webSalvador, a Web Tool for the Luria-Delbrük Experiment

ABSTRACT Existing Web tools for the Luria-Delbrück fluctuation experiment do not offer many desirable capabilities that are vital to mutation research. webSalvador offers these capabilities via a user interface that allows researchers to access most of the functions in the R package rSalvador without having to learn the R language.

A Bayesian approach for correcting for partial plating in fluctuation experiments

SummaryThe fluctuation experiment is the preferred method for estimating microbial mutation rates. A difficult task facing the data analyst is to infer the mean number of mutations from the number of mutant cells that only indirectly reflects the number of mutations. Partial plating, commonly practised in the laboratory, renders this task even more challenging by allowing only a portion of the mutant cells to be counted. In this paper, we propose a Bayesian approach to correcting for partial plating in the analysis of fluctuation experiments.

Fluctuation analysis: the probability distribution of the number of mutants under different conditions.

In the 47 years since fluctuation analysis was introduced by Luria and Delbrück, it has been widely used to calculate mutation rates. Up to now, in spite of the importance of such calculations, the probability distribution of the number of mutants that will appear in a fluctuation experiment has been known only under the restrictive, and possibly unrealistic, assumptions: (1) that the mutation rate is exactly proportional to the growth rate and (2) that all mutants grow at a rate that is a constant multiple of the growth rate of the original cells. In this paper, we approach the distribution of the number of mutants from a new point of view that will enable researchers to calculate the distribution to be expected using assumptions that they believe to be closer to biological reality. The new idea is to classify mutations according to the number of observable mutants that derive from the mutation when the culture is selectively plated. This approach also simplifies the calculations in situations where two, or many, kinds of mutation may occur in a single culture.

Waves with harmonic structure below and above the lower hybrid resonance observed on the CENTAUR 35.001 and 35.002 rockets

Measurements obtained from the 0- to 10-kHz electric-wave experiment and the 0- to 4.2-kHz electron-density fluctuation experiment on the CENTAUR 35.001 and 35.002 rockets launched in December 1981 are presented. The observations include (i) spectacular narrow-banded signals with harmonic structure related to the proton gyrofrequency. The bands are observed both below and above the lower hybrid frequency, from 2 to 10 kHz, and are modulated in frequency during a rocket spin, typically by 2 kHz. The character of the signals indicate that they are generated by the presence of the payload in the plasma. The source of free energy remains unidentified; we suggest, however, a perpendicular ion beam emitted by or created by the presence of the payload, generating flute-mode ion cyclotron harmonic waves. (ii) Bursts of harmonic waves are received primarily below 2 kHz. The fundamental frequency of these emissions varies over a large range, from 47 to 700 Hz, and is rarely at any of the characteristic frequencies of the plasma. (iii) Waves with a clear lower cutoff at the lower hybrid frequency are also observed. The lower hybrid frequency estimated from magnetic field and density measurements and from the wave measurements agree within 6% (500 Hz).