CHAP. XVII] BACTERIOLOGY, ANTISEPTICS AND DISEASE 



197 



Yeast cells are a little larger than most 

 bacteria. 



Moulds and fungi decompose dead matter 

 underground and inside trees, where bacteria, 

 etc., cannot gain access. Mushrooms are a form 

 of fungi. The fermentations of sauerkraut and 

 those of bread, etc., are due to yeasts. 



Vegetable micro-organisms are not alone in 

 helping along daily life, because protozoa carry 

 out their portion of work ; they eat up bacteria 

 after the bacteria have done their work. Pro- 

 tozoa are found in all waters and in grass, hay, 

 etc., and in any infusion of grass, hay, herbs, 

 plants, etc. The chalk cliffs of Dover, England, 

 are composed of millions of fossilised protozoa, 

 showing that these cliffs were once inundated. 

 It will be understood from the above remarks 

 what an important part micro-organisms play 

 in the routine of daily life independently of that 

 of producing disease. 



773. A very short while ago it was considered 

 that bacteria were necessary for the actual 

 existence of life, but it has of late been proved 

 in the Pasteur laboratories that chickens can 

 live, and live abnormally well, in an atmosphere 

 absolutely free from micro-organisms, and on food 

 also free from any microbe. Whether inverte- 

 brates can live without bacteria is doubtful. 

 Grass-eating animals (herbivora), which devour a 

 great deal of cellulose, require bacteria (bacilli 

 colli) in their stomach to assist in digesting the 

 cellulose. 



But even if all animals, vertebrates and in- 

 vertebrates, could live without microbes, plants 

 could not, and the lowest forms of life could not, 

 so that our food supply would soon become 

 exhausted. 



774. Protoplasm is the substance of which all 

 cell life is composed. When any animal or 

 vegetable dies, it is split up by bacteria, etc., 

 into its original elements. Plants have a power, 

 in the presence of sunlight, by means of the 

 chlorophyll (the green colouring matter in 

 plants) that all plants contain, of building up 

 protoplasm again from the elements that they 

 collect from the air. In this way the elements 

 are used over and over again to form animal 

 life. 



An animal dies, and is decomposed by bac- 

 teria, who eat some of the elements and liberate 

 the rest. They themselves may be eaten by pro- 

 tozoa. Plants gather some of the elements and 

 reform protoplasm. Animals (e.g. cattle) eat the 

 plants, we eat the cattle, and thus obtain proto- 

 plasm. Again, fish eat protozoa, we eat the fish, 

 and thus obtain more protoplasm. We also 

 require starch and fat besides protoplasm ; these 

 substances we get from plants. Plants have the 

 power to produce starch and fat from the 

 elements. By plants are meant all grasses, 

 herbs, cereals, etc. 



775. The micro-organisms that live on dead 



material, as described above, are called sapro- 

 phytes. 



Those that live on living matter, such as 

 worms, mistletoe, certain ivies, etc., are called 

 parasites ; they do not necessarily kill, but live 

 on the living matter in order to exist. 



There are others, called infective agents, 

 which have not yet learned to live without doing 

 harm to living tissue ; these cause disease. 

 Vegetable infective agents attack animal tissue 

 mostly, whilst animal infective agents (harmful 

 protozoa) attack animal and vegetable tissue. 



It is difficult to differentiate between living 

 and dead material, because when an animal dies 

 there is at the moment of death only a very 

 small portion of his body really dead — for 

 example, all his muscles, if healthy, remain alive 

 until decomposed by bacteria, whilst iron, sugar, 

 etc., are dead materials. The best definition of 

 dead and living material, I think, is the follow- 

 ing : living material can construct more material, 

 e.g. more protoplasm and more fat can be 

 formed from the elements by protoplasm and 

 fat respectively ; dead material cannot construct. 



776. This construction is carried out by 

 agents, know as ferments (enzymes), that are 

 always present ; these ferments do not them- 

 selves become used up. Thus, a bacterium 

 attacks a piece of muscle ; by means of the fer- 

 ment it breaks up the elements and digests those 

 that it requires, and liberates the rest into the 

 atmosphere in the form of gases. The process is 

 the same whether it lives on dead or living 

 material. 



All infective agents, i.e. all micro-organisms 

 that produce disease, whether vegetable or 

 animal, are called pathogenic (disease-produc- 

 ing), whilst those that do not produce disease are 

 called non-pathogenic. 



Pathogenic micro-organisms, or microbes, 

 have not yet been discovered that will produce 

 disease in every form of life ; in fact, most 

 pathogenic microbes produce disease in only one 

 or two varieties of hosts. For instance, those 

 that produce disease in mammals, as a rule, do 

 not in cold-blooded animals, and vice versa. 

 Some are restricted to sheep, some to dogs, 

 whilst some are restricted to a certain kind of 

 tissue, e.g. the liver. The typhoid bacteria when 

 swallowed by a man produce disease, or may 

 do so, but if swallowed by a horse or ox have 

 no effect, as a rule. Thus, e.g. an organism that 

 is pathogenic to some animals or to human beings 

 is non-pathogenic to others and plants. There 

 are other factors in the patient that affect the 

 pathogenicity of a microbe, as age, hunger and 

 thirst, fatigue, exposure to extremes of heat or 

 cold, unsuitable diet, general health, nervous- 

 ness, condition of blood, the habits of life, etc. 

 Then, again, the pathogenicity of a microbe 

 depends upon its virulence and upon the number 

 that gain access to the body. 



