Bacterial growth kinetics estimation by fluorescence in situ hybridization and spectrofluorometric quantification

The aim of this study was to develop a specific and rapid method to identify and quantify relevant bacterial populations in mixed biomass by spectrofluorometric quantification (SQ) of whole cells hybridized with fluorescently labelled oligonucleotide probes targeting mature 16S ribosomal RNA (rRNA). Probe targeting the precursor of rRNA synthesis was also employed because it was being suggested as more indicative of the activity state of the micro-organisms.

Original fluorescence in situ hybridization protocol was modified to be applied to liquid samples and the fluorescence emission from the Cy3-labelled cells was measured by spectrofluorometry. The method was calibrated on an exponentially growing cell suspension of Acinetobacter johnsonii and was successfully applied to generate kinetic data. No substantial difference in the estimated maximum specific growth rate (mu(max)) values was found between the SQ method and the classical method, using absorbance at 420 nm (6.2 d(-1)). The preliminary validation tests showed their direct applicability to target enriched cultures.

This study demonstrated the validity of the SQ method to easily quantify the concentration and to determine the growth rate of specific micro-organisms present in mixed cultures.

The proposed method can be directly utilized for quantification and kinetic characterization of microbial enrichments. It has the advantage of being easily applicable using simple, inexpensive equipment suitable for routine analysis.