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SUMMARY : Details are given of the 'auxanographic' method for identifying growth- factor requirements of micro-organisms, and particularly of mutant strains differing from a parent strain in having additional growth-factor requirements. Cells are distributed in a nutritionally deficient agar medium which is allowed to set in a Petri dish. Possible required nutrients are then spotted on the agar surface, thus enabling the corresponding exacting mutants to grow at those localities. The technique is only a labour-saving device which can be extremely efficient in shortening the pre- liminary steps in the work. It can also be applied to other uses, such as the identification of competitive interactions between nutrilites. Sixty years ago, in 1889, Beyerinck (Beijerinck) described a technique, simple and efficient, for the study of the nutritional requirements of micro-organisms. The 'auxanographic' technique, as he called it, has gone almost unnoticed; the present author became aware of Beyerinck's paper only after having redevised the technique, developed it for biochemical genetics, and used it for several years (Pontecorvo, 1947). Its efficiency in saving labour and its many uses make worthwhile a description of its applications to the study of the genetics of micro-organisms and certain other problems. On a densely seeded agar plate of a medium lacking a required growth factor for a given micro-organism, no visible growth can take place. If, however, we place at a point of the agar surface a minute amount of the missing growth factor, this will diffuse out, and on incubation growth will take place in a circular area centred on that point. Instead of a growth response other kinds of visible response can be used; Beyerinck suggested, for instance, luminescence, secretion of a pigment, etc. The technique can be used in reverse, as, indeed, it has been used in Heatley's cylinder assay for penicillin and its numerous variations. If we place a minute amount of an inhibitory substance at a point on the agar surface of a seeded nutrient medium, a circular clear area without growth, surrounded by a general background with growth, will remain after incubation. The first kind of test, based on the promotion of growth, could perhaps be termed 'positive ' auxanography ; the second, based on inhibition of growth, ' negative' auxanography. The two are sometimes used in combination, for instance, for investigating the mode of action of a metabolite analogue (see below), and are suitable for quantitative as well as qualitative tests. An example of quantitative positive auxanography is the plate assay for growth factors (see below), and an example of quantitative negative auxanography is the cylinder assay for penicillin. Qualitative negative auxanography has been used by a number of authors especially for detecting the presence of antibiotics in culture filtrates and their spectrum of activity. Qualitative positive aux- anography has been extensively used by the present author for a quick

Auxanographic Techniques in Biochemical Genetics