In microbiology, monitoring the growth of any microorganism in culture is important for studying and optimizing the growth kinetics, the biomass and the metabolite production. In this work, we show that laser speckle imaging is a reliable technique that can be used to perform real-time monitoring of bacteria growth kinetic in liquid culture media. Speckle parameters, specifically speckle grain size and the spatial contrast of the speckle images, and standard analytical parameters (optical density, pH and colony forming units) were measured during the culture of different strains of Bacillus thuringiensis. Our results show that both speckle grain size and spatial contrast decrease with bacterial growth. Furthermore, speckle parameters are sensitive to the fermentation conditions. Statistical analysis revealed a relatively high correlation between speckle and analytical parameters.

The speckle imaging technique has been proven to be a reliable and effective method for real-time monitoring of the growth kinetics of any bacterium in suspension. To understand the interaction between the light and the bacterial density, a simulation of the bacterial growth of Bacillus thuringiensis was performed using calibrated microspheres of different concentrations and sizes. Results show that the decrease of speckle grain size with the increase of the medium scattering coefficient reveals the two essential phases of the bacterial growth: the exponential phase where the number of the bacteria increases and the stationary phase where sporulation and cell lysis occur.

In this paper, we report a morphological study of the crystals and spores of different shapes synthesized by seven different strains of Bacillus thuringiensis. Crystals and spores were separated after 48 h of culture on T3 agar medium and imaged under a scanning electron microscope (SEM). Sizes of the crystals and spores were determined using Image J software. The results showed that crystal and spore sizes were normally distributed. In addition, the volumes and aspect ratios of the crystals and spores were calculated. The statistical analysis of the data showed the variability of the size distribution and morphological data of the crystals produced by the analyzed strains. Furthermore, variations in spore size and shape within the same serovar were observed, indicating that, perhaps, there are still some unexplored differences between strains of this serovar, making them less identical than what was believed so far.