Paromita Banerjee, IISER- Kolkata, Mohanpur Campus, India
Jalpa Soni, IISER- Kolkata, Mohanpur Campus, India
Nirmalya Ghosh* , IISER- Kolkata, Mohanpur Campus, India
Tapas K Sengupta*, IISER- Kolkata, Mohanpur Campus, India
Bacteria, as living organisms, are unicellular in nature, but in order to cope with environmental conditions, particularly under unfavorable one, bacterial cells of same species have evolved with time to develop cooperative behavior among them as survival strategy, which results in complex growth patterns in bacterial colony . The nature and growth pattern of such colony is not only influenced by the type or species of bacteria, but also by environmental conditions such as nutrient availability, temperature, thickness and hardness of the supportive surface and presence of toxic materials etc . Development of methods for quantification of cellular association and patterns in growing bacterial colony is of considerable current interest, because such methods may provide useful information on the cellular and molecular basis of signaling network for bacterial competition and cooperation. This in tern may help in detection and identification of a bacterial species in a given space and under a given set of condition(s). We have explored a generalized method for polarimetry analysis in combination with light scattering spectroscopy, for simultaneously extraction / quantification of the intrinsic polarimetry characteristics and the fractal micro-optical parameters of growing bacterial colonies, to probe the structural changes taking place during the colony formations under different conditions.
The method is based on measurement of spectral 3 x 3 Mueller matrices from bacterial colony samples and its subsequent analysis via polar decomposition of Mueller matrix and fractal-based inverse light scattering approach. The use of the 3 x 3 Mueller matrix (instead of the full 4 x 4 matrix) is motivated by the fact that it requires measurements using linear polarizers alone and is thus particularly suitable for spectroscopic measurements (measurement at multiple wavelengths which is employed in this study) [2, 3]. Briefly, in this approach, the recorded spectral 3 x 3 Mueller matrices are decomposed into the constituent basis matrices of the individual polarimetry effects (diattenuation, depolarization and retardance). The intrinsic medium polarization parameters, namely the linear diattenuation (d (lambda)), linear retardance (delta (lambda)) and linear depolarization (depol(lambda)) coefficients, are subsequently extracted from the decomposed basis matrices [2,3]. The decomposition procedure was further exploited to extract the polarization preserving component of the scattered light, which was subsequently subjected to inverse light scattering model based on Born approximation to yield the fractal micro-optical parameters (namely, the Hurst parameter – H, fractal dimension - Df) of the bacterial colony samples. Studies were conducted on Bacillus thuringiensis colonies, grown on agar plates under three different conditions (in absence or presence of 1% glucose or 3 mM sodium arsenate) . Interesting differences were noted in the fractal micro-optical parameters and the intrinsic polarization parameters of the growing bacterial colonies under different conditions. The bacterial colony growing in presence of 1% glucose exhibited the highest fractal dimension (lowest value of H), whereas that for the colony growing in presence of 3 mM sodium arsenate showed the weakest fractality. Moreover, the values for (delta (lambda)) and (d (lambda)) were also found to be considerably higher for the colony growing in presence of glucose, indicating more structured growth pattern. The findings of the polarimetry and light scattering spectroscopic measurements / analysis were corroborated further with optical and electron microscopic studies conducted on the same samples. The details of these results will be presented.
1. Manas K. Roy et al, Journal of Theoretical Biology, 265, 389 – 395 (2010).
2. Nirmalya Ghosh and I. Alex Vitkin, Journal of Biomedical Optics, 16, 110801 (2011).
3. M.K. Swami et al, Optics Express, 14, 9324 – 9337 (2006).
4. M.Hunter et al, Physical Rev. Lett, 97, 138102 (2006).
Dr. Nirmalya Ghosh
IISER - Kolkata
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