38 BACTERIOLOGY 



same group of bacteria frequently show a very similar appear- 

 ance on culture, and require special tests for their differentia- 

 tion. In a fluid medium such as nutrient broth, the organism 

 may produce a general turbidity, or it may grow as a scum on 

 the surface, or as a deposit at the bottom. On the surface or 

 in the substance of a jelly, typical appearances are often seen; 

 and, if gelatin be the substance employed to make the jelly, 

 some organisms are found to liquefy it by the action of 

 digestive ferments which they produce, whilst others do not 

 a very important test whereby bacteria are divided into two 

 great groups. 



The colour of the growth is often characteristic. Many of 

 the organisms commonly found in air and water, but com- 

 paratively few pathogenic bacteria, produce brightly coloured 

 growths. The various tints formed by such chromogenic 

 bacteria include all the colours of the spectrum, and the fol- 

 lowing interesting table (taken from Frost and M'Campbell's 

 General Bacteriology) gives one example of each : 



Red . . . Bacillus prodigiosus. 



Orange . . . Sarcina aurantiaca. 



Yellow . . . Micrococcus fla/vus. 



Green . . . Bacterium viridis. 



Blue . . . Pseudomonas syncyanea. 



Indigo . . . Pseudomonas indigofera. 



Violet . . . Bacillus violaceus. 



Bacteria producing all the intermediate shades are known. 

 The first-mentioned organism, B. prodigiostis, produces a rich 

 blood-red colour, and it is supposed to have been the explana- 

 tion of the " bleeding bread " of the Middle Ages. The pig- 

 ment produced by chromogenic bacteria may be contained in 

 their substance, or may be excreted into the surrounding 

 medium. 



Fluorescence a peculiar bluish appearance on exposure to 

 light is seen in some cultures ; and phosphorescence or light 

 production is also caused by some bacteria, as well as by 

 certain Protozoa and moulds. The latter process is really a 

 slow combustion or burning up of organic matter, and an 

 abundant supply of oxygen is required for its production. 

 Such phosphorescence is a common phenomenon in sea-water, 

 and it may also often be well seen in decaying animal or 

 vegetable material. 



Some media are coagulated by the action of bacteria, the 

 most familiar example of this being the curdling of milk. The 

 chemical action of bacteria upon their surroundings is a very 

 complicated subject, and it is sufficient to say here that many 



