38 BACTERIA. 



in salt water, the traces of organic material which the sulphur 

 water contains proving sufficient for them. The cellulose-fermenta- 

 tion, to which the sulphur wells in all probability owe their origin, mainly 

 procures them suitable conditions for existence. The CaC0 3 and H 2 S, 

 formed during the cellulose fermentation by the reduction of CaS0 4 is 

 again changed into CaS0 4 and C0 2 by the Sulphur-bacteria (Winogradsky, 

 1887). Other Sulphur-bacteria, the so-called purple Sulphur-bacteria, e.g. 

 B. roseo-persicina, Spirillum sanguineum (Fig. 23), Bacterium sulfuratum, etc., 

 have their protoplasm mixed with a red colouring matter (bacterio-purpurin) 

 which, like chlorophyll, has the power, in the presence of light, of giving off 

 oxygen (as proved by T. W. Englemann, 1888, in oxygen-sensitive Bacteria). 

 The three purple Sulphur-bacteria mentioned, are, according to Winogradsky, 

 not pleomorphic kinds but embrace numerous species. 



Many Spirilli (Spirillum tenue, S. undula, S.plicatile, and others) 

 are found prevalent in decaying liquids. 



Bacteria (especially Bacilli) are the cause of many substances 

 emitting a foul odour, and of various changes in milk. 



Parasitic Bacteria live in other living organisms ; but the 

 relation between " host " and parasite may vary in considerable 

 degree. Some parasites do no injury to their host, others produce 

 dangerous contagious diseases ; some choose only a special kind 

 as host, others again live equally well in many different ones. 

 There are further specific and individual differences with regard 

 to the predisposition of the host, and every individual has not the 

 same receptivity at all times. 



THE HARMLESS PARASITES OF HUMAN BEINGS. Several of the 

 above mentioned saprophytes may also occur in the alimentary 

 canal of human beings; e.g., the 'Hay-bacillus, the Butyric-acid- 

 bacillus, etc. ; but the gastric juice prevents the development of 

 others, at all events in their vegetative condition. Sarcina ven- 

 tricidi, "packet-bacterium," is only known to occur in the stomach 

 and intestines of human beings, and makes its appearance in cer- 

 tain diseases of the stomach (dilation of the stomach, etc.) in great 

 numbers, without, however, being the cause of the disease. It 

 occurs in somewhat cubical masses of roundish cells (l^ig. 25). 



LESS DANGEROUS PARASITES. In the mouth, especially between 

 and on the teeth, a great many Bacteria are to be found (more 

 than fifty species are known), e.g. Leptothrix buccalis (long, brittle, 

 very thin filaments which are united into bundles), Micrococci in 

 large lumps, Spirochcete cohnii, etc. Some of them are known to 

 be injurious, as they contribute in various ways to the decay of the 

 teeth (caries dentiuni) ; a Micrococcus, for instance, forms lactic acid 



