A comparison of methods for counting viruses in aquatic systems

Area of Science:

• Environmental microbiology
• Virology
• Microscopy techniques

Background:

• Accurate viral enumeration is crucial for understanding aquatic ecosystem dynamics.
• Traditional methods for virus counting can be time-consuming and vary in efficiency.

Purpose of the Study:

• To compare the efficiency of different epifluorescence microscopy (EM) and nucleic acid labeling methods for virus counting.
• To identify the optimal protocol for quantifying viruses in freshwater ecosystems and phage cultures.

Main Methods:

• Comparison of transmission electron microscopy with epifluorescence microscopy (EM).
• Evaluation of nucleic acid stains: 4',6'-diamidino-2-phenylindole (DAPI), YOPRO-1, and SYBR Green I.
• Analysis of samples from freshwater ecosystems and T7 phage cultures.

Main Results:

• EM counting using YOPRO-1 as the label demonstrated the highest efficiency and fluorescence stability across all tested ecosystems.
• SYBR Green I showed rapid photobleaching, making it unsuitable for high viral load samples (>10^8 particles/ml).
• DNase and RNase treatments indicated a ~15% overestimation by EM due to free nucleic acid interference.

Conclusions:

• The YOPRO-1 labeling protocol, particularly a modified version, is recommended for efficient and routine virus enumeration in aquatic samples.
• The study provides a validated, optimized method for viral quantification in environmental microbiology.
• Careful consideration of potential interference from free nucleic acids is necessary for accurate viral counts.