_Beggiatoa_ (parallel with the Blue-green Alga _Oscillaria_). Long filaments formed of cylindrical cells which are attached by one of the ends, but which are nearly always free when observed. The filaments, like those of _Oscillaria_, describe conical figures in their revolutions, the free filaments slide upwards and parallel with one another; sheaths are wanting; strongly refractive sulphur drops are found in the interior. The Beggiatoas are the most prevalent _Sulphur-bacteria_. They occur, very commonly in large numbers, wherever plant or animal remains are decaying in water in which sulphuretted hydrogen is being formed; thus, for example, _B. alba_ (Fig. 31) occurs frequently as a white covering or slimy film on mud containing organic remains. ~_B. mirabilis_ is remarkable for its size and its strong peristaltic movements.~ The Sulphur-bacteria oxidize the sulphuretted hydrogen, and accumulate sulphur in the shape of small granules of soft amorphic sulphur, which in the living cell never passes over into the crystalline state. They next oxidize this sulphur into sulphuric acid, which is immediately rendered neutral by absorbed salts of calcium, and is given off in the form of a sulphate, thus CaCO_{3} is principally changed into CaSO_{4}. In the absence of sulphur the nutritive processes are suspended, and consequently death occurs either sooner or later. The Sulphur-bacteria may exist and multiply in a fluid which only contains traces of organic matter, in which organisms devoid of chlorophyll are not able to exist. The Beggiatoas very frequently form white, bulky masses in sulphur wells and in salt water, the traces of organic material which the sulphur water contains proving sufficient for them. ~The cellulose-fermentation, to which the sulphur wells in all probability owe their origin, mainly procures them suitable conditions for existence. The CaCO_{3} and H_{2}S, formed during the cellulose fermentation by the reduction of CaSO_{4} is again changed into CaSO_{4} and CO_{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.