L-forms of group A streptococci Size of viable particles and growth requirements
Filtrability is one of the characteristics of the L-phase of bacteria. The results of the filtration experiments reported in the literature indicate a diameter of the minimal reproductive unit of approximately 0.3t~. However, by measurements on photographs of slide cultures of the L-phase of several bacteria the diameter of the reproductive elements was found to be > 0.6/~ to 0.7t~ (Weibull and Lundin, 1962; Weibull, 1963). With the L-phase of two strains of group A streptococci the same discrepancy between the results of both techniques was obtained. Dependent on the expelimental conditions the filtration endpoint varied between 0.25/~ and 0.45u.. Slide cultures, observed by phasecontrast microscopy and photographed at regular intervals during incubation, showed that only elements having a diameter > 0.8~ developed into colonies. The possibility that the L-phase, due to its plasticity, is capable of passing through filters of smaller pore size than their diameter was investigated. Filtrates of 0.45~. filters were conceotrated by ultracentrifugation. Slide cultures inoculated with the sediment showed that the elements responsible for growth had a diameter ranging from 0.7tL-1.7tz. The results obtained indicate that the reproductive elements of the L-phase of the group A streptococci have a diameter > 0.8/~. Another property in which the L-phase differs from the parent strains is the need for special growth requirements (Sharp, 1954). However, results were obtained which indicate that these special conditions are not required during subculture. It proved to be possible to get luxuriant growth in a chemically defined medium of both the L-phase and the parent strains.
Growth of elements of various sizes found in cultures of a stable Proteus L form.
SUMMARY: Samples of a liquid culture of Proteus L9 were transferred to agar blocks containing the same medium as the liquid culture. Photomicrographs were made of the slide cultures thus obtained, and the diameters of a large number of individual L elements determined. The measurements indicated that the elements of the L cultures studied could be assigned to two classes (I and II) with regard to their size. The elements of class I had an average diameter of 0.33 μ (standard deviation 0.071; standard error of the mean 0.006 μ); the corresponding figures for class II were 0.94, 0.21 and 0.02 μ, respectively. The growth of individual L elements in the slide cultures was followed during incubation at 30°. Only elements having a diameter > 0.6 to 0.7 μ, i.e. elements belonging to class II, enlarged measurably or formed microcolonies. Calculations based on measurements made on L elements growing on streak plates confirmed these observations.
Microstructure of the solder-casting zone in bridges of dental gold alloys.
The composition and structure were studied in the cast and in the solder of bridges which had failed clinically. Both materials were gold alloys but with different contents of Pt, Ag and Zn. The metallographic investigation revealed defects mostly in the solder, situated in the subsurface layer. SEM studied of the fracture surface revealed large porosites and a structure of dense parallel lines--striations--, indicating that the material had failed from fatigue. The materials in the fractured bridges were identified by micro-probe measurements. It was stated that cast material and solder material used together, were of different composition.