Quantitative spectral light scattering polarimetry for monitoring fractal growth
pattern of Bacillus thuringiensis bacterial colonies
Paromita Banerjee1, Jalpa Soni2, Nirmalya Ghosh2* and Tapas K Sengupta1*
1
Dept.of Biological Sciences, IISER- Kolkata, BCKV Main Campus, Mohanpur 741 252, Nadia, West Bengal, India
Dept. of Physical Sciences, IISER- Kolkata, BCKV Main Campus, Mohanpur 741 252, Nadia, West Bengal, India
*senguptk@iiserkol.ac.in, nghosh@iiserkol.ac.in
2
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 [1]. 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 [1]. 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  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  3 Mueller matrix (instead of the full 4  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  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 ()), linear retardance ( ()) and linear depolarization (()) 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) [4]. 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 (
()) and (d ()) 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.
References
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2.
3.
4.
Manas K. Roy et al, Journal of Theoretical Biology, 265, 389 – 395 (2010).
Nirmalya Ghosh and I. Alex Vitkin, Journal of Biomedical Optics, 16, 110801 (2011).
M.K. Swami et al, Optics Express, 14, 9324 – 9337 (2006).
M.Hunter et al, Physical Rev. Lett, 97, 138102 (2006).