Acetamide Agar Medium Selective for Pseudomonas aeruginosa

A synthetic base of acetamide and salts in agar permitted isolation of small numbers of Pseudomonas aeruginosa from swarming Proteus and other gram-negative species.     

Methacarn (methanol-Carnoy) fixation

Summary According to chemical data, methanol raises the shrinkage temperature of collagen significantly more than ethanol (86° C versus 70° C). Since increase of shrinkage temperature appears desirable in tissues to be embedded in paraffin, methanol was substituted for ethanol in Carnoy's fluid. This methanol-Carnoy mixture is referred to as methacarn solution. The fixation-embedding procedure was similar to that described in the study of Carnoy fixation. Methacarn-fixed sections showed little or no shrinkage and compared well with material fixed in Carnoy's or Zenker's fluid. Myofibrils, especially in endothelial and epithelial cells, were more prominent in methacarn- than in Carnoy-fixed tissues.A review of the chemical literature showed that methanol, ethanol and chloroform stabilize or even enhance helical conformations of proteins, presumably by strengthening of hydrogen bonds. Interference with hydrophobic bonds causes unfolding and/or structural rearrangements in globular proteins. The twin-helical structure of DNA collapses in alcoholic solutions. Hence, methacarn fixation can be expected to preserve the helical proteins in myofibrils and collagen, but the conformations of globular proteins and DNA will be significantly altered. Literature on conformational effects produced by fixatives used in electron microscopy was also reviewed. Glutaraldehyde and OsO₄ cause considerable loss of helix (22–29% and 39–66% respectively). KMnO₄ and glutaraldehyde followed by OsO₄ produce extensive transitions from helical to random-coil conformations similar to those seen in powerful denaturants such as 8 M urea. Evidently these fixatives are unsuitable for studies of helical proteins. In contrast ethylene glycol preserves helical conformations.     

Photosynthetic Membrane System in Anacystis nidulans

Cultures of Anacystis nidulans were grown under conditions of varying light intensity and temperature. Changes in pigment content were compared with changes in the fine structure of these cells. Pigment concentration and lamellar content varied inversely with the light intensity in cells grown with 100 and 1,000 foot candles of fluorescent light. Estimations of the relative area and volume of lamellae in cells showed that the amount of double membrane was directly proportional to the chlorophyll content of whole cells. Continuity of double membranes with cytoplasmic membrane was observed.