title: Laser-induced speckle scatter patterns in Bacillus colonies.
abstract: Label-free bacterial colony phenotyping technology called BARDOT (Bacterial Rapid Detection using Optical scattering Technology) provided successful classification of several different bacteria at the genus, species, and serovar level. Recent experiments with colonies of Bacillus species provided strikingly different characteristics of elastic light scatter (ELS) patterns, which were comprised of random speckles compared to other bacteria, which are dominated by concentric rings and spokes. Since this laser-based optical sensor interrogates the whole volume of the colony, 3-D information of micro- and macro-structures are all encoded in the far-field scatter patterns. Here, we present a theoretical model explaining the underlying mechanism of the speckle formation by the colonies from Bacillus species. Except for Bacillus polymyxa, all Bacillus spp. produced random bright spots on the imaging plane, which presumably dependent on the cellular and molecular organization and content within the colony. Our scatter model-based analysis revealed that colony spread resulting in variable surface roughness can modify the wavefront of the scatter field. As the center diameter of the Bacillus spp. colony grew from 500 to 900 μm, average speckles area decreased two-fold and the number of small speckles increased seven-fold. In conclusion, as Bacillus colony grows, the average speckle size in the scatter pattern decreases and the number of smaller speckle increases due to the swarming growth characteristics of bacteria within the colony.
authors.affiliation: Applied Optics Laboratory, School of Mechanical Engineering, Purdue University West Lafayette, IN, USA.
authors.forename: Huisung
authors.initials: H
authors.lastname: Kim
authors.affiliation: Molecular Food Microbiology Laboratory, Department of Food Science, Purdue University West Lafayette, IN, USA.
authors.forename: Atul K
authors.initials: AK
authors.lastname: Singh
authors.affiliation: Molecular Food Microbiology Laboratory, Department of Food Science, Purdue University West Lafayette, IN, USA.
authors.forename: Arun K
authors.initials: AK
authors.lastname: Bhunia
authors.affiliation: Applied Optics Laboratory, School of Mechanical Engineering, Purdue University West Lafayette, IN, USA.
authors.forename: Euiwon
authors.initials: E
authors.lastname: Bae
keywords: Bacillus; bacterial colony; diffraction; phase modulation; speckle