158 BREAKDOWN OF NITROGENOUS MATERIAL 



highly active proteases into their environment. This can 

 be demonstrated by filtering such organisms from culture, 

 when it will be found that the cell-free filtrate contains an 

 active proteolytic enzyme which can be concentrated and 

 precipitated by suitable protein precipitants. The lique- 

 faction of tissues around a wound is due to the proteolytic 

 activities of contaminants of this type. Other genera, such 

 as Proteus and Pseudomonas, have less marked proteolytic 

 activities, while Streptococci are sometimes feebly proteolytic. 

 Even proteolytic organisms will fail to grow when inoculated 

 into a medium containing native protein as sole source of 

 nitrogen, as they require some utilisable source of nitrogen 

 from which to synthesise the extracellular protease necessary 

 to initiate the hydrolysis of the protein. 



Once native protein has been hydrolysed to peptones, the 

 majority of the heterotrophic organisms are able to utilise 

 these peptones as sources of nitrogen and/or energy. Thus 

 media in common use in the laboratory for general growth 

 purposes are prepared with a basis of peptone. It is highly 

 probable that genera and species differ widely in the proteolytic 

 enzymes which they produce, but there have not as yet been 

 sufficient studies of this aspect of the subject to make any 

 generalisations possible. The end-products of the breakdown 

 of proteins by bacterial proteases are amino-acids, and dis- 

 cussion of nitrogen metabolism must at present hinge mainly 

 on amino-acid metabolism, as it is here that the widest variety 

 of further breakdown products occur. 



AMINO-ACID BREAKDOWN 



If we consider the general formula of an amino-acid: 



R.CHNH2.COOH 



we find that there is no theoretically possible mode of attack 

 which is not employed by some organism or other under 

 some condition or other. The molecule can be degraded in 

 three main ways: (1) by removal of the — NHg group, or 

 deamination; (2) by removal of the — COOH group, or 



