54 PHYSIOLOGY OF NUTRITION 



would contain only y^ milligram of mineral salts, so that it is evident that 

 very minute quantities of such salts are sufficient for nutritive media, say 

 from 01 to 0-2 per cent. 



Of the elements may be mentioned as indispensable : sulphur, phos- 

 phorus, calcium, magnesium, potassium, and sodium, with traces of chlorine 

 and iron. Whether other alkalies such as rubidium or caesium can be 

 substituted for potassium and sodium, or other alkaline earths, such as barium 

 or strontium, for calcium, is doubtful. Recent experiments on the metabolism 

 of mould-fungi render it improbable. 



Full details as to the most suitable proportions of mineral salts in nutri- 

 tive media will be found in practical treatises. In the following brief account 

 of the nutrition of bacteria, the words ' necessary salts ' will be understood 

 to mean o-i per cent. K 2 HPO 4 , 0-02 per cent. Mg SO 4 , and o-oi per cent. 

 CaCl 2 . Sodium and iron are also necessary, but are always present in 

 traces in the other chemicals used unless these have been specially purified ; 

 they are likewise present in tap-water. For pathogenic forms the addition 

 of from 01 per cent, to 0-7 per cent. NaCl is advisable ; but if the media 

 have been prepared with infusion or extract of meat it is not necessary. 



These few mineral substances will be found sufficient probably for all 

 bacteria, be they prototrophic, metatrophic, or paratrophic. The require- 

 ments in carbon and nitrogen compounds, on the other hand, are widely 

 different for each group. 



The prototrophic nitrifying bacteria grow well in a fluid of the following 

 composition : 



Water, H 2 O loo- 

 Potassium Nitrite, KNO 2 .... -05 

 Potassium Hydrogen Phosphate, K. 2 HPO! . -02 

 Magnesium Sulphate, MgSO 4 .... -03 

 Sodium Carbonate, Na 2 CO 3 .... -05 

 Sodium Chloride, NaCl -05 



The nitrous acid supplies the nitrogen, and the carbon is derived, not from 

 the Na 2 CO 3 , but from the carbonic acid of the atmosphere. Other proto- 

 trophic soil bacteria are able to fix atmospheric nitrogen, but require some 

 organic compound as a source of carbon ; sugar for instance. 



The growth of various metatrophic bacteria upon solutions of different 

 carbon and nitrogen compounds is illustrated by the following table. All 

 the solutions contained the same amounts of mineral substances, and, with 

 the exception of three cases, had an alkaline reaction ; free acid, as the 

 figures show, generally retarding development. The cultures were kept at 

 the most suitable temperature in each case and were watched for fourteen 

 days, so that delayed growth might not be overlooked. The symbols 

 employed have the following meanings : 



