A genetic and biochemical study of streptomycin- and spectinomycin-resistance in Salmonella typhimurium

Summary In Salmonella typhimurium, streptomycin resistance can occur by mutation at the strA or the strB mutants have altered ribosomes which are refractory to the drug in cell-free amino acid incorporation systems, and in ³H-dihydrostreptomycin binding studies. StrB mutants, unlike the strA mutants, are resistant to several aminoglycoside antibiotics and resistance is not due to a mutational change in the cell's protein synthetic machinery. Spectinomycin resistant mutants of S. typhimurium also fall into two classes, only one of which is ribosomal in mechanism. The spcA and spcB loci are closely linked to strA, aroC, and argG on the S. typhimurium linkage map.     

Cellular Studies of X-Ray Induced Inhibition of Lens Regeneration

Whole-body X-irradiation of adult newts 0 to 3 days after lentectomy inhibits transformation of the dorsal iris epithelium into a lens in all cases. The first question raised was whether irradiation affects infiltration of the iris area by macrophages, and the phagocytic activities of these cell types in the iris epithelium (prominent phenomena in this system). The number of macrophages infiltrating into the iris epithelium, and their phagocytic activities (indicated by uptake of melanosomes) were not affected by irradiation under those conditions. The second group of experiments concerns the possible effects of irradiation on DNA replication of iris epithelial cells, which become transformed into lens cells in the non-irradiated system. Autoradiographic studies of iris epithelial cells in vivo revealed a significant suppressive effect of irradiation on the frequencies of cells incorporating ³H-thymidine 7 and 14 days after lentectomy. When autoradiography was applied to the primary pure culture of iris epithelial cells at different time intervals after the start of culture and irradiation in vitro , significant and persistent reduction of cell labelling due to irradiation, was demonstrated. Multiplication of spread cells in the iris epithelial culture was strongly and persistently inhibited throughout a period of 2 months. Inhibition of cell labelling and of cell multiplication was always accompanied by reduction in the extent of de-pigmentation of iris epithelial cells. De-pigmentation is one of the requirements for the cells become transformed into lens cells. The possible mechanism of radiation-induced inhibition of lens regeneration is discussed.     

Continuous production of L-aspartic acid by immobilized aspartase

Various methods were tried for the immobilization of aspartase, and the preparation having the highest activity was obtained when partially purified aspartase from Escherichia coli was entrapped into polyacrylamide gel Iattice. Enzymatic properties of the immobilized aspartase were investigated and compared with those of the native aspartase. With regard to optimum pH, temperature, concentration of Mn⁺⁺, kinetic constants and heat stability, no marked difference was observed between the native and immobilized aspartases. By employing an enzyme column packed with the immobilized aspartase, conditions for continuous production of L -aspartic acid from ammonium fumarate were investigated. When a solution of 1M ammonium fumarate (pH 8.5, containing 1mM MnCl₂) was passed through the aspartase column at the flow rate of SV = 0.08 at 37°C, the highest rate of reaction was attained. From the column effluents, L-aspartic acid was obtained in a good yield.     

Clinical Use of a New Mitral Disc Valve

A disc valve of new design was used successfully for the replacement of the mitral valve in patients with rheumatic mitral valve disease. This valve would appear to have the following advantages over the mitral ball valve prosthesis:• Lower left atrial pressure after replacement.• Elimination of the hazard of left ventricular outflow tract obstruction with mitral valve replacement.• Decreased incidence of thromboembolization.• Abolition of possibility of ventricular septal irritation.Despite the better outlook for this valve compared with the ball valve for mitral valve substitution, the mitral valve should always be repaired whenever feasible. Repair is possible in the majority of patients.