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.     

Studies on the origin of bacterial viruses. I-IV

I. The Incidence of Phage-Producing Cells in Various B. megatherium Cultures Analyses of small samples containing a few cells each show that lysogenic B. megatherium produces phage particles in groups of from 10 to 1000 depending on the megatherium strain and the culture medium. These groups probably correspond to the number of particles produced by a single cell. The proportion of such phage-producing cells varies from <1 x 10(-10) to about 1 x 10(-2) depending on the megatherium strain and the culture medium. If a culture produces two types of phage, the different types usually appear in separate samples. If mixed samples occur, the number of such samples is about what would be expected for the probability that two separate groups would appear in one sample. This result indicates that the appearance of a distinct phage type is the result of a change in the bacterial cell rather than a change in a phage particle, since in the latter case a mixture of the two types would result. II. The Effect of Ultraviolet Light on the Incidence of Phage-Producing and of Terramycin-Resistant Cells in Various B. megatherium Cultures Low intensity of ultraviolet light increases the proportion of both phage-producing cells and of terramycin-resistant mutants. The increase in phage-producing cells is greater than the increase in terramycin-resistant cells. High intensities of ultraviolet light cause practically all the cells of some B. megatherium strains to produce phage. The number of terramycin-resistant mutants cannot be determined under these conditions. The effect of ultraviolet light varies, depending on the megatherium strain and the culture medium. III. The Effect of Hydrogen Peroxide on the Incidence of Phage-Producing and of Terramycin-Resistant Cells in Various B. megatherium Cultures Low concentrations of hydrogen peroxide increase the number of phage-producing cells and of terramycin-resistant cells, concomitantly, from two to five times. High concentrations of hydrogen peroxide cause almost all the cells of some strains of megatherium to produce phage. IV. Calculation of the Incidence of Phage-Producing Cells The time rate of the appearance of phage particles in normal cultures, or in cultures treated with ultraviolet light or hydrogen peroxide, may be calculated by the same equations which predict the occurrence of terramycin-resistant mutants in B. megatherium cultures. These equations predict that the number of mutants will increase more or less in proportion to the concentration of mutagenic agent, so long as the mutation rate remains very small compared to the growth rate. As the mutation rate approaches the growth rate, there will be a very rapid increase in the proportion of mutants. This explains the striking effect of higher concentrations of mutagenic agents. In order to calculate the results after exposure to strong ultraviolet light or hydrogen peroxide, it is necessary to assume that the change from normal to phage-producing cell occurs without cell division.     

Relationship between temperature and growth rate of bacterial cultures.

The Arrhenius Law, which was originally proposed to describe the temperature dependence of the specific reaction rate constant in chemical reactions, does not adequately describe the effect of temperature on bacterial growth. Microbiologists have attempted to apply a modified version of this law to bacterial growth by replacing the reaction rate constant by the growth rate constant, but the modified law relationship fits data poorly, as graphs of the logarithm of the growth rate constant against reciprocal absolute temperature result in curves rather than straight lines. Instead, a linear relationship between in square root of growth rate constant (r) and temperature (T), namely, square root = b (T - T0), where b is the regression coefficient and T0 is a hypothetical temperature which is an intrinsic property of the organism, is proposed and found to apply to the growth of a wide range of bacteria. The relationship is also applicable to nucleotide breakdown and to the growth of yeast and molds.     

Studies on the culicine mosquito host range of Bacillus sphaericus and Bacillus thuringiensis var. israelensis with notes on the effects of temperature and instar on bacterial efficacy

Toxicity tests of three strains of Bacillus sphaericus against late instars of 12 culicine mosquito species indicated a wide range of susceptibility. Culex pipiens and C. salinarius were highly susceptible (LC₅₀s < 10⁴ spores/ml) to strain 1593, and C. pipiens and C. restuans were highly susceptible to strain 2013-4. The potency of strain SSII-1 was approximately one-tenth that of strains 1593 and 2013-4 against C. pipiens. Susceptibility of Aedes species to strain 1593 was highly variable. At temperatures ≥ 20°C, A. fitchii, A. intrudens, A. stimulans, and A. vexans were moderately to highly susceptible (LC₅₀s 6 × 10³−4 × 10⁴ spores/ml), A. triseriatus was only slightly susceptible (LC₅₀ > 10⁶ spores/ml), and A. aegypti was refractory. Susceptibility of Aedes mosquitoes to strain SSII-1 was less variable, with LC₅₀s against A. aegypti, A. canadensis, A. stimulans, and A. triseriatus all being between 10⁴ and 10⁶ vegetative cells + spores/ml. All species of mosquitoes tested were, in general, highly susceptible to B. thuringiensis var. israelensis (LC₅₀s 2.3 × 10³−2.5 × 10⁴ spores/ml). In B. sphaericus toxicity tests, decreased temperatures resulted in up to a 16-fold increase in LC₅₀ and a substantial reduction in probit line slope. First-instar A. aegypti larvae were more susceptible to B. sphaericus strain SSII-1 than the three later instars, which were approximately equally susceptible; however, no significant difference was observed in the susceptibility of the four instars of A. triseriatus.     

The effect of ultraviolet and white light on growth rate,lysis, and phage production of Bacillus megatherium

Cultures of megatherium 899a, growing under different conditions, were exposed to ultraviolet or white light. 1. Cultures exposed to ultraviolet light and then to white light continue to grow at the normal rate. Cultures exposed to ultraviolet light and then placed in the dark grow at the normal rate for varying lengths of time, depending on conditions, and then lyse with the liberation of from 5 to 1000 phage particles per cell, depending on the culture medium. 2. Increasing the time of exposure to ultraviolet light results in an increase in the fraction of cells which lyse in the dark. The lysis time decreases at first, remains constant over a wide range of exposure, and then increases. The lysis can be prevented by visible light after short exposure, but not after long exposures. 3. The time required for lysis is independent of the cell concentration. 4. Effect of temperature. After exposure to ultraviolet the cell concentration increases about 4 times at 20°, 30°, or 35°C., but only 1.5 to 2.0 times at 40–45°. This is due to the fact that the growth rate of the culture reaches a maximum at 38° while the lysis rate increases steadily up to 45°. 5. Terramycin decreases the growth rate and lysis rate in proportion. 6. At pH 5.1, the cultures continue to grow slowly in the dark after exposure to ultraviolet light. 7. Megatherium sensitive cells infected with T phage lyse more rapidly than ultraviolet-treated 899a, and visible light does not affect the lysis time. The results agree with the assumption that exposure to ultraviolet results in the production of a toxic (mutagenic) substance inside the bacterial cell. This substance is inactivated by white light.