Measuring diameters of rod-shaped bacteria in vivo with polarized light scattering.

The angular function for elements of the Mueller matrix for polarized light scattering from suspensions of microorganisms is known to be reproducible for different growths of a given bacterial strain in the log (or exponential) phase of growth. The reason for this, the stability of the size and shape distribution for cells, is briefly discussed. Experiments were performed using suspensions of two different strains of Escherichia coli cells in log phase and measuring the angular dependence of the Mueller matrix ratio S34/S11. Calculations were then performed using the coupled dipole approximation to model electromagnetic scattering from particles where the shape of an individual cell was approximated by a cylinder capped with hemispheres of the same radius as the cylinder. Using previously measured values for the length distribution and index of refraction of the cells, the calculated scattering curve was found to fit the measured curve very well. The values obtained for the cell diameters were quite close to diameters previously measured by optical microscopy. Thus this method provides a rapid and convenient method for monitoring bacterial diameters in vivo even when there is an appreciable distribution of bacterial lengths in the population.

Radiation pattern of fluorescence from molecules embedded in small particles: general case.

A method is presented for calculating and analyzing the angular distribution of fluorescent emission from randomly oriented anisotropic molecules embedded in small dielectric particles with the nonzero reorientation angle between absorption and emission moments suggested by physical considerations now taken into account. Calculations performed on the basis of this method are compared with some of the available experimental data for fluorescent dye molecules embedded in microspheres, and good quantitative agreement is found. It is shown how fitting the computed results to experimental data determines an effective reorientation angle between absorption and emission transition moments. A more definitive test to which the model could be subjected is described.

Raman and fluorescent scattering by molecules embedded in spheres with radii up to several multiples of the wavelength.

When molecules which give rise to Raman and fluorescent light scattering are distributed within a small dielectric particle, the signal is affected by the morphology and optical properties of the particle and by the distribution of active molecules within it. This effect is considered to arise from the influence of the particle boundary both upon the internal field at the incident frequency and upon the emissions at the shifted frequency rather than from any alteration of the molecular transitions. This study presents numerical results over a broad range of refractive indexes and for larger size parameters than heretofore. It includes explicit consideration of components polarized in the same plane as the incident field, H(h) and V(nu), as well as the depolarized components V(h) and H(v). Quantitative estimates of the concentration of active species may be in error if these effects are not considered.