MIXED CULTURES 67 



of carbon dioxide, immediately above the liquid, preventing the free access to the 

 latter of the copious supply of oxygen without which the oxidation of the alcohol 

 cannot proceed. Now, however, that both substances are present, the liquid 

 commences to undergo a second alteration, and turns sour, the acetic acid bac- 

 teria being now on the surface ; and this condition endures so long as there is 

 any alcohol left. When this is exhausted, a third group of organisms comes 

 to the front, thread fungi establish themselves in the strongly acid liquid and 

 consume the acetic acid, carbon dioxide and water being found. This accom- 

 plished, the once again altered nutrient medium is attacked by putrefactive 

 bacteria, which have been carried into the vessel along with the dust in the 

 atmosphere, but can only develop now that the alcohol and acid, which are 

 poisonous to them, are wanting. The liquid is seized upon by these Schizomycetes, 

 and, with their activity, the series of metabiotic phenomena which the wine-must 

 presents to our notice closes. 



The mutual influence of two or more species may be of such a nature that it 

 is impossible for them to live together, the presence of the one species retarding 

 the development of the other. This set of conditions is termed antagonism, a 

 number of examples of which will be given in subsequent sections. 



65.-Mixed Cultures. 



When a nutrient medium is inoculated with two or more species of symbiotic 

 organisms, we obtain a mixed culture. Such a culture may, under certain cir- 

 cumstances, yield fermentation products that cannot be obtained from any of 

 the component species cultivated singly, but owe their origin partly to the 

 coalescence of the normal products of the individual species, and partly to the 

 reciprocal stimulative action exerted by the associated organisms. A few highly 

 instructive examples of this are given below. 



The first of these which was discovered by NENCKI (I.) is afforded by the 

 bacillus of symptomatic anthrax (Rauschbrand) and Micrococcvs acidi paralactici. 

 Fuller information concerning the individual behaviour of these two Schizomycetes 

 will be found in subsequent paragraphs, which we will here anticipate in respect 

 of the fact now coming under consideration, viz., that the first-named bacillus 

 yields, in nutrient solutions containing cane-sugar, the following fermentation 

 products : hydrogen, carbon dioxide, normal butyric acid, and inactive lactic 

 acid. On the other hand, Micrococcus acidi paralactici forms, almost exclusively, 

 optically active paralactic acid, and that, too, in a quantity almost identical with 

 the theoretical yield from the sugar eliminated. If, now, both these organisms 

 be cultivated together in the nutrient solution aforesaid, fermentation proceeds 

 much more rapidly, and the final products consist not only of the already men- 

 tioned substances (yielded by the organisms singly), but also of a large amount 

 of normal butyl-alcohol. This substance, therefore, owes its production in this 

 case to the co-operation of two species of bacteria, neither of which singly is 

 capable of such power. 



Interesting as this fact, that new fermentation products can be formed by 

 the association of organisms, may be, the following one, which was first estab- 

 lished by BUKRI and STUTZER (I.), is so in a still greater degree. In this case 

 two organisms are concerned, neither of which is capable singly of liberating 

 nitrogen from nitrates ; but, when acting conjointly, they decompose the sam< 

 nutrient medium with violent disengagements of gas. The one organism is the, 

 Bacterium coli commune, already mentioned, and very abundant in human fseces 

 and that of domestic animals, whilst the second microbe was named by the 

 above-named naturalists Bacillus denitrificans I. A bouillon containing three 

 grams of sodium nitrate per litre, inoculated with both these organisms and 



