strains B and K12 could grow in very limiting conditions of divalent cation deficiency. Growth curves showed a long lag period of about 30 h, followed by an exponential phase bringing the bacterial concentration to about 10 ml, with a 24 min doubling time, while the growth curves of control cultures were characterized by short lag periods, maximum populations of about 10 ml and an 18 min doubling time. The DNA/protein ratio in bacteria grown in deficient medium was 0·48 compared with 0·21 for control bacteria. Significant differences were found in the ultrastructure of the two types of bacteria. Freeze-etched control cells showed the typical appearance with the protoplasmic fracture face of the cytoplasmic membrane (PFC) having a random distribution of intramembranous particles. Bacteria growing in deficient medium in exponential phase presented several particle-free areas on the PFC. At the beginning of the stationary phase, the particle-free zones became larger and crystalline structures were formed. These structural modifications, which increased with culture age, were never observed in bacteria grown in control medium. Optical diffraction analysis of the crystalline structures in freeze-etched cells revealed regular periodic arrays with a rhomboid repeating unit approximately 7·6 × 5·4 nm in dimension and an angle between the axes of about 73°. Negative staining of isolated membranes of bacteria grown in deficient medium showed a more complex organization of the crystalline arrays, each unit being clearly composed of subunits.


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