1 62 . PHOTOGENIC BACTEKIA. 



luminosity, but a higher percentage arrests both growth and phos- 

 phorescence, a circumstance due not to any injurious effect of the 

 carbohydrate itself, but to the acids produced therefrom by the 

 vital activity of the organism, the luminous bacteria thriving solely 

 on neutral or faintly alkaline media. Concerning the extent of 

 the injurious content of sugar, and also the varying influence 

 exerted by the different saccharides, Beyerinck arrived at note- 

 worthy conclusions in respect of his six species. Thus, for example, 

 maltose is taken up by Ph. phosphor escens, but is discarded by 

 Ph. Pflugeri ; Ph. Fischeri is very susceptible to cane-sugar, a 

 content of 0.5 per cent, sufficing to retard growth and suppress 

 phosphorescence, whereas, on the other hand, Ph. balticum will 

 stand 5.0 per cent, without injury. A similar relation in respect 

 of glucose obtains between Ph. luminosum and Ph. indicum, the 

 luminosity of the former ceasing when the medium contains i per 

 cent, of this sugar, whilst the latter produces light even in presence 

 of 4 per cent. 



The six species of photobacteria examined by Beyerinck are 

 halophil, i.e. absolutely require sodium chloride, of which the 

 medium must contain at least 3.5 per cent. Consequently it 

 follows that none of these species is able to thrive on the flesh of 

 land animals slaughtered for human food. The luminosity appear- 

 ing on this latter substance is caused by other species, among them 

 being the above-named Micrococcus Pflugeri discovered by Ludwig. 

 The presence of oxygen is essential to the production of phos- 

 phorescence, but is not requisite for mere growth. For preparing 

 pure cultures a nutrient gelatin made from fish bouillon is em- 

 ployed, and, for cultivation on a large scale, boiled salt-water fish 

 forms an advantageous medium. 



101. The Luminous Bacteria as Tests for Enzymes. 



The different behaviour of Ph. phospliorescens and Ph. Pflugeri 

 towards maltose can be utilised when it is desired to ascertain 

 whether this sugar is produced in any diastatic process. For this 

 purpose plate cultures of the two organisms are prepared, a mixture 

 of sea-water with 8 per cent, of gelatin, i per cent, of peptone, and 

 \ per cent, of boiled potato-starch being used. On these cultures 

 are placed small drops of a solution of the substance whose 

 saccharifying properties are to be tested. Then, if this elaborates 

 glucose or levulose from the starch, the (thickly sown) plates will 

 shortly become luminous at the points affected, whereas if maltose 

 alone is formed, the culture of Ph. Pflugeri will remain dark. In 

 point of delicacy this reaction has but one compeer, viz., the 

 Bunsen flame reaction, whilst in respect of the duration of the 

 phenomenon it is unequalled. The method may also be used with 

 advantage in the solution of the question whether any given 

 microbe has the power of elaborating an enzyme, and if so, of 



