Most of the mycoplasma media use in California comes from the Media Laboratory within the School of Veterinary Medicine at UC Davis. For direct culture, the milk sample is plated directly on to the plates. After incubation for 24-36 hours, the plates are checked for growth. Usually the plates are kept for up to 7 days before there are declared to be "no growth". For enrichment, the a sample of the milk is inserted into the mycoplasma enrichment broth. The broth is then incubated for 48 hours at 37^oC in a non-CO₂ incubator. It is then subcultured on to a fresh mycoplasma plate and handled as a direct culture plate.
Enrichment culture methods are used to isolate a particular organism that may be present in low numbers in the original sample such as milk from a cow or bulk tank. They require additional media (about $0.50 for mycoplasma) and increase the time until the final reading of the culture is made (+48 hours).
For samples from individual cows (quarters or composites), enrichment methods for mycoplasma provides additional assurance as to whether the cows in infected or not. Research indicates that enrichment may increase the probability of isolating mycoplasma in an individual cow sample by 70% over direct culture (Thurmond, 1989). This may be of value when trying to find all the mycoplasma infected cows in a herd for culling or segregation.
The is no published research on the comparison of direct culture and enrichment culture for bulk tank milk. It can only be assumed that enrichment will detect mycoplasma in small numbers than will direct culture. At one point, it was widely felt that direct plating would detect one infected cow in a herd of about 800 cows.
Part of the answer about bulk tank culture will depend on what other means of monitoring the herd for mycoplasma are currently being used. In herds where the bulk tank is routinely cultured for mycoplasma using direct plating; fresh cows are routinely cultured for mycoplasma; clinical cases of mastitis are routine cultured for mycoplasma; and perhaps high SCC or cows with non-responsive treatments are cultured, little benefit may be gained by enrichment culture.
With several monitoring methods in place and routinely being used, any mycoplasma outbreak will be quickly detected. When only bulk tank culture is being used without other means of monitoring, enrichment may detected mycoplasma when only present in small amount.
The downside of enrichment culture of the bulk tank milk may be to detect non-pathogenic mycoplasma in small numbers within the sample. This may be particularly true during periods of high rain fall. If care is not used in the interpretation of such a situation, a major campaign may be started within the herd to eradicate mycoplasma that may not be warranted. This also points out the importance of speciating mycoplasma isolates.
Thurmond MC, Tyler JW, Luiz DM, Holmberg CA and Picanso JP. The effect of pre-enrichment on recovery of Streptococcus agalactiae, Staphylococcus aureus and mycoplasma from bovine milk. Epidem Inf 1989;103:465-474.