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VARIOUS substances added to nutrient agar media are known to enhance colonial pigmentation of Staphylococcus aureus. These include sterile cow's milk1,2, fermentable carbohydrate3, sodium chloride4, glycerophosphoric acid5and glycerol monoacetate6. Not all these are equally effective, however, the most intense pigmentation being developed on milk agar and on glycerol monoacetate agar. Glycerol monoacetate agar has the additional advantage that strains of Staph. aureus growing on it can be readily differentiated into a number of colour varieties—yellow, orange, buff and white—after incubation for 48 h. In our hands, milk agar has given irregular results. Thus, although the medium has consistently induced strongly pigmented growth, some batches of it have shown little or no differentiation between the colour types, even after prolonged incubation, while other batches have developed striking colour differentiation after incubation for only 24 h. These inconsistencies led us to examine different components of cow's milk in order to determine the nature of its pigment-stimulating fraction.

METHODS of differentiating pathogenic species of Staphylococci (Staphylococcus aureus) from commensal strains (Staph. albus) are of prime importance to the medical bacteriologist. The method generally used to effect this separation is one of the many modifications of the coagulase test, and it is usually considered that new tests adopted for this purpose should correlate highly with the coagulase index. Among the alternatives that have been proposed to the coagulase test are colonial pigmentation, phenolphthalein phosphatase activity and the possession of a fibrinolysin.

COBALT ferrite (CoO.Fe2O3) and magnetite (FeO.Fe2O3) both crystallize at room temperature with the inversed-spinel-type of structure1. A magnetic transition consisting of an abrupt decrease of saturation magnetization takes place in magnetite2at 116° K., and is accompanied by a slight deformation of the structure cell from cubic symmetry above the transition to approximately rhombohedral symmetry below3. As cobalt ferrite4undergoes a similar magnetic transition at 90° K., we might expect that, like magnetite, it is no longer cubic below its magnetic transition temperature. We have shown by means of X-ray powder photography that this is indeed true, but the departure from cubic symmetry is even smaller than in the case of magnetite. Our results indicate that the structure cell of cobalt ferrite at low temperatures is approximately tetragonal.

NAGLER1showed that the surface of colonies of Clostridium oedematiens type A growing on egg-yolk–sheep-blood agar, is covered by a pearly layer. Subsequent investigations by McClung et al.2and McClung and Toabe3showed that this phenomenon was not restricted to Cl. oedematiens type A, but was also shown by all types of Cl. botulinum and by Cl. sporogenes, and was more evident if the blood was omitted from the medium. All the Clostridia which show this pearly layer on egg-yolk agar also produce an opalescence within the medium, somewhat similar in appearance to that produced by anaerobes such as Cl. welchii and Cl. bifermentans, which produce lecithinase. Oakley et al.4showed that cultures and culture-filtrates of Cl. oedematiens type A produce two distinct types of opalescence in egg-yolk emulsions: a diffuse opalescence due to a hæmolytic lecithinase (Cl. oedematiens γ-toxin), and a more confined opalescence due to a lipase (Cl. oedematiens ɛ-antigen). They suggested that the pearly layer also produced by Cl. oedematiens type A might be caused by the lipolytic ɛ-antigen (see also ref. 5). Sussman (personal communication) showed that cultures of Cl. sporogenes were lipolytic when grown on tributyrin agar (‘Oxoid’), and that this organism developed a pearly layer when grown on an agar medium containing triolein.

HAYWARD1suggested in 1941 that the Nagler reaction might be used for the rapid identification of Clostridium welchii. She expanded this work later2and directed attention to the difficulty of distinguishing between Cl. welchii and Clostridium bifermentans on serum agar plates. Both these organisms produced opalescence in the medium, and it was inhibited by Cl. welchii antitoxin. She suggested that distinction between these organisms may be made by making use of the hydrolysis of phenolphthalein phosphate as described by Bray and King3.

IN a recent letter in Nature, Amelinckx1has given evidence for the existence of screw dislocations in mica. He observed a step, 140 A. high, which terminated on the surface : the point of termination of the step defined the point of emergence of a screw dislocation of Burgers's vector 140 A.

STRIATIONS perpendicular to the Z-axis frequently occur on prism faces of quartz. They are often attributed to an ‘oscillatory combination’ of the prism and adjacent rhombohedron faces; but although this sometimes undoubtedly occurs (leading to tapered and ‘candle’ crystals), we believe that it is not the correct explanation of striations on well-developed crystals. A study of the surface topography of a number of natural crystals and a few synthetic crystals shows that the striations can be attributed to a difference in the rate of spreading of growth sheets parallel and perpendicular to the Z-axis.

VARIOUS bacteria are known, to produce opalescence in egg-yolk emulsions due to lecithinase or lipase activity. Among these are the lecithinase-producing clostridia and aerobic spore-forming bacilli, and the lipolytic clostridia and staphylococci. Little attention seems to have been paid to the effects produced by Gram-negative organisms growing on egg-yolk media. Felsenfeld1showed that the opalescence produced by four strains of Vibrio comma and one strain of Vibrio el tor was due to lecithinase activity, and Crook2made a passing reference to one strain of Pseudomonas pyocyanea (Ps. aeruginosa) which produced an opalescence on egg-yolk agar. Willis3noted tho opalescence changes produced by four strains of Ps. pyocyanea.

THE bactericidal effect of ionizing radiation has been recognized for many years; but it is only recently that serious attention has been given to this method of sterilization for foodstuffs1and for pharmaceutical and other products2,3. This is due mainly to the increased availability of radioactive isotopes. In this respect γ-radiation seems to be the most useful due to its effective depth of penetration, and the most convenient sources of γ-radiation are cobalt-60 and cæsium-137.

WILSON1has calculated the nature of the diffraction from a cylindrical crystal with a single axial screw dislocation. Diffraction from an edge dislocation is more difficult to treat theoretically, because of the more complex nature of the elastic field surrounding the dislocation. An approximate treatment has been given by Wilson2, who assumed that the displacements of the atoms were parallel to the Burgers vector. We have succeeded in solving the diffraction problem for a cylindrical crystal with an axial edge dislocation, in which the displacements correspond to isotropic elastic theory. Two independent methods were used : straightforward calculation (by T. S.), and direct observation in a Lipson diffractometer of the optical diffraction pattern of a two-dimensional grating representing the crystal (B. T. M. W.). In Fig. 1 we reproduce the optical diffraction pattern given by a grating representing (a) a perfect crystal (no dislocation) and (b) a simple-cubic crystal with an edge dislocation normal to the plane corresponding to the plane of the grating.