Putrefactive bacteria are numerous. These split up proteins into the following: -

1. Fatty acids and amino-acids, e.g., leucine, tyrosine, indol, skatol.

2. Other aromatic bodies, e.g. phenol and cresol.

3. Various ptomaines, e.g., murine, choline, cadaverine, and other ammoniacal compounds.

4. Various gases, e.g. sulphuretted hydrogen.

Putrefactive bacteria also split up fats into fatty acids, which are later changed into volatile acids.

In health the decomposition products arising from the influence of bacteria on carbohydrates and proteins are either removed or neutralised by the liver or other gland, or are changed into harmless compounds, as in the ethereal sulphates in the urine. As already mentioned, the organic acids produced from the decomposition of the carbohydrates exercise an inhibiting influence on the putrefactive bacteria.

In various diseased states these decomposition products are produced in excess, with the result that they are absorbed into the system and induce various manifestations of disease (see Auto-intoxication). The products of bacterial activity may be roughly grouped as follows: -

1. Gases and acids which are produced chiefly from carbohydrates.

2. Alkaloids produced from decomposition of proteins.

The recently published results of Herter and Kendall on the influence of dietary alternations on the types of intestinal flora are of great interest. Experimenting with monkeys and cats, these observers showed that on a protein diet the bacterial flora was of a strongly proteolytic character, but that on a change to a carbohydrate diet these organisms were rapidly replaced by others of a non-proteolysing type. Associated with this change there was a change in the putrefactive products of the urine and faeces, and a reduction in the indol, skatol, phenol, and sulphuretted hydrogen, and diminution in the indican in the urine. These observations are of great value, as from analogy there is every likelihood that similar results hold good for man.

It will be obvious from the foregoing that diet may profoundly modify the bacterial activity in the intestinal tract. Speaking generally, a restriction in the amount of protein reduces the formation of putrefactive decomposition products, and a reduction in the carbohydrates lessens the development of gases and acids which are injurious to the system. Some authors consider that the administration of any protein above 70 grammes is bad, as inevitably tending to auto-intoxication. Further, the kind of food has to be considered. In some patients a milk diet may be unsuitable because of the large amount of lactose leading to the formation of lactic acid, and later, butyric acid, carbonic acid, and water. Again, certain carbohydrates such as potatoes are by many digested with difficulty, being carried well down into the ileum before digestion, thus forming a suitable nidus for abnormal bacterial activity. On the other hand, in certain cases the administration of stewed fruits may be advisable, on account of the organic acids present. These points are further referred to in discussing different diseases.