Comparison of methods for monitoring bacterial transport in the subsurface

The purpose of this study was to compare in a laboratory experiment, a suite of methods developed to track viable bacteria during field transport experiments. The criteria for development and selection of these methods included: (1) the ability to track bacteria within the environment from which they were isolated; (2) the lack of any effect upon the viability or the transport characteristics of the strain; (3) low detection limits; (4) a quantification range that covered several orders of magnitude; and (5) an analytical cost and turnover time commensurate with the analysis of several thousands of samples in a few months. The approaches developed included: enumeration of bacteria labeled with a vital fluorescent stain (CFDA/SE) using microplate spectrofluorometry, flow cytometry, and ferrographic (immunomagnetic) capture; enumeration of highly (13)C-enriched bacteria using combustion-IRMS; and quantitative PCR. These methods were compared to direct microscopic enumeration and plate counts during a bacterial transport experiment performed in an intact sediment core and designed to simulate the field experiment. Four of the seven methods had equivalent recoveries for the breakthrough of a pulse of bacteria eluting from a 50-cm long sediment core, and all of the methods detected the arrival of cells in the effluent prior to the conservative tracer. Combustion IRMS and ferrographic enumeration had the lowest quantification limits (approximately 2 to 20 cells/ml), whereas microplate spectrofluorometry had the highest quantification limit (approximately 10(5) cells/ml). These methods have the potential for numerous applications beyond tracking bacteria injected into the subsurface.