Comparison of fluorescence microscopy and solid-phase cytometry methods for counting bacteria in water

Area of Science:

• Environmental microbiology
• Water quality analysis
• Bacteriology

Background:

• Accurate bacterial abundance counts are crucial for assessing water biomass and quality in ecological and industrial settings.
• Variability in bacterial counts using fluorescent microscopy is often due to insufficient cells per filter for statistical confidence.
• Previous methods aimed for ~10(6) cells/filter, but the lower limit for reliable estimates was not well-defined.

Purpose of the Study:

• To establish the minimum bacterial cell count per filter for statistically reliable abundance estimates.
• To compare the accuracy and consistency of microscopic methods versus solid-phase cytometry for bacterial enumeration.
• To identify the threshold below which microscopic bacterial counts become statistically unreliable.

Main Methods:

• Investigated bacterial cell abundance counts using fluorescent microscopy and solid-phase cytometry.
• Determined the lower limit of bacterial cells per filter for acceptable statistical reliability.
• Compared overestimation ranges between microscopic and cytometer methods as cell counts decreased.

Main Results:

• Microscopic methods increasingly overestimated bacterial abundance (15.0% to 99.3%) when cell counts dropped below 10(5) cells/filter.
• Solid-phase cytometry showed minimal and consistent overestimation (8.9% to 12.5%) across the same low cell count range.
• Microscopic counts from 5 fields/3 filters were statistically equivalent to 20 fields/1 filter.

Conclusions:

• Solid-phase cytometry is a less biased and superior method for total direct bacterial counts compared to microscopy.
• Microscopic bacterial enumeration methods become unreliable and overestimate true abundance below 10(5) cells per filter.
• Optimized microscopic field counting strategies can yield statistically equivalent results to traditional methods.