Enumerating ammonia-oxidizing bacteria in environmental samples using competitive PCR

Area of Science:

• Environmental microbiology
• Molecular biology
• Biogeochemistry

Background:

• Ammonia-oxidizing bacteria (AOB) are crucial for nitrification in various ecosystems.
• Detecting and quantifying AOB is essential for understanding nitrogen cycling.
• Existing methods for AOB detection may lack precision or specificity.

Purpose of the Study:

• To develop and validate a quantitative polymerase chain reaction (PCR) method for accurate enumeration of ammonia-oxidizing bacteria (AOB).
• To assess the performance of a competitive PCR (cPCR) technique using the ammonia-monooxygenase gene (amoA) primer pair.
• To determine the abundance and distribution of AOB in a waterlogged rice paddy soil environment.

Main Methods:

• Development of a competitive PCR (cPCR) assay incorporating an internal standard for amoA gene quantification.
• Evaluation of cPCR efficiency, primer degeneracy, and flanking region effects.
• Calibration using known amounts of Nitrosomonas europaea in soil samples.
• Analysis of vertical distribution of ammonia-oxidizers alongside oxygen and nitrate profiles in rice paddy soil.

Main Results:

• The cPCR method demonstrated a detection limit below 1000 cells g(-1) soil with a linear response across a broad range of cell additions.
• Specificity of the amoA primers was confirmed through cloning and sequencing, yielding no false positives.
• The study successfully mapped the vertical distribution of ammonia-oxidizers in rice paddy soil, correlating with nutrient and oxygen gradients.

Conclusions:

• The developed cPCR technique provides a reliable and sensitive tool for quantifying ammonia-oxidizing bacteria (AOB) in environmental samples.
• This method enhances our ability to study microbial communities involved in nitrogen cycling.
• The findings offer insights into the ecological distribution of ammonia-oxidizers in specific soil environments.