Search

Spirodela oligorrhiza grown in axenic culture was able to use either ammonium, nitrate, or nitrite as sole source of nitrogen, although the morphology of the plants was affected. Plants utilizing ammonium contained higher levels of NH4, arginine, asparagine and glutamine than did those utilizing NO3, whereas concentrations of other amino acids were similar.The utilization of NH4 inhibited that of NO3 by inhibiting, at least partially, NO3 uptake, and by inhibiting almost completely the reduction of NO3 to NO2. NO2 also inhibited the utilization of NO3. NH4 and NO2 were taken up and assimilated simultaneously when they were supplied together in the medium.

Wheras ammonium inhibited almost completely utilization of nitrate (NO3) by Spirodela oligorrhiza, asparagine, aspartate, glutamine, and glutamate had only a sparing effect. Asparagine and glutamine were used more rapidly than NO3; aspartate and glutamate at about the same rate. These differences in utilization of NO3 were not determined by differences in the levels of nitrate reductase.

Nitrate reductase (NO3R) in Spirodela oligorrhiza has a specific requirement for NAD, and no activity could be detected when reduced NADP was supplied as a cofactor. NO3R and nitrite reductase (NO2R) are adaptive enzymes, being present only when the plant is supplied with NO3 or NO2. There is significant correlation between the concentration of NO3 in plants, and the levels of NO3R that they contain. When plants not containing NO3R or NO2R are supplied with NO3, there was a rapid increase in the levels of the two enzymes and in the concentration of NO3 in the plants. These increases were retarded but not prevented by NH4 in the medium. When NO3-grown plants were depleted of N, there was a rapid decrease in the levels of NO3R and NO2R.The utilization of NH4 prevented the assimilation of NO3 even by plants that contained high levels of NO3R and NO2R. It was therefore concluded that NH4, or the processes or products of its assimilation must inhibit the activity of NO3R. NH4, arginine, asparagine or glutamine, tested separately or together, had little effect on the in vitro activity of NO3R.

A selective and differential medium, Shahidi-Ferguson Perfringens agar (SFP agar), and a confirmatory medium, lactose-motility agar (LM agar), were developed for the enumeration and identification of Clostridium perfringens in foods. These media provide a rapid, specific, and direct diagnosis of C. perfringens. SFP agar contains sodium metabisulfite and ferric ammonium citrate to demonstrate H(2)S production and egg yolk to demonstrate lecithinase production by C. perfringens. On SFP agar, C. perfringens produces black colonies, 2 to 3 mm in diameter, surrounded by zones of opaque precipitate. The typical colonies are confirmed on LM agar. Enumeration and identification are completed within 48 hr. All of the ingredients of SFP agar are stable to heat and storage conditions. SFP agar also contains two antibiotics, kanamycin and polymyxin B, which are inhibitory to many bacteria commonly occurring in foods. A comparative study of SFP agar and noninhibitory media showed that SFP agar did not inhibit any of the 16 strains of C. perfringens tested. Recovery of C. perfringens added to foods averaged 90.6% for SFP agar as compared with 69.8% for sulfite polymyxin-sulfadiazine (SPS) agar (BBL) and 60.2% for SPS agar (Difco). The colonies on the SFP agar, were much larger and were consistently black. Of 464 food samples tested, C. perfringens was found in 27 samples with SFP agar and in 5 samples with SPS agar (Difco), with a recovery ratio considerably higher on SFP agar. SFP agar is a more specific presumptive medium for the enumeration of C. perfringens and in conjunction with LM agar should save considerable time, effort, and materials toward the final identification of the species.