut off. Such organisms are termed _obligate aerobes_.

Some bacteria appear to thrive equally well whether supplied with or deprived of oxygen. These are termed _facultative anaerobes_.

A third cla.s.s will only live and multiply when the access of free oxygen is completely excluded. These are termed _obligate anaerobes_.

2. _Temperature._--Practically no bacterial growth occurs below 5C, and very little above 40 C. 30C. to 37 C is the most favorable for the large majority of micro-organisms.

The maximum and minimum temperatures at which growth takes place, as well as the optimum, are fairly constant for each bacterium.

Bacteria have been cla.s.sified, according to their optimum temperature, into--

MIN. OPT. MAX.

1. Psychrophilic bacteria (chiefly water organisms) 0 C. 15 C. 30C.

2. Mesophilic bacteria (includes pathogenic bacteria) 15 C. 37 C. 45C.

3. Thermophilic bacteria 45 C. 55 C. 70C.

The thermal death-point of an organism is another biological constant; and is that temperature which causes the death of the vegetative forms when the exposure is continued for a period of ten minutes (see pages 298-301).

3. _Light._--Many organisms are indifferent to the presence of light. On the other hand, light frequently impedes growth, and alters to a greater or lesser extent the biochemical characters of the organisms--e. g., chromogenicity or power of liquefaction. Pathogenic bacteria undergo a progressive loss of virulence when cultivated in the presence of light.

4. _Movements._--Movements, if slight and simply of a flowing character, do not appear to injuriously affect the growth of bacteria; but violent agitation, such as shaking, absolutely kills them.

A condition of perfect rest would seem to be that most conducive to bacterial growth.

~The Metabolic Products of Bacteria.~--_Pigment Production._--Many micro-organisms produce one or more vivid pigments--yellow, orange, red, violet, fluorescent, etc.--during the course of their life and growth.

The colouring matter usually exists as an intercellular excrement.i.tious substance. Occasionally, however, it appears to be stored actually within the bodies of the bacteria. The chromogenic bacteria are therefore cla.s.sified, in accordance with the final destination of the colouring matter they elaborate, into--

_Chromoparous_ Bacteria: in which the pigment is diffused out upon and into the surrounding medium.

_Chromophorous_ Bacteria: in which the pigment is stored in the cell protoplasm of the organism.