i 3 4 METHODS OF PURE CULTURE. 



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 test-tubes can be treated 

 by moistening the cotton plug with formalin and then covering it 

 with a rubber cap to prevent desiccation. Plates and cultures in 

 Petri dishes can be kept for some time by covering them with 

 filter-paper moistened with the same antiseptic, which kills the 

 cultures without destroying their form. It also penetrates the 

 medium, hardens the gelatin, and makes it unsuitable for the 

 further development of organisms, so that the preparations thus 

 treated are exceedingly durable. Fuller information on the pre- 

 servation of pure cultures of fermentative organisms, and practical 

 hints concerning the arrangement of mycological museums, which 

 are very useful both for teaching and for reference purposes, will 

 be found in the treatises prepared by J. SOYKA (I.), F. KRAL (I.), 

 H. PLAUT (I.), E. CZAPLEWSKI (I.), and E. KRUCKMANN (I.). 



The necessity will not infrequently arise for reliable (living) 

 pure cultures of authoritatively named species of bacteria, &c., 

 either for use as a starting-point for study or for comparing, and, 

 as far as possible, identifying some newly-discovered species. Krai's 

 Bacteriological Laboratory (n Kleiner Ring, Prague I.) can be 

 recommended as a source from whence to obtain them. This in- 

 stitution supplies living pure cultures, streak-cultures on oblique 

 solidified agar-agar, in test-tubes at the moderate price of one to 

 two marks ( = shillings) per tube. 



Koch's plates can also be used with advantage when it is a 

 question of ascertaining which nutrient media are suitable for a 

 given microbe. For this purpose BEYERINCK (VIII.) has devised 

 a method which he calls Auxanography. A 10 per cent, gelatin 

 or 2 per cent, agar-agar in distilled water is prepared, both of 

 which substances in the pure state form very bad media, whether 

 for bacteria or higher fungi. Plate cultures of the micro-organism 

 whose nutritive requirements form the object of the investigation 

 are then prepared. These, if left to themselves, will not exhibit 



