Ecological implications of an improved direct viable count method for aquatic bacteria

Area of Science:

• Marine microbiology
• Bacteriology
• Environmental science

Background:

• The direct viable count (DVC) method is crucial for enumerating viable microbial cells.
• Traditional DVC methods using single antibiotics like nalidixic acid are often insufficient for natural marine communities.
• Limitations include incomplete growth inhibition and difficulty distinguishing substrate-responsive cells.

Purpose of the Study:

• To improve the direct viable count (DVC) method for marine environments.
• To enhance the detection and enumeration of metabolically active marine bacteria.
• To investigate the ecological significance of previously uncharacterized viable cells.

Main Methods:

• Screening 100 marine isolates for antibiotic sensitivity to quinolones and beta-lactams.
• Developing an improved DVC method using a combination of five antibiotics.
• Applying the enhanced DVC method to natural marine microbial communities.

Main Results:

• A single antibiotic was ineffective for inhibiting growth across all marine isolates.
• An antibiotic cocktail proved highly efficient for growth inhibition, allowing lower concentrations and longer incubation.
• The improved DVC method revealed a 1- to 2-orders-of-magnitude greater fraction of substrate-responsive cells compared to traditional methods.

Conclusions:

• The enhanced DVC method with an antibiotic cocktail is effective for studying natural marine microbial communities.
• This method increases the detection of viable cells, including slow-growing and starved cells.
• The findings have significant ecological implications for understanding the metabolic contributions of nonculturable marine bacteria.