July 1, 1899.] 



KNOWLEDGE 



151 



MICROBES IN CO-OPERATION. 



By G. Clarke Nuttall, b.so 



OF late years botanists have been made very familiar 

 with that kmd of relationship shown to exist in 

 certain instances in the plant world which is 

 known as Symbiosis. In these instances we 

 find two organisms living a common life of mutual 

 benefit, each supplying the other with some requisite of its 

 existence, and receiving in return some essential to its own 

 well-being. The arrangement does not necessarily benefit 

 both sides equally, and never is the advantage of the same 

 nature on either hand ; the essential point is that there is 

 a common life of two organisms with a mutual advantage of 

 some kind or other. A lichen is, perhaps, the best and 

 most striking illustration of a symbiotic union, where a 

 minute rudimentary green plant — an alga^and a fungus, 

 living and growing in intimate connection, give rise to that 

 product which we term a lichen thallus. In this case the 

 mutual benefit lies in the fact that the alga provides food for 

 the fungus, and the fungus stimulates and shelters the alga. 

 Metabiosis — we owe the name to the French chemist 

 Garro — is another kind of relationship existing in certain 

 cases which is of later recognition and not so generally 

 known at present. For one thing, all the instances yet 

 found of this condition are confined to the world of the 

 bacteria, and hence do not furnish so obvious or so familiar 

 examples as the lichens do of symbiosis ; moreover, there 

 is lacking in metabiosis that suggestion of sensationalism 

 which caused so much interest and controversy when the 

 double nature of the hchen household was first put forward 

 for acceptance. Nevertheless, metabiosis, as a condition 

 of life, has, for an interested observer, a fascination 

 peculiarly its own. 



Now, metabiosis maybe broadly defined as that relation- 

 ship which exists between two organisms when for one of 

 the two to flourish and live in a certain medium it is 

 necessary that the other should have preceded it and pre- 

 pared the way for it. The development of the one with 

 its consequent reaction on the environment is a necessary 

 condition of the development of the other. The first is 

 independent of the second and in no way touches it in any 

 intimate way ; the second is wholly dependent upon the 

 good offices of the first, for without its predecessor had 

 hved and developed and through its living changed the 

 character of its environment it could never have been 

 called into active life. 



An example will perhaps best serve to illustrate the 

 point. It was found some time ago that certain of the 

 very finest wines produced in the vineyards of the Rhine- 

 land were made from grapes that, after they had been 

 gathered, were allowed to stand and go mouldy and " bad," 

 and apparently become absolutely useless and disagreeable. 

 Yet it was from these very grapes that the wines 

 possessing the best flavour resulted. Now, a clever German 

 chemist, MiiUer Thergau, examined the matter from a 

 scientific point of view, and he found that the mouldiness 

 which appeared on the grapes after standing was a 

 fungus which lived on the contents of the grapes, and 

 which in absorbing its food changed the chemical con- 

 stitution of those contents, so that when the fermentation 

 processes began through the agency of yeast organisms, 

 they were favoured and affected for the better by the 

 changes which had already been brought about by the 

 mould fungus. Here then the yeast stands in a metabiotic 

 relationship to the mould fungus. The mould is absolutely 

 independent of the yeast and appears under any circum- 

 stances, the yeast organism can only take that particular 

 line of development with the resulting production of I 



" bouquet " when the way has been prepared for it by the 

 mould fungus. It is dependent upon its predecessor for 

 its particular action — that is to say, we have here a 

 condition of metabiosis. 



Prof. Lafar gives an instance of a whole series of 

 metabiotic relationships when he explains, with much 

 lucidity, the sequence of events in the evolution of wine 

 from the grape. 



The skin of the grape, he says, is naturally the 

 home of many varieties of fungi, especially bacteria, and 

 when the grapes are gathered and pressed down these 

 germs are naturally also to be found in the " must." 

 And since the germs are of many varieties, they, of 

 necessity, differ as to the conditions most favourable to 

 their development, and as to the length of time they 

 demand for the various stages in their life processes. 



The first to develop are the yeasts — the organisms which 

 bring about ordinary alcoholic fermentation. These seize 

 upon the sugar of the grape for food and split it up into 

 carbon dioxide and alcohol, and thus at once the original 

 constitution of the "must" is greatly altered. Another 

 species of germs, hitherto lying dormant, then spring 

 into activity, as the conditions are now those which most 

 favour its development. These are the organisms which 

 bring about acetic fermentation, for their food is the 

 alcohol which the yeast brought into existence, and 

 they, in acting upon it, oxidise it by means of the free 

 oxygen in the air, and thus produce acetic acid. It is 

 obvious that the relationship between the two species is 

 here again one of metabiosis ; the bacteria producing 

 acetic acid could not have developed in the " must " as it 

 was in its earliest form, for no alcohol was then to be 

 found in it ; the germs might be abundantly present in 

 the liquid, but their growth was wholly dependent on 

 the yeast preceding tnem and preparing the way for 

 them. Thus they are under an absolute obligation to 

 the yeast, though the yeast is entirely independent of 

 them. The result of this second stage in the proceedings 

 is therefore a strongly acid liquid. 



But now a third kind of bacteria come into play, which 

 themselves stand in a metabiotic relationship to the 

 second. An acid liquid is their natural home, and as the 

 originally sweet "must" has become distinctly acidified, 

 the dormant germs of this species are aroused into activity. 

 They seize upon the acetic acid present, and under their 

 influence it rapidly becomes split up into carbon dioxide 

 and water. The bacteria accomplishing this are the 

 thread fungi (the vinegar eel is one of the members of this 

 variety), who thus form a third link in the metabolic chain, 

 and their special work is the elimination of the acid. 



And the thread fungi are themselves the forerunners of 

 yet another new-comer. Certain Schitzomycetes — bacteria 

 which promote putrefaction — had entered tlie " must " with 

 the dust and air in the first instance, but had had no chance 

 of growth until this late stage, for alcohol is poison to them 

 and acetic acid distinctly injurious. So while the yeasts 

 were producing alcohol and the second species was re- 

 placing it with acetic acid they were compelled to remain 

 quiescent, and it was not until the alcohol had been 

 removed by the bacteria promoting acetic fermentation, 

 and the acetic acid had in its turn been reduced by the 

 thread fungi, that the way was clear for their appearance. 

 The above sequence ol changes thus gives us several 

 instances of metabiotic relationship, and indeed it is pro- 

 bable that in most cases of fermentation and putrification 

 we have much the same sort of thing happening ; in all 

 cases, that is, where several varieties of bacteria have 

 their habitat in the same medium. 



It is difficult to over-estimate the practical value which 



