KOCH'S PLATE CULTURES 103 



Pure cultivation with solidified media is almost exclusively applicable to fungi 

 alone, its application to other groups having been successful in the case of a few 

 only of the lowest unicellular algae. In this manner BEYERINCK (VI.) prepared 

 pure cultures of the last-named organisms (frequently found in the microscopic 

 examination of river- water), viz., Chlorella vulgar is, Scenedesmus acutus, Chloro- 

 sphcera limicola, Chlorococcum (Cystococcus) humicola, Stichococcus major, and a 

 second species of Chlorella. WILHELM KRUGER (I.) prepared by the plate method 

 pure cultures of two algse, which he named Chlvrella protothecoides and Chlorothe- 

 cium saccharophilum, from the sap of the lesser maple. Quite recently BEYERINCK 

 (VII.), A. CELLI (I.), FR. SCHARDINGER (I.) and others, have also successfully 

 attempted the cultivation of Amceba in the same manner. 



The solid nutrient media in question are, except those containing chalk, 

 transparent, so that the plates prepared therefrom can be laid on the stage of 

 the microscope and their colonies examined under a low power by transmitted 

 light. The differences thereby observable form valuable indications for the 

 identification of the individual species. A number of them, Bacillus subtilis for 

 example, excrete a peptonising enzyme, in consequence of which the gelatin is 

 liquefied as far as the solvent enzyme proceeding from the colonies is able to 

 diffuse, and thus a liquefactive colony is obtained. The converse was supplied 

 by such organisms as produce no enzyme capable of dissolving gelatin, and which 

 therefore do not liquefy the medium, but grow as solid colonies. The develop- 

 ment of these latter may proceed in various ways ; the colonies of Bacterium 

 aceti, for instance, gradually assuming a stellar form, whilst those of the lactic 

 acid bacteria have a circular outline. Bacillus ramosus a fission fungus 

 frequently occurring in soil and in natural waters, and generally known as 

 wurzel (root) bacillus which M. WARD (IV.) subjected to exhaustive 

 morphological and physiological examination, grows on agar-agar to colonies 

 built up of entangled, branched, and plaited threads resembling the roots of a 

 tree. A comprehensive description of these characteristics, as presented by the 

 separate species of bacteria, will be found in Eisenberg's work. 



If a platinum wire, previously heated to redness and then dipped in a 

 bacterial culture, be thrust into a solid medium contained in a test-tube, the 

 cells so implanted in the passage formed by the wire will develop to a so-called 

 puncture culture, the appearance of which also affords valuable indications for 

 the recognition of individual species. Organisms requiring air grow only on 

 the surface, whereas those shunning the air will develop only in the deepest 

 part of the channel, and those dissolving the gelatin will forma liquefied funnel. 

 This latter indication is one developed in a highly characteristic manner by the 

 cholera bacillus, and is, therefore, made use of in the bacteriological analysis of 

 water. If a test-tube containing about 8 or 10 c.c. of liquefied nutrient gelatin 

 or agar-agar be held at a sharp angle, the contents will set in the form of a 

 wedge, and if the plane surface be stroked over with a" small quantity of a 

 culture, then a so-called streak culture will be developed. This also in many 

 cases assumes a characteristic form that should be taken into consideration in 

 the identification of a species. The potato cultures already referred to are 

 nothing more than streak cultures on the cut surfaces of steamed potatoes. We 

 are indebted to C. FRAENKEL and R. PFEIFFER (I.) for an excellent atlas of 

 photographs of the colonies, streak cultures, cover-glass preparations, &c., of a 

 number of (mostly pathogenic) bacteria. An atlas of coloured plates of these 

 objects has been issued by K. B. LEHMANN and R. NEUMANN (I.). 



It may be valuable, for purposes of instruction, to preserve in the cultures 

 the appearance they present at the time of their most vigorous growth. 

 According to G. HAUSER (II.), the preservation of cultures on gelatin or agar- 

 agar can be most conveniently ensured by means of formaldehyde. Cultures in 



