Adhesion of Rhodococcus ruber IEGM 342 to polystyrene studied using contact and non-contact temperature measurement techniques.

Adhesion of industrially important bacteria to solid carriers through the example of actinobacterium Rhodococcus ruber IEGM 342 adhered to polystyrene was studied using real-time methods, such as infrared (IR) thermography and thermometry with platinum resistance (PR) detectors. Dynamics of heat rate and heat production was determined at early (within first 80 min) stages of rhodococcal cell adhesion. Heat rate was maximal (1.8 × 10-3-2.7 × 10-3 W) at the moment of cell loading. Heat production was detected for the entire length of adhesion, and its dynamics depended on concentration of rhodococcal cells. At high (1 × 1010 CFU/ml) cell concentration, a stimulative (in 1.7 and 1.4 times consequently) effect of polystyrene treatment with Rhodococcus-biosurfactant on the number of adhered rhodococcal cells and cumulative heat production at rhodococcal cell adhesion was revealed. The values of heat flows (heat rate 0.3 × 10-3-2.7 × 10-3 W, heat production up to 8.2 × 10-3 J, and cumulative heat production 0.20-0.53 J) were 5-30 times higher than those published elsewhere that indicated high adhesive activity of R. ruber IEGM 342 towards polystyrene. To analyze experimental results and predict effects of boundary conditions on the temperature distribution, a mathematical model for heating a polystyrene microplate with distributed heat sources has been developed. Two independent experimental methods and the numerical modeling make it possible to verify the experimental results and to propose both contact and non-contact techniques for analyzing kinetics of bacterial adhesion.

Biosurfactant-enhanced immobilization of hydrocarbon-oxidizing Rhodococcus ruber on sawdust.

Immobilization of microorganisms on/in insoluble carriers is widely used to stabilize functional activity of microbial cells in industrial biotechnology. We immobilized Rhodococcus ruber, an important hydrocarbon degrader, on biosurfactant-coated sawdust. A biosurfactant produced by R. ruber in the presence of liquid hydrocarbons was found to enhance rhodococcal adhesion to solid surfaces, and thus, it was used as a hydrophobizing agent to improve bacterial attachment to a sawdust carrier. Compared to previously used hydrophobizers (drying oil and n-hexadecane) and emulsifiers (methyl- and carboxymethyl cellulose, poly(vinyl alcohol), and Tween 80), Rhodococcus biosurfactant produced more stable and homogenous coatings on wood surfaces, thus resulting in higher sawdust affinity to hydrocarbons, uniform monolayer distribution of immobilized R. ruber cells (immobilization yield 29-30 mg dry cells/g), and twofold increase in hydrocarbon biooxidation rates compared to free rhodococcal cells. Two physical methods, i.e., high-resolution profilometry and infrared thermography, were applied to examine wood surface characteristics and distribution of immobilized R. ruber cells. Sawdust-immobilized R. ruber can be used as an efficient biocatalyst for hydrocarbon transformation and degradation.