Using Genetic Markers to Identify Bacterial Populations in Dairy Wastewater

Jeffrey McGarvey

USDA/ARS/FCR

Research on a California dairy used molecular methods to determine the type of bacteria that inhabit a wastewater treatment system, how they travel through the system and whether pathogenic organisms are a risk. Total DNA was extracted from samples from three major components of the system, manure from the free stall barn, separator pit water and circulated holding lagoon water, and 16S rDNA genes were amplified using PCR and universal primers. The PCR products were cloned, the 16S rDNA genes sequenced then compared to known 16S rDNA sequences. The analysis indicated a decrease in the bacterial density and diversity as the manure moved through the system. Few pathogenic bacteria were detected and none were identified with a specific PCR. A second study compared the bacterial communities between a stagnant and a circulated lagoon from two dairies located within a 1-mile radius both using a balanced feed. Although gross plate counts showed similar microbial and nutrient content, rarefraction analysis indicated that the stagnant lagoon contained a more diverse population. Additional analysis revealed that green non-sulfur bacteria were more prevalent in the stagnant lagoon whereas purple sulfur bacteria were more common in the aerated lagoon. The observance that the aerated lagoon had minimal odors as compare to the stagnant lagoon may be due to the purple sulfur bacteria which have been shown to degrade volatile organic carbons. Additional research is planned to evaluate volatile organics on a dairy farm that has the option of diverting wastewater to either a stagnant or an aerated lagoon.

Discussion:

Srinand Sreevatsan (OSU) asked if the bacterial population fluctuates with the season. McGarvey noted that the amount of photosynthetic purple sulfur bacteria decreases with decreasing sunlight but that others go up.  The weather plays a role as does the fact that the animals are on pasture more often during the summer.

Sreevatsan asked whether the farm has a Johnes testing program.  McGarvey responded that according to the producer, Johnes is not a problem.  While McGarvey’s research tested for but did not detect Johnes, it did not enrich the sample for a stronger analysis.

Fred Michel (OSU) asked if the photosynthetic bacteria are phototrophic and heterotrophic and whether they can use different substrates.  What effect on the bacterial populations would lagoon covers have?  McGarvey suggested that the photosynthetic bacterial populations would be expected to decrease.  Lagoon covers can be effective in trapping odors but can cause other problems.

A Columbus participant asked about the differences in bacteria between the manure, separator pit and lagoon in the first study.  McGarvey noted that the most common bacterial groups identified, unknown and firmicutes, were the most common from all three locations so are fit for each environment.  However, this is not true for all the bacteria as the third most common bacteria in the manure and separator pit, actinobacter, differed from that in the holding lagoon, proteobacter.

Notes by Mary Wicks