256 THE FERMENTATION OF UREA 



completes its task even when the nutrient solution contains 140 grams of urea 

 per litre. Morphologically similar to this, but differing greatly in physiological 

 character, is the Urobacillus Freud enreichii, found with particular frequency in 

 the sweepings of the streets of Paris. This bacillus can only hydrolise 0.3 gram 

 of urea per hour, and cannot ferment a larger quantity than 45 grams per litre 

 of nutrient solution. It is an actively motile rod, 1.0-1.3 M Droa d and of 

 variable length. 



High degrees of speed and power of fermentation are not always found in 

 association. This is well exemplified by Urobacillus Schiitzenbergii, a rod i JLI 

 long and 0.5/1 broad, incapable of producing spores It was found by Miquel 

 both in natural and drainage waters, but never in the air. This species is very 

 energetic, i.e. transforms a large amount of urea per unit of time, but its activity 

 ceases as soon as the liquid has become somewhat enriched with ammonium 

 carbonate. That this is actually the cause of the cessation follows from the fact 

 that the fermentation proceeds further when this salt is removed by aerating the 

 medium. 



To this injurious influence of ammonium carbonate towards which different 

 degrees of susceptibility are exhibited by the various species is due the rapid 

 dying of cultures of the bacteria under consideration, in liquids containing urea. 

 If it is desired to prolong their existence or to refresh debilitated cultures, they 

 must be transferred to nutrient media that are free from urea, and consequently 

 enable growth to proceed only at a slow rate, but, just for this reason, ensure a 

 longer life. 



Even greater than their sensitiveness towards ammonium carbonate, of which 

 even the most delicate species can support a very considerable quantity, is the 

 susceptibility of the urea bacteria to the presence of free acid in the nutrient 

 medium. Burri and his collaborators ascertained (in their above-mentioned 

 researches) that 0.4 per cent, of sulphuric acid produces a fatal effect. This 

 observation can be practically utilised in the protection of stall manure from 

 early decomposition. 



Into the remaining distinguishing characteristics of Miquel's urobacteria 

 especially the degree of resistance to heat exhibited both by the vegetative forms 

 and the spores produced by all but the last-named species we cannot enter 

 further. One common characteristic peculiar to the group must not, however, 

 be omitted from mention, and that is the aureole with which the colonies 

 surround themselves when grown on 2 per cent, urea-gelatin, and whereby they 

 are distinguishable from all other bacterial species, even at an early age, by 

 macroscopical examination or under a low power. The colonies are closely 

 surrounded, for a distance exceeding their diameter, with numerous biscuit- 

 shaped bodies, embedded in the gelatin and for the most part so close together 

 that they envelop the colony as in a cloud. These bodies are facetted crystals, 

 each composed of two combined globules, containing lime, carbonic acid, and 

 phosphoric acid, and formed by the reaction of ammonium carbonate (liberated 

 by the bacteria), on the salts of the alkaline earths contained in the medium. 



Whether and how far the nitrogen in the urea contributes to the structure of 

 these bacteria is also an interesting point. Jaksch in his treatise asserted that 

 his bacillus preferentially takes up nitrogen from urea, other sources, such as 

 peptone, being less suitable ; but Miquel came to exactly the opposite conclusion, 

 the species examined by him greatly preferring peptone, or any similar 

 substance, before urea as a source of nitrogen. 



Urobacteria are of very frequent occurrence in nature. For quantitative 

 determinations on this point we are indebted to Miquel, according to whom the 

 average number of such bacteria in the air of Paris was (in 1891) 151 germs per 

 cubic metre. The smallest number (90) was found in autumn and the highest 



