Salmonella: Bacterial Culture and PCR Testing Explanation
The AHDC performs Salmonella testing using two methods, bacterial culture (including blood cultures) and PCR (molecular testing). Routine Salmonella bacterial culture on enteric (fecal or intestinal) samples is the recommended test at the AHDC for most Salmonella requests, with environmental surveillance testing being performed by the PCR method. Blood cultures for Salmonella may be also be indicated when attempting to isolate Salmonella Dublin in untreated dairy calves or in sepsis/fever of unknown origin cases in other species, i.e. foals.
After performing extensive internal testing at the AHDC, we have found that enteric samples for both Salmonella bacterial culture and PCR must follow an overnight enrichment process, which assists the Salmonella organism's growth, recovery, and detection. While moderate to heavy Salmonella infections may be detected by direct plating on culture media, light infections or bacteria inhibited by treatment often need incubation in an enrichment media to enhance discovery. (Please note that Salmonella can be cultured from "clean" non-enteric samples such as the lung or lymph node, on a typical aerobic bacterial culture. This is due to lower amounts of other bacterial contaminants in those sites and the systemic nature of the disease.)
Salmonella PCR is used primarily for environmental surveillance testing. If a Salmonella PCR is requested on an enteric sample, both a bacterial culture and PCR are required to increase the testing sensitivity from the enrichment process. Two advantages to Salmonella bacterial culture over PCR are the ability to perform antimicrobial susceptibility testing and the ability to determine the serogroup and serotype of the recovered Salmonella isolate. Determining the serotype of Salmonella isolates assists in epidemiology and preventative measures.
Salmonella Testing Chart
| Type of Sample | Type of Test | Turn-around Time | Test Code |
|---|---|---|---|
| Enteric (fecal or intestinal) samples | Salmonella Culture | 8-72 hours | SALM |
| Enteric (fecal or intestinal) samples | *Salmonella PCR and Culture | 36-72 hours | SALMFPCR |
| Environmental surveillance | Salmonella PCR | 36-72 hours | SPCR |
| Non-enteric samples (lung, lymph node, spleen, body fluids, etc.) | Aerobic Culture | 48-72 hours | AER |
| Whole blood (in blood culture bottle) septic cases | Aerobic Blood Culture | Up to 5 days | BLDAER |
Note: *Salmonella fecal PCR is available but must be run in conjunction with Salmonella bacterial culture (SALMFPCR)
References
Hapuarachchi, et al., Journal of Medical Microbiology 2019;68:395–395 DOI 10.1099/jmm.0.000923
Pusterla N, Byrne BA, Hodzic E, Mapes S, Jang SS, Magdesian KG. Use of quantitative real-time PCR for the detection of Salmonella spp. in fecal samples from horses at a veterinary teaching hospital. Vet J. 2010;186(2):252-255. doi:10.1016/j.tvjl.2009.08.022
Salmonella Dublin, animal-health-diagnostic-center/news/salmonella-dublin