Milk acidification to control the growth of Mycoplasma bovis and Salmonella Dublin in contaminated milk

Bacterial contamination of milk fed to calves compromises calf health. Several bacterial pathogens that infect cows, including Mycoplasma bovis and Salmonella enterica ssp. enterica serovar Dublin, are shed in milk, providing a possible route of transmission to calves. Milk acidification lowers the milk pH so that it is unsuitable for bacterial growth and survival. The objectives of this study were to (1) determine the growth of M. bovis and Salmonella Dublin in milk, and (2) evaluate the efficacy of milk acidification using a commercially available acidification agent (Salstop, Impextraco, Heist-op-den-Berg, Belgium) to control M. bovis and Salmonella Dublin survival in milk. For the first objective, 3 treatments and a positive control were prepared in 10 mL of milk and broth, respectively, and inoculated with M. bovis or Salmonella Dublin to an approximate concentration of 104 cfu/mL. Each treatment was retained at 5, 23, or 37°C with the positive control at 37°C. Aliquots were taken at 4, 8, 24, 28, 32, 48, 52, and 56 h after inoculation and transferred onto agar medium in triplicate following a 10-fold dilution series in sterile phosphate-buffered saline. All plates were incubated and colonies counted. For the second objective, 4 treatments and a positive control were prepared with 100 mL of milk and inoculated with M. bovis or Salmonella Dublin to an approximate concentration of 106 cfu/mL. With the use of Salstop, treatments were adjusted to an approximate pH of 6, 5, 4, or 3.5. The positive control was left untreated. At 1, 2, 4, 6, 8, and 24 h after treatment, triplicate aliquots were taken, the pH measured, and then the aliquots were transferred onto agar medium and into broth for enrichment. Following incubation, agar colonies were counted, while broths were plated and incubated prior to colonies being counted. All trials were repeated. Mycoplasma bovis did not grow in milk, but Salmonella Dublin proliferated. The pH of all acidification treatments remained stable for 24 h. No viable M. bovis organisms were detected at 1 h of exposure to pH 3.5 and 4 or at 8 h of exposure to pH 5. Following 24 h of exposure to pH 6 M. bovis remained viable. No viable Salmonella Dublin organisms were detected at 2 and 6 h of exposure to pH 3.5 and 4, respectively. Salmonella Dublin remained viable following 24 h of exposure to pH 5 and 6. These results demonstrate that milk acidification using Salstop is effective at eliminating viable M. bovis and Salmonella Dublin organisms in milk if the appropriate pH and exposure time are maintained.