Showing papers in "Microbiology in 1962"

The establishment of vesicular-arbuscular mycorrhiza under aseptic conditions.

Abstract: SUMMARY: The establishment of vesicular-arbuscular mycorrhizal infections by inoculation with germinated resting spores of an Endogone sp. was investigated under microbiologically controlled conditions; pure two-membered cultures were obtained for the first time. Seedlings were grown in a nitrogen-deficient inorganic salt medium; in these conditions the fungus failed to form an appressorium and to penetrate the plant roots unless a Pseudomonas sp. was also added. Adding soluble nitrogen to the medium completely inhibited root penetration, even in the presence of the bacteria. Various sterile filtrates could be used to replace the bacterial inoculum but these substitutes induced only few infections per plant. Mycorrhizal roots grew more vigorously than non-mycorrhizal roots of the same seedling. They were longer and more profusely branched. At first mycorrhizal infections were predominantly arbuscular, but many prominent vesicles developed as the seedlings declined, and then the fungus grew out of infected roots and colonized the agar. The fungus could not be subcultured without a living host. The possible interpretation of these results is considered with reference to the specialized nutritional conditions under which test plants were grown.

Growth, Cell and Nuclear Divisions in some Bacteria

Abstract: SUMMARY: The timing of cell and nuclear division of certain enteric bacteria was determined under conditions of balanced growth. Organisms were grown in a high refractive index medium and photographed at frequent intervals with a phase-contrast microscope. This allowed an estimation of the time between successive divisions of nuclei and cells (interdivision times) and the growth rate of each individual. The interdivision times of cell and nuclear divisions had a similar degree of variation (coefficients of variation of about 20%). The interdivision times of sister cells and sister nuclei were positively correlated to a significant degree. The correlation between mothers and daughters was negative to a significant degree in some, but not all, experiments. The correlation between interdivision times of a cell and that of its corresponding nucleus was positive in most experiments. The rate of mass increase of individual cells was estimated by measuring the rate of elongation. Within the limitations of the method of observation, it could be concluded that cells grew exponentially between successive divisions. Different individuals grew with very nearly the same rate constant. The variation in size of cells at the time of nuclear and cell division was smaller (coefficients of variation of about 10%) than that of the interdivision times. Some observations on the morphological changes of nuclei during growth and division are presented.

The survival of starved bacteria.

Abstract: SUMMARY: Samples of populations of Aerobacter aerogenes grown in continuous culture on a limited supply of glycerol died linearly with time without significant cryptic growth when aerated in buffered physiological saline at their growth pH and temperature. Death was uninfluenced by atmospheres varying from pure O2 to 2% O2 in N2 but was accelerated under N2 in media of various Eh values. Death was accelerated in environments of higher or lower tonicity, in unbuffered media, at pH values above 7, temperature above 40°, by strong illumination. Within limits, lower pH values or temperatures prolonged survival. Death was not immediately accompanied by breakdown of the osmotic barrier. Populations more dense than equiv. 20 μg. dry wt./ml. survived longer, sparser ones died more rapidly. The death rates of the populations studied were not influenced by 30% i-erythritol or by certain high molecular weight materials. Several metabolic inhibitors were tested; most of them accelerated death or had no effect; three protected transiently. Mg2+, Ca2+ or Fe3+ ions delayed death. A variety of trace elements, inorganic ions, growth factors, or an amino acid mixture, had no significant action; glycerol or intermediate compounds accelerated death. Dying populations showed rapid initial breakdown of intracellular RNA with release of phosphate and base fragments into the medium; most of the ribose was metabolized. Intracellular protein was degraded after a lag; intracellular polysaccharide and DNA were scarcely degraded at all. Endocellular reserves declined much more rapidly than did viability; the Qo2 with glycerol and ‘glycerol dehydro-genase’ activity declined in parallel with viability. Populations permitted to grow at different rates died more rapidly the slower the growth rate; steady states were obtained by slow continuous culture in which constant proportions of the organisms were dead. For a given growth rate the nature of the limiting nutrient influenced the form of the survival curve: carbon, phosphorus, sulphur or oxygen limitations gave almost linear curves, N-limitation gave a sigmoid curve and Mg-limitation a concave form. In some instances the growth rate influenced the form of the survival curve and the death rate; Mg-limited organisms apparently died faster the faster they grew. The survival of stationary phase populations derived from batch cultures depended markedly on which chemical component of the environment limited growth; ‘logarithmic phase’ and ‘stationary phase’ C-limited organisms differed insignificantly in death rates. Partially synchronized populations from batch cultures died marginally more rapidly when harvested just after division ceased.

A model for statistics of the cell division process.

Abstract: SUMMARY: A model for the statistics of cell division is proposed. The model assumes: (1) that growth at the cellular level is deterministic; (2) that the mean size of a cell at division is under cellular and environmental control; (3) that the distribution of sizes of cells at division has a small coefficient of variation, and is independent of the size at previous divisions; (4) that the cell divides nearly into equal halves. The observed coefficient of variation of the life-length distributionf f (t) results from a twofold smaller coefficient of variation in the distribution of cellular mass at division, g (c). The magnification of the coefficient of variation results from the fact that the mass variable enters twice, once in determining the size of cell formed at division, and, secondly, in determining the size of the cell when it in turn divides. In the expression for T, these sizes enter as the logarithm of their ratio. These mathematical operations contribute to the increased coefficient of variation of the f (T) distribution over the g (c) distribution. The skewed nature of the f (T) distribution is attributable to a number of causes. We feel that an important source of the skewness is due to deviations from equi-partition of cell constituents at division. In this respect the present model is in disagreement with previous models which presume that the skewness results from the statistics of a small number of molecular events taking place inside each cell. The experimentally observed positive sister-sister life-length correlation and the negative mother-daughter life-length correlation are explained by the model. Deviations from the predicted values of + 0.5 and —0.5, respectively may be explained in part by deviation from equal distribution of cell contents at division. Deviations of the correlations in excess of these, observed in some cases, imply the existence of the other biological processes. The size distribution of cells in balanced growth cultures based on this model is given.

The kinetics of the synthesis of photopigments in Rhodopseudomonas spheroides.

Abstract: SUMMARY: The magnitude of self-shading in light-grown cultures of Rhodopseudomonas spheroides has been estimated. When a culture of R. spheroides is transferred from bright light to dim light there is preferential synthesis of bacteriochlorophyll and carotenoid pigments relative to net protein synthesis. Protein turnover does not account for this. The kinetics of the synthesis of bacteriochlorophyll relative to the incorporation of amino acids into protein have been estimated under a variety of conditions. On the basis of these findings a mechanism for the cellular control of pigment synthesis is presented.

Rate of growth of Bacillus cereus between divisions.

Abstract: SUMMARY: Bacillus cereus organisms growing exponentially have a stable length distribution. This length distribution can be analysed by the method described to give the mean rate of increase in length of organisms at any given length. The validity of the method was confirmed by observing the growth of clones of B. cereus in the culture chamber. Both methods showed that the rate of increase in length increased as the organisms got longer, and that there was no hesitation before or after division. Possible applications of this general method to other parameters that can be measured in samples of bacteria taken from stable populations are suggested.

The type species of the genus Nocardia.

Abstract: SUMMARY: Expanded descriptions of Nocardia asteroides (Eppinger) Blanchard, based on 142 strains, and of N. brasiliensis (Lindenberg) Castellani & Chalmers, based on 62 strains, are presented and compared. Illustrations are given of the variation in morphology and appearance of the growth among strains of both species. The morphological and physiological similarity of ATCC strain no. 3318, the accepted type strain of N. farcinica Trevisan, to strains of N. asteroides is shown, and the resulting problem of nomenclature is discussed.

Chemical composition and antigenic structure of cell walls of Corynebacterium, Mycobacterium, Nocardia, Actinomyces and Arthrobacter.

Abstract: SUMMARY: A comparison has been made between the chemical composition of the cell walls of strains from the genera Corynebacterium, Mycobacterium, Nocardia, Actinomyces and Arthrobacter, and the antigenic composition of the same cell-wall fractions as judged by an agglutination test. A common antigenic component was identified in all those strains of corynebacteria, mycobacteria and nocardias which have arabinose and galactose as their principal cell-wall sugars. Some strains of corynebacteria, and 3 strains of Nocardia pelletieri which have a different pattern of cell-wall components, appear to lack this cell-wall antigen. The antigen was not present in the cell walls of 8 strains of actinomyces and 6 of arthrobacter. Seven of the 8 strains of actinomyces had a cell-wall composition identical with that previously described for Actinomyces israelii, and these 7 strains formed a homogeneous group serologically: the other actinomyces strain differed in cell-wall wall composition and its cell walls did not agglutinate with the ‘israelii’ serum, nor did those of any of the 6 strains of arthrobacter, although some of the latter resemble A. israelii in cell-wall structure. These results show that the antigenic studies tended to confirm relationships suggested by cell-wall composition alone, especially among corynebacteria, nocardias and mycobacteria: they also suggest that Corynebacterium as at present constituted is a mixture of species of diverse origin.

Some factors influencing the rumen microbial population.

Abstract: SUMMARY: In sheep fed once daily, the concentrations of micro-organisms in the rumen changed with the time after feeding, some organisms fluctuating in numbers more than others; peak concentrations were reached at different times for different organisms. These changes in concentration were reflected in changes in the proportion of dividing cells, for example, in Dasytricha ruminantium this proportion varied from less than 0.2% throughout most of the day to a maximum of 23 % a few hours before feeding; for Entodinium spp. the variation was less: it was possible to calculate the minimum doubling time for these latter organisms as about 5 hr. One animal at different times, or different animals, on the same ration and dietary regime had very different rumen microbial populations, these differences being particularly marked in the case of some organisms. Reasons why these marked differences in microbial population are not reflected in similarly marked differences in over-all rumen metabolism are discussed. Feeding different quantities of the same ration had little effect on the concentration of rumen microbes, provided the ration was above a minimal level; it is, however, suggested that output of microbial cells from the rumen may have varied with the amount of feed given. Starvation for a few days or prolonged under-nutrition had a marked effect, some organisms being drastically reduced in numbers or dying out completely. When the qualitative nature of the diet was changed, about 10 days were needed to complete the major adjustments in the rumen microbial population.

The Development of Rumen Microbial Populations in Lambs and Calves Under Various Conditions of Management

Abstract: SUMMARY: The development of the rumen flora and fauna has been followed by microscopic observations of rumen fluid from 58 young lambs and calves kept under different conditions of management; including remaining with the dam, early weaning and artificial inoculation into the rumen. Ciliate-free animals were successfully maintained by isolation from direct contact with other ruminants from an early age, but it was noted that greater space should be left between cattle than between sheep. The general effect of different diets and treatments proved similar in calves and lambs but certain differences were noted in their flora. Certain large bacteria developed in isolated lambs without direct contact between animals. Some generalizations, particularly the effect of a high concentrate diet, were applicable to all animals examined, but there were variations between individual animals. In the establishment of rumen ciliates diet was the governing factor and artificial inoculation with ciliates proved as effective as constant contact with the dam. Although the ciliate-free animals showed no differences in performance certain differences in the rumen flora common to isolated calves and lambs were noted.

Inter-Relationships between Certain Rumen Ciliate Protozoa

Abstract: SUMMARY: The establishment and components of the rumen ciliate population in a series of young animals has been followed using intra-ruminal inoculation with rumen material, and for comparison purposes the ciliate population in a number of adult ruminants has been examined. The fact that the ciliates Polyplastron multivesiculatum, Eudiplodinium maggii and Epidinium spp., though not host specific, did not form a stable mixed population was noted and experiments were carried out to examine the antogonism between certain of these organisms. The cause of the antagonism was not determined but cannibalism, food competition, or gross bacterial change did not seem to be responsible. It was found that the population of an adult animal could be changed by inoculation but the relationship between the ciliates appeared to be in some way affected by the host. It is concluded that inter-relationships of the type described may play an important role in determining the components of a particular rumen microfauna.

An inducible amidase produced by a strain of Pseudomonas aeruginosa.

Abstract: SUMMARY: A strain of Pseudomonas aeruginosa was obtained which was able to grow on acetamide or propionamide as sole source of carbon and nitrogen. When grown on these amides, whole bacteria and cell-free extracts rapidly hydrolysed acetamide, glycollamide, acrylamide and propionamide and slowly hydrolysed formamide and butyramide. N-Methylformamide, N-methylacetamide, N-ethylacetamide, N-acetylacetamide, N-methylpropionamide, N-ethylpropionamide, lactamide and methyl carbamate were found to be non-substrate inducers of the amidase when the organism was grown in succinate + ammonium chloride medium. N-Methylformamide, N-methylacetamide, lactamide and methyl carbamate did not inhibit propionamide hydrolysis by whole bacteria, but under the same conditions glycine amide, iodoacetamide and urea were effective inhibitors of amidase activity. N-Phenylacetamide, cyanoacetamide, glycine amide, sarcosine amide, β-hydroxy-propionamide and thioacetamide were neither substrates nor inducers of the amidase in this strain, but inhibited amidase induction by N-methylacetamide in succinate + ammonium chloride medium. Formamide also inhibited amidase induction under the same conditions.

Adansonian classification of mycobacteria.

Abstract: SUMMARY: The physiological properties of 229 strains of mycobacteria (photochromogens, scotochromogens, non-photochromogens and rapid growers) and others already classified were analysed. According to their metabolic capacities and using the method proposed by Sneath, three branches were established. Branch I is formed by micro-organisms with a high metabolic capacity and rapid rate of growth. It includes Mycobacterium smegmatis, M. phlei, M. peregrinum sp.nov. and many unnamed strains with great disparity in their characteristics, that were placed in a separate branch labelled ‘irregular’. Branch II includes micro-organisms that utilize only a limited number of carbohydrates. Belonging to this branch are Mycobacterium fortuitum, M. marinm, M. piscium and M. thamnopheos. Three new species are described, two of them rapid growers, M. acapulcensis sp.nov. and M. runyonii sp.nov. and a scotochromogen, M. flavescens sp.nov. Branch III, formed by Mycobacterium kansasii (photochromogen), M. avium (non-photochromogen), M. marianum (scotochromogen) and M. gordonae sp.nov. (scotochromogen). All are slow growing micro-organisms. Neotype strains of old species are proposed and holotypes of the new species are designated. A key for the identification of these species and a taxonomic tree of mycobacteria are described.

Transmission of colicinogeny between strains of Salmonella typhimurium grown together.

Abstract: SUMMARY: Ability to produce colicines I, E1, E2, K or B was transferred to Salmonella typhimurium strain LT2 by growth in broth with suitable colicinogenic strains of Escherichia coli or Shigella sonnei. When LT2 (colI), i.e. carrying the colicine I factor, or LT2 (colB) were grown overnight in broth with LT2 col - (non-colicinogenic), c. 50 % of the latter became colicinogenic; LT2 (colE2) and LT2 (colK) did not transmit; LT2 (colE1) transmitted to only c. 0.1 % of the acceptor population. But LT2 carrying either colI or colB in addition to colE2, colK or colE1, transmitted both factors. When overnight broth cultures of LT2 (colI) and LT2 col - were mixed and incubated c. 40 % of the latter acquired colI by 20 hr. (when the viable count had doubled); but only c. 0.02 % acquired colI in 3 hr. The low initial transfer results from the fact that in a stock culture of LT2 (colI) only c. 1/5000 bacteria are ‘competent donors’, able to transmit colI. The later large increase in the proportion of colicinogenic bacteria probably results from ‘epidemic spread’ of the colI factor amongst the acceptor population, initiated by the few acceptor bacteria which originally receive it. It is supposed that most bacteria which have just acquired colI become competent donors. In a doubly colicinogenic strain most competent donors transmit both colicine factors. Aeration by shaking during incubation interfered with transmission of colicinogeny, probably by abolishing the prolonged phase of slow growth of unaerated cultures. Growth in the presence of acriflavine did not ‘cure’ LT2 (colI) or LT2 (colI) (colE2) of colicinogeny, nor of ability to transmit. LT2 (colE1) and LT2 (colE2) supported the epidemic spread of colI or colB about as well as did LT2 col -; but in LT2 (colK) the spread of colI was greatly reduced and that of colB somewhat reduced. The prior presence in an acceptor strain of one of the readily transmissible factors, colI or colB, did not interfere with the epidemic spread of the other. But LT2 (colI) did not become a competent donor on accepting colE2 and, by inference, colI from LT2 (colI) (colE2).

Properties and behaviour of a virus depending for its multiplication on another.

Abstract: SUMMARY: The Rothamsted culture of tobacco necrosis virus contains two serologically unrelated viruses one of which, called the ‘satellite virus’ (SV), causes no lesions and multiplies detectably only when the other (TNV) is present. It decreases the size of necrotic local lesions formed by TNV. Inocula containing both viruses gave the same, fewer or more lesions than inocula containing only TNV; the results depended on the ratio of the two viruses in the mixed inoculum, on the species and age of the test plants and, particularly, on the temperature at which the plants were kept after the inoculation. The concentration of TNV decreased when SV was also present; the extent of the decrease depended on the ratio of the two viruses in the inoculum and other factors. Inoculation with SV 5 days before inoculation with TNV still decreased lesion size and led to the multiplication of SV. Phenol-disrupted SV affected lesion size only when inoculation with TNV followed within a few hours, but when inoculation with TNV was delayed by up to 2 days, SV sometimes multiplied detectably. SV has the smallest particle reported for any plant virus; diameter c. 17 mμ and weight equivalent to a molecular weight of about 1·9 x 106, of which 20% is nucleic acid. It is exceptionally stable, retaining infectivity after 17 years at 3° and after heating for 10 min. at 90°. TNV was much more susceptible to inactivation by heat and by ultraviolet light than SV. The two differed greatly in electrophoretic mobility and were readily separated from mixed preparations by electrophoresis.

Studies on the utilization of nitrate by Micrococcus denitrificans.

Abstract: SUMMARY: Micrococcus denitrificans is capable of carrying out assimilatory and dissimilatory nitrate reduction, though only the assimilatory process occurs under both aerobic and anaerobic conditions. Aeration affects the dissimilatory activity (reduction of nitrate to nitrogen) of a growing culture in at least three ways: (a) it prevents the adaptive formation of the system, (b) it partially represses any further synthesis if the system is already present, and (c) it inhibits the activity of the preformed system completely. To some extent these effects of oxygen are reflected in the control which it exerts upon the organism's content of nitrate reductase (the enzyme responsible for the initial reduction of nitrate to nitrite) and upon its activity during growth. Ammonium ions partially inhibit the transformation of nitrate into cell nitrogen but have no detectable effect on the nitrate reductase activity of crude extracts of this organism.

Enumeration of rumen micro-organisms.

Abstract: SUMMARY: Techniques are described for counting and differentiating the rumen microbial population. The rumen liquor is fixed and preserved in formol saline, and counts are made in two counting chambers, whose dimensions depend on the size of the organism to be counted; stained smears of the bacteria are also examined. Counts made on small samples of rumen liquor obtained through a fistula have been compared with counts on samples of the entire rumen contents; the ratio of these counts seemed to depend on the particular organism counted, the ration and the time after feeding. The importance of differential as opposed to total counts is emphasized.

Effect of chilling on Aerobacter aerogenes in aqueous suspension.

Abstract: SUMMARY: The lethal effect of cold shock on Aerobacter aerogenes suspensions depended on the time of exposure to low temperature, the growth phase, the concentration of bacteria, the diluent. No death occurred when weak suspensions of susceptible bacteria (about 108/ml.) in buffered saline (pH 6.5) were rapidly cooled to 0° and immediately warmed to 20°, but loss of viability was progressive during 1 hr. at 0°. Bacteria harvested from defined medium at intervals during the exponential growth phase varied in sensitivity to chilling but were more susceptible than stationary phase organisms. While growing in partially synchronized culture the sensitivity of bacteria did not increase significantly during the division lag phase. The viability of dense suspensions (about 1010 bacteria/ml.) in buffered saline was little affected by chilling for 1 hr. at 0°, irrespective of the growth phase. A bacteria-free filtrate from a chilled concentrated suspension of exponential-phase organisms substantially protected a dilute suspension from the lethal effect of chilling. Substances found in protective filtrates were amino acids, adenosine triphosphate and nucleic acid constituents. When added to the diluent in which susceptible bacteria were chilled, a mixture of amino acids afforded some protection; small amounts of adenosine triphosphate had no effect. Other substances found to protect susceptible bacteria were sucrose (0.3 M), magnesium or calcium ions (5 x 10−3M) and, to a much smaller extent, spermine (10−5M). The present results support the suggestion that the lethal effect of chilling is at least partly due to interference with the functioning of a bacterial permeability control mechanism.

A taxonomic study of certain bacteria currently classified as Vibrio species.

Abstract: SUMMARY: Twenty-five strains currently classified into 14 serological types or species of the genus Vibrio obtained from the NCIB and NCTC have been studied in detail for morphological, cultural and biochemical characters in two independent studies. The results indicate that these strains include members of three distinct taxonomic groups or genera, Vibrio, Pseudomonas and Comamonas. The new generic name Comamonas, type species C. percolons, replaces Lophomonas (Galarneault & Leifson) which is invalid.

Further studies on the isolation of proteolytic bacteria from the sheep rumen.

Abstract: SUMMARY: A survey of the proteolytic bacteria present in the rumens of sheep on different kinds of diets has been made, with special emphasis on the isolation of anaerobic types. The results suggest that proteolytic activity is not confined to a single kind of rumen bacterium, but that it is a variable property possessed by strains of many kinds of bacteria which can be active in the breakdown of other feedstuff constituents. The properties of the bacteria isolated are given.

Influence of calcium and magnesium on the growth of rhizobium.

Abstract: SUMMARY: Growth of Rhizobium trifolii in a defined medium reflected the supply of Ca2+ and Mg2+ (subsequently Ca and Mg, respectively) in distinctive fashion. Deficiency of Ca, in the presence of sufficient Mg, caused reduction in growth rate, the level of maximum growth and the proportion of viable cells. Such Ca-deprived cells were markedly swollen and vacuolated. On the other hand, although shortage of Mg (Ca sufficient) was without effect on growth rate down to the lowest concentration at which growth occurred, maximum growth and the proportion of viable organisms were markedly decreased. Mg-deficient organisms were appreciably elongated. Signs of Ca deficiency became apparent at less than 0.025 mM, and Mg deficiency at less than 0.1 mM, most markedly in the range below 0.5 mM. Additionally there was a need for total divalent cations of the order of 0.4-0.6 mM. This could be met by either Ca or Mg provided both were sufficient for their maximum specific effect.

The effect of growth conditions on oxidative and dehydrogenase activity in Staphylococcus aureus.

Abstract: SUMMARY: Suspensions of Staphylococcus aureus grown aerobically on nutrient broth oxidized glucose, acetate and intermediates of the tricarboxylic acid cycle, but glucose-grown organisms oxidized glucose only The inability of glucose-grown staphylococci to oxidize intermediates of the tricarboxylic acid cycle was correlated with diminished succinate and isocitrate dehydrogenase activity in cell-free extracts as compared with extracts from organisms grown without glucose Suspensions of aerobically-grown staphylococci fermented glucose anaerobically at only about one third the rate observed with anaerobically-grown organisms The nicotinamide-adenine dinucleotide-linked lactate dehydrogenase activity in extracts of the anaerobically-grown organisms was about ten times higher than that in extracts of aerobically-grown staphylococci

A study of the negative staining process.

Abstract: SUMMARY: The effectiveness of a number of different materials in the negative staning method for electron microscopy of viruses is evaluated. Shadowing was used to study the degree of distortion suffered by the specimen. Changes in pH value which occurred while the negative staining solution was trying were measured in those materials which gave the most satisfactory results in the electron microscope. The ultimate resolution of the method is discussed and demonstrated. It was important to de-grease supporting film: perforated carbon films were found valuable for obtaining good contrast.

The Effects of Partial Pressure of Oxygen upon Respiration and Nitrogen Fixation by Soybean Root Nodules

Abstract: SUMMARY: Increased oxygen tension (pO2) caused increased respiration by excised soybean nodales of all ages. The increase took place in two steps, the first maximum occurring at about 50% O2 and the second at 90–100% O2 for actively nitrogen-fixing nodules. With increasing nodule age the first maximum occurred at decreasing pO2 until, when fixation ceased at about 6 weeks, this maximum had disappeared. This effect was more marked at 30° than at 23°. The respiration of bacteroids increased with increasing pO2 with a maximum at 2–3% O2; the curve indicated a simple saturation of the terminal respiratory pathway with O2. Increased pO2 raised nitrogen fixation by excised nodules to a maximum which corresponded to the first maximum of the respiratory response to raised pO2; higher pO2 than this decreased nitrogen fixation. Sliced nodules showed the same effect but the stimulation of fixation at the lower pO2 levels was not as great as with intact nodules. The Michaelis constant (Km ) for nitrogen fixation by intact excised nodules was relatively unaffected by pO2 until this reached the pO2 for maximum fixation when the Km rose sharply. At external pO2 of 80%, oxygen was shown to be a competitive inhibitor of nitrogen fixation. An explanation of these results is offered; it is suggested that the first part of the nodule O2 consumption/pO2 curves is due to O2 consumption by plant tissue and the second part to O2 consumption by bacteroids. The two components are separated by an O2 permeability barrier. When this barrier permits a rise in the pO2 at the bacteroids, nitrogen fixation is inhibited as oxygen competes with nitrogen for the reducing power of the bacteroids.

Regulation of flavin synthesis by Escherichia coli.

Abstract: SUMMARY: An approach has been made to the problem of how the synthesis of coenzymes is regulated. Two aspects of the problem have been studied, especially as they concern the synthesis of flavins by bacteria: (1) How are coenzymes prevented from being synthesized as fast as amino acids or nucleic acid bases? (2) How is coenzyme synthesis adjusted to the often changing physiological needs of bacteria? Evidence is presented that flavins cannot inhibit the activity of enzymes in the flavin biosynthetic pathway of Escherichia coli, but the amount of these enzymes can be made to vary by a factor of at least two. Repression might, therefore, account for the low rate of flavin synthesis. The possibility that repression rather than feedback inhibition also accounts for the low rate of synthesis of other coenzymes is discussed. Flavin synthesis is not as precisely adjusted to the physiological needs of bacteria as are syntheses of major metabolites for the following reasons: (1) Flavins are greatly overproduced by bacteria during exponential growth; the ratio of flavins excreted to flavins retained in the cells is between 0.8 and 8 for all strains and cultural conditions tested. (2) Flavin synthesis is not tightly geared to growth; thus, flavin synthesis goes on uninterrupted for more than an hour when the growth rate of E. coli or Pseudormonas fluorescens is abruptly reduced from a rapid rate to zero; also growth goes on uninterrupted for over an hour when the flavin supply is abruptly cut off from rapidly growing lactic acid bacteria. Evidently the control mechanism in the flavin pathway is not very sensitive to physiological needs. This conclusion probably applies to other coenzymes as well. Some incidental findings of interest from other points of view were: (1) Although internal flavins can get out of E. coli, external flavins apparently cannot enter. This could account for the absence of flavinless mutants. (2) After brief treatment with penicillin, E. coli becomes permeable to external flavins while remaining both impermeable to inulin and capable of synthesizing flavins. (3) Less than 4 % of the intracellular flavins of E. coli are free, in a form that can be extracted with n-butanol (5 %, v/v), toluene (0.05 %, v/v), cetyl trimethylammonium bromide (0.001 %, w/v) or distilled water. The remaining flavins are bound, in a form that can be extracted with trichloracetic acid (5 %, w/v). The intracellular concentration of free flavins in E. coli is estimated to be less than 4 x 10-6M.

Indifferent and haemolytic streptococci possessing group-antigen F.

Abstract: SUMMARY: ‘Indifferent’ strains of streptococci-i.e. strains which give no haemolysis or greening on blood-agar plates-frequently occur in cultures from dental root-canals. A serological study of over 200 strains of these streptococci showed that about half of them belonged to Lancefield groups F, G or C. It was shown by cross-absorption that the group-antigen of indifferent streptococci of group F is identical with the group-antigen of haemolytic strains of group F. Apart from the group-antigen, five independent carbohydrate type-antigens, localized in the cell wall, were demonstrated in group F strains. These type-antigens were found in groups other than F, e.g. the type-antigen I was observed in haemolytic and indifferent strains of group G; the type-antigen III occurred in indifferent strains of group C. Several strains with a type-antigen but without group-antigens were observed. The presence of carbohydrate type-antigens in formamide extracts can cause confusing cross-reactions in the grouping procedure, unless strains without type-antigen are used for the preparation of sera.

Transformability of Haemophilus influenzae.

Abstract: SUMMARY: A method is described for the preparation of highly competent (transformable) bacteria of Haemophilus influenzae. After three successive treatments, i.e. aerobic growth, anaerobic incubation and incubation in saline containing a few per cent broth, transformation frequencies close to 5% have been obtained. Competent bacteria absorb DNA very fast. Untransformable ones lack this ability. Absorption is inhibited by dinitrophenol or arsenate; it is not inhibited by chloramphenicol. This suggests the participation of an enzyme system. Development of competency is blocked by dinitrophenol, arsenate or chloramphenicol. It is strongly dependent upon temperature. It is not influenced by the absorption of one or two DNA molecules per bacterium.

An environmentally-induced transition from the flagellated to the non-flagellated state in Salmonella typhimurium: the fate of parental flagella at cell division.

Abstract: SUMMARY: Bacteria in cultures of Salmonella typhimurium LT2 were peritrichously flagellated when grown in nutrient broth at 370; but most were non-flagellated when grown for 6 mean-generation times or more at 440. When a culture growing exponentially at 370 was transferred to 440, growth continued at about the same rate; but the synthesis of new flagella was largely curtailed. The fate of the parental flagella was studied by staining and counting flagella on bacteria from samples taken during growth at 440 of cultures first grown at 370. After 3 mean-generation-times the average number of flagella/flagellated bacterium had fallen from about 8 to about 2 and the proportion of flagellated bacteria from about 100 % to about 60 %. The distribution of numbers of flagella/bacterium was at all times unimodal, with the mode decreasing from about 8 to 0. In non-growing cultures at 440 there was little or no change in the average number of flagella/bacterium, in the proportion of flagellated bacteria, or in the distribution of numbers of flagella/bacterium. It is inferred that parental flagella are neither rapidly shed at 440 nor retained entirely by one daughter cell at each division but are distributed about equally between the two daughter cells.

The Nucleic Acid Contents of Viruses

Abstract: SUMMARY: The nucleic acid contents of purified suspensions of mouse pneumonitis organisms (Nigg), pox viruses, adenovirus type 5, the iridescent virus of Tipula and avian myeloblastosis virus were analysed. From these results and published information the absolute amounts of nucleic acid in each virus particle were calculated and are presented in tabular form. Some generalizations can be made about the nucleic acid composition of different groups of organisms reproduced in cells; these are discussed in relation to the possible evolutionary origins of the organisms.