TRANSIENT MODIFICATIONS 111 



bacillus prodigiosus, and yet other forms which produce a red 

 colouring matter vary in the intensity of their colour production 

 according to the acidity or alkalinity of the medium of growth. 

 The most beautiful example of temporary modifications of 

 easy production by alteration of environment is supplied 

 in Gessard's very full study of the Bacillus pyocyaneus. 1 

 Ordinary beef-broth sown with this bacillus soon takes on a 

 greenish-blue slightly fluorescent appearance, and the blue colour 

 becomes more marked if there be a relatively large surface 

 exposed to the air, or if at intervals the culture be shaken, and 

 the pellicle of surface growth be made to sink to the bottom of 

 the vessel. Evidently then there are at least two pigments 

 produced, and Gessard succeeded in causing the independent 

 development of either. For he found that upon solidified white 

 of egg the bacillus gives rise to the bright green colour alone, 

 while upon the agar-agar, to which glycerine and a fair quantity 

 of peptone had been added, no green is developed, the medium 

 rapidly becoming tinged throughout an intense deep blue. It 

 may be added that the absence of free oxygen causes the develop- 

 ment of a colourless growth. These observations, I repeat, 

 can be easily confirmed. 2 



(3) Modifications of Ferment Production. — Similarly modifica- 

 tions can be induced in the ferment production of microbes. 

 Thus many micro-organisms, which elaborate a ferment capable 

 of liquefying gelatine, do not manifest that power if glycerine 

 be added to the medium of growth, or (from a different cause) 

 lose that power, as Cartwright Wood has shown 3 in the case of 

 the Cholera spirillum, of the Bacillus prodigiosus and the Micro- 

 coccus indicus, if minute quantities of carbolic acid be added to 

 the culture medium. Again, as indicated by Lauder Brunton 

 and Macfadyean, 4 the bacteria which form a peptonizing 

 enzyme on proteid soil can also produce a diastatic enzyme on 

 carbohydrate soil. 



(4) Modifications of Pathogenicity. — With regard to the 



1 Gessard, Annates de VInst. Pasteur, iv., 1891, p. 737; and v., 1891, p. 65. 



2 Adami, Meeting of East Anglian Branches of British Medical Assoc. 

 Cambridge, Lancet, July 24, 1891. 



3 CartAvright Wood, Proc. Royal Soc. Edin. xvi., 1889 ; and Laboratory 

 Report, R.G.P. Edin. ii., 1890, p. 253. 



4 Lauder Brunton and Macfadyean, Proceedings Royal Society, xlvi., 1890 

 p. 542. 



