Use of fluorescent microorganisms to perform in vivo and in situ local characterization of microbial deposits

This study presents an original method for in situ characterisation of the structure and organization of a microbial deposit accumulated on a membrane surface during filtration operations. The strategy is based on coupling macroscopic measurements (filtration results) and local observations at the microscopic level. This approach has been performed thanks to the use of self-fluorescent microorganisms (modified microorganisms) and the design of a specific filtration cell allowing a direct microscopic visualisation of the deposit to be performed without any treatment which could damage or modify the structure. Indeed the use of living fluorescent cells has allowed a non-destructive, in situ and in vivo, study of the deposits. As an example of application, the influence of microorganism size and morphology on the filtration performances and the deposit characteristics were examined in the case of dead-end microfiltration of model suspensions (yeasts and bacteria) obtained under controlled conditions. For these compact deposits the upper part of the deposits, corresponding to a 30 μm thickness slice, could be precisely analysed at the microscopic level and for yeasts/bacteria mixed deposit, the local microorganisms organization in the slice could be analysed. For pure yeasts deposit, the porosity obtained by image analysis, 12%, was in agreement with the calculated value by Carman–Kozeny equation using permeate flow measurements.