Inactivation and reactivation of B. megatherium phage

Preparation of Reversibly Inactivated (R.I.) Phage.- If B. megatherium phage (of any type, or in any stage of purification) is suspended in dilute salt solutions at pH 5-6, it is completely inactivated; i.e., it does not form plaques, or give rise to more phage when mixed with a sensitive organism (Northrop, 1954). The inactivation occurs when the phage is added to the dilute salt solution. If a suspension of the inactive phage in pH 7 peptone is titrated to pH 5 and allowed to stand, the activity gradually returns. The inactivation is therefore reversible. Properties of R.I. Phage.- The R.I. phage is adsorbed by sensitive cells at about the same rate as the active phage. It kills the cells, but no active phage is produced. The R.I. phage therefore has the properties of phage "ghosts" (Herriott, 1951) or of colicines (Gratia, 1925), or phage inactivated by ultraviolet light (Luria, 1947). The R.I. phage is sedimented in the centrifuge at the same rate as active phage. It is therefore about the same size as the active phage. The R.I. phage is most stable in pH 7, 5 per cent peptone, and may be kept in this solution for weeks at 0 degrees C. The rate of digestion of R.I. phage by trypsin, chymotrypsin, or desoxyribonuclease is about the same as that of active phage (Northrop, 1955 a). Effect of Various Substances on the Formation of R.I. Phage.- There is an equilibrium between R.I. phage and active phage. The R.I. form is the stable one in dilute salt solution, pH 5 to 6.5 and at low temperature (<20 degrees C.). At pH >6.5, in dilute salt solution, the R.I. phage changes to the active form. The cycle, active right harpoon over left harpoon inactive phage, may be repeated many times at 0 degrees C. by changing the pH of the solution back and forth between pH 7 and pH 6. Irreversible inactivation is caused by distilled water, some heavy metals, concentrated urea or quanidine solutions, and by l-arginine. Reversible inactivation is prevented by all salts tested (except those causing irreversible inactivation, above). The concentration required to prevent R.I. is lower, the higher the valency of either the anion or cation. There are great differences, however, between salts of the same valency, so that the chemical nature as well as the valency is important. Peptone, urea, and the amino acids, tryptophan, leucine, isoleucine, methionine, asparagine, dl-cystine, valine, and phenylalanine, stabilize the system at pH 7, so that no change occurs if a mixture of R.I. and active phage is added to such solutions. The active phage remains active and the R.I. phage remains inactive. The R.I. phage in pH 7 peptone becomes active if the pH is changed to 5.0. This does not occur in solutions of urea or the amino acids which stabilize at pH 7.0. Kinetics of Reversible Inactivation.- The inactivation is too rapid, even at 0 degrees to allow the determination of an accurate time-inactivation curve. The rate is independent of the phage concentration and is complete in a few seconds, even in very dilute suspensions containing <1 x 10(4) particles/ml. This result rules out any type of bimolecular reaction, or any precipitation or agglutination mechanism, since the minimum theoretical time for precipitation (or agglutination) of a suspension of particles in a concentration of only 1 x 10(4) per ml. would be about 300 days even though every collision were effective. Mechanism of Salt Reactivation.- Addition of varying concentrations of MgSO(4) (or many other salts) to a suspension of either active or R.I. phage in 0.01 M, pH 6 acetate buffer results in the establishment of an equilibrium ratio for active/R.I. phage. The higher the concentration of salt, the larger proportion of the phage is active. The results, with MgSO(4), are in quantitative agreement with the following reaction: See PDF for Equation Effect of Temperature.- The rate of inactivation is too rapid to be measured with any accuracy, even at 0 degrees C. The rate of reactivation in pH 5 peptone, at 0 and 10 degrees , was measured and found to have a temperature coefficient Q(10) = 1.5 corresponding to a value of E (Arrhenius' constant) of 6500 cal. mole(-1). This agrees very well with the temperature coefficient for the reactivation of denatured soy bean trypsin inhibitor (Kunitz, 1948). The equilibrium between R.I. and active phage is shifted toward the active side by lowering the temperature. The ratio R.I.P./AP is 4.7 at 15 degrees and 2.8 at 2 degrees . This corresponds to a change in free energy of -600 cal. mole(-1) and a heat of reaction of 11,000. These values are much lower than the comparative one for trypsin (Anson and Mirsky, 1934 a) or soy bean trypsin inhibitor (Kunitz, 1948). Neither the inactivation nor the reactivation reactions are affected by light. The results in general indicate that there is an equilibrium between active and R.I. phage. The R.I. phage is probably an intermediate step in the formation of inactive phage. The equilibrium is shifted to the active side by lowering the temperature, adjusting the pH to 7-8 (except in the presence of high concentrations of peptone), raising the salt concentration, or increasing the valency of the ions present. The reaction may be represented by the following: See PDF for Equation The assumption that the active/R.I. phage equilibrium represents an example of native/denatured protein equilibrium predicts all the results qualitatively. Quantitatively, however, it fails to predict the relative rate of digestion of the two forms by trypsin or chymotrypsin, and also the effect of temperature on the equilibrium.     

Growth and phage production of lysogenic B. megatherium

Cell multiplication and phage formation of lysogenic B. megatherium cultures have been determined under various conditions and in various culture media. 1. In general, the more rapid the growth of the culture, the more phage is produced. No conditions or culture media could be found which resulted in phage production without cell growth. 2. Cultures which produce phage grow normally, provided they are shaken. If they are allowed to stand, those which are producing phage undergo lysis. Less phage is produced by these cultures than by the ones which continue to grow. 3. Cells plated from such phage-producing cultures in liquid yeast extract medium grow normally on veal infusion broth agar or tryptose phosphate broth agar, which does not support phage formation, but will not grow on yeast extract agar. 4. Any amino acid except glycine, tyrosine, valine, leucine, and lysine can serve as a nitrogen source. Aspartic acid gives the most rapid cell growth. 5. The ribose nucleic acid content is higher in those cells which produce phage. 6. The organism requires higher concentrations of Mg, Ca, Sr, or Mn to produce phage than for growth. 7. The lysogenic culture can be grown indefinitely in media containing high phosphate concentrations. No phage is produced under these conditions, but the cells produce phage again in a short time after the addition of Mg. The potential ability to produce phage, therefore, is transmitted through cell division. 8. Colonies developed from spores which have been heated to 100°C. for 5 minutes produce phage and hence, infected cells must divide. 9. No phage can be detected after lysis of the cells by lysozyme.     

The effect of various culture media on infection,growth, lysis,and phage production of B. megatherium

B. megatherium cells were grown in various culture media, centrifuged and washed, and suspended in other culture media containing "C" or "T" phage. The per cent of infection, rate of growth, lysis, and phage production were determined. The behavior of the system depends on the culture medium in which the cells were grown and also on the culture medium in which they were mixed with phage. With the T phage it is possible to set up systems which yield the following results: 1. No infection, normal growth, no phage production. 2. Infection, normal growth, no lysis) phage production. 3. Infection, growth for several hours, lysis, and phage production. 4. Infection, no growth, lysis, and phage production. The C phage system is less affected by changes in the culture medium. The change in the behavior of the cells with T phage probably is not due to selection since it occurs without much growth of the culture, and is readily reversible.     

Growth and phage production of B. megatherium. I. Growth of cells after infection with C phage. II. Rate of growth,phage yield,and RNA content of cells. III. Effect of various substances on growth rate and phage production

I. Lysogenic B. megatherium 899a (de Jong, 1931) produces two types of phage (Gratia, 1936 c) T and C. The T phage forms cloudy plaques and gives rise to fresh lysogenic strains (Gratia, 1936 b) when added to the sensitive strain of megatherium. It may or may not cause lysis, depending on the media (Northrop, 1951). The C phage occurs very rarely) forms clear plaques, does not give rise to lysogenic strains, and causes complete lysis of the sensitive strain under all conditions tested, provided infection occurs. If C phage is added to the sensitive strain, and the mixture allowed to stand, or made into a hanging drop preparation, the infected cells stop growing and lyse completely after 60 to 80 minutes with the liberation of from 50 to 200 phage particles per cell. If, however, C phage is added to a rapidly growing culture of B. megatherium and the suspension shaken at 34°, the cells continue to grow and divide for 50 to 60 minutes, after infection has occurred. They then lyse, with the liberation of from 1000 to 2500 phage particles per cell. II. The following determinations have been made on megatherium sensitive cells growing in 5 per cent peptone at different stages of growth. (1) Growth rate of infected and uninfected cells; (2) RNA, DNA, and protein content; (3) volume of the cell; (4) phage yield per cell by plaque count; (5) phage yield per cell by cell and plaque count; (6) lysis time. The growth rate decreases as the cell concentration increases. The lysis time and the protein N per cell are nearly independent of the growth rate; all the other values increase as the growth rate increases. The ratio See PDF for Equation is nearly constant. RNA and DNA per cell increase less rapidly than the volume, so that NA per unit volume is not constant, but decreases as the size of the cell increases. The phage yield measured under conditions in which the infected cells do not grow (by plaque count) is very nearly proportional to the size of the cell. The phage yield per cell, under conditions in which the infected cells do grow, increases more rapidly than the size of the cells. The phage yield per cell under these conditions may be calculated by the equation See PDF for Equation The determining factor for the variation in phage yield is the growth rate of the cells. This, in turn, is determined by the composition of the medium. III. The growth and phage production of megatherium 899a have been determined in the presence of the following substances: aureomycin, bacitracin, chloromycetin, gramicidin, Merck AB631, Merck AB191, Merck AB624, penicillin, streptomycin, terramycin, tyrothricin, usnic acid, acetone, chloroform, ethyl alcohol, formaldehyde, gentian violet, glycerin, maleic hydrazide, methyl alcohol, phenyl mercuric acetate, sodium fluoride, sulfanilamide, toluene, and urethane. In every case, the lowest concentration of the substance which completely inhibits growth, is also the lowest concentration which completely inhibits phage production. One antibiotic, Merck AB81, causes increased phage production in concentrations which partially inhibit growth, and low phage production in concentrations which completely inhibit growth (as determined by turbidity). Short exposure to ultraviolet light also decreases the growth rate, with increase in phage production. Longer exposure, which completely inhibits growth (as determined by turbidity) results in lysis and phage liberation.     

Appearance of new phage types and new lysogenic strains after adaptation of lysogenic B. megatherium to ammonium sulfate culture medium

1. Lysogenic B. megatherium 899a was adapted to growth in a minimal ammonium sulfate medium (ASCM). 2. Adaptation took place slowly and the following changes in the culture occurred: (a) The growth rate increased from 0.5 to 1.5–2.0/hr. (b) The culture changed from diffuse to mucoid. (c) The total phage titer, and the gelatinase concentration decreased to 1/100 or less. (d) The types of phage produced changed from >99 per cent T (wild type) to 30 to 60 per cent miscellaneous clear types. The original T phage was replaced by a different smaller t, never observed in the original 899a culture. (e) Several new colony types also appeared, but the colony morphology was not correlated with the phage types produced. None of the colony types was stable on repeated transfer either in peptone or ASCM, but continued to disassociate into different colony types (cf. Ivánovics, 1955). 3. Control experiments showed that these changes in phage production and colony types could not be brought about by growing sensitive B. megatherium in the presence of the various new phages, in ASCM. It is therefore unlikely that the changes observed in adapted culture were due to infection of a sensitive cell with phage. 4. Continued growth of the ASCM-adapted strain in peptone resulted in increasing the total phage titer, and also the gelatinase concentration. The growth rate returned to its original value and the ability to grow rapidly in ASCM was soon lost. The phage types, however, remained the same as in the ASCM. 5. An improved cell for steady state growth is described.     

The proportion of terramycin-resistant mutants in B. megatherium cultures

The number of terramycin-resistant mutants in Bacillus megatherium cultures, their mutation rate, and the growth rate of the wild and mutant cells have been determined under various conditions. These values are in agreement with the following equations (Northrop and Kunitz, 1957):— See PDF for Equation λ = mutation rate, A = growth rate constant of wild cells, B = growth rate constant of mutants, See PDF for Equation equilibrium. The value of the mutation rate as determined from equation (6) agrees with that found by the null fraction method.     

Factors Controlling the Production of Lysogenic Cultures of B. megatherium

(1) The proportion of infected B. megatherium cells which develop lysogenic colonies depends on the number and kind of infecting virus particles and on the culture medium in which the cells are growing. (2) Cells infected with 100 or more T virus particles (from megatherium 899) in yeast extract peptone disintegrate, produce very few virus particles, and less than one lysogenic colony per 10⁷ infected cells. Cells infected with one or a few particles produce 500 to 1000 virus particles each and about 30 lysogenic colonies per 10⁷ infected colonies. (3) T phage obtained from lysogenic magatherium KM cultures produces many more lysogenic cells than does the original megatherium 899 virus. (4) Cells infected with megatherium 899 T virus in peptone medium and then transferred to asparagine medium give rise to 10⁶ lysogenic colonies per 10⁷ infected cells and this transformation will occur even after the infected cells have been in peptone for 60 to 90 minutes and are beginning to produce virus particles. (5) Continued growth of KM strain with either C or T virus from megatherium 899 for several hundred generations in the steady state apparatus results in a lysogenic strain which produces several different types of virus.