Futuristic veterinary lab analyzing calf health with PCR technology.

Gut Check: How PCR Tests Could Revolutionize Detection of Salmonella in Calves

Kai Mendoza in Health & Wellness August 2025 • 5 min read.

"Discover how advanced PCR technology is transforming Salmonella detection in livestock, offering faster and more reliable results than traditional methods."

Salmonella, a bacterium found worldwide, poses a significant threat to various animal species, including humans. It's a notorious cause of foodborne illnesses, often linked to consuming contaminated meat, milk, and their derivatives. Among the many serotypes, Salmonella Dublin and Salmonella Typhimurium are the most prevalent in cattle, with Salmonella Dublin particularly adapted to bovine species. This bacterium is a primary culprit behind diarrhea in calves, causing significant health and economic concerns for farmers.

Traditional methods for detecting Salmonella rely on bacterial culture, which involves using selective media followed by biochemical and serological tests to identify suspicious colonies. While these methods are well-established, they're also time-consuming, often taking four to seven days to yield results. This delay can be particularly problematic in managing outbreaks, where rapid detection is crucial to prevent further spread. Additionally, traditional methods can be labor-intensive and costly, especially when applied on a large scale.

Polymerase Chain Reaction (PCR) has emerged as a game-changing tool for diagnosing salmonellosis, prized for its speed and sensitivity. PCR amplifies specific DNA segments, allowing for the detection of even small amounts of the bacteria. However, the direct use of PCR on fecal samples is often hindered by high concentrations of inhibitory substances, which can interfere with the amplification process. To overcome this obstacle, selective enrichment—culturing the sample in a selective medium before DNA extraction—has shown promise. This step not only increases the number of target bacteria but also dilutes inhibitory substances, paving the way for more reliable PCR results.

PCR vs. Traditional Methods: A Head-to-Head Comparison

Futuristic veterinary lab analyzing calf health with PCR technology.

A study meticulously compared the effectiveness of PCR combined with selective enrichment against traditional bacteriological isolation in detecting Salmonella Dublin in fecal samples from experimentally infected calves. Seventy-six fecal samples were collected from six Holstein calves aged 10 to 15 days. These calves were intentionally infected with 10° colony-forming units (CFU) of Salmonella Dublin. Samples were taken in duplicate, immediately before inoculation and at twelve-hour intervals for seven days post-infection. This controlled setup allowed for a precise comparison of the two detection methods.

In the traditional bacteriological isolation method, rectal swabs were inoculated into selective enrichment broths—specifically selenite cystine (SC) and tetrathionate Muller-Kauffman (TKM)—and incubated at 37°C for 24 hours. Following incubation, the broths were streaked onto modified brilliant green agar plates containing nalidixic acid. Suspect colonies were then inoculated into triple sugar iron agar and lysine iron agar. Presumptive Salmonella colonies underwent slide agglutination tests using polyvalent somatic (O) and flagellar (H) antisera to confirm the presence of Salmonella. This multi-step process is typical of traditional Salmonella detection, highlighting its complexity and the time required to complete.

Key Steps in the Traditional Method:

Inoculation of rectal swabs into selective enrichment broths (SC and TKM).
Incubation at 37°C for 24 hours.
Streaking onto modified brilliant green agar plates with nalidixic acid.
Confirmation via slide agglutination tests.

For the PCR method, DNA was extracted from aliquots of both selective enrichment broths using the salting-out technique, known for its effectiveness in purifying DNA from complex samples. The PCR assay itself was a multiplex reaction, targeting the rfbE and fliC segments present in Salmonella serogroup D strains. This multiplex approach allowed for the simultaneous detection of multiple target sequences, increasing the assay's efficiency. The PCR reaction was performed in a thermal cycler with specific parameters: denaturation at 95°C, annealing at 56°C, and extension at 72°C, each for 30 seconds over 35 cycles. The amplified DNA products were then separated by gel electrophoresis, stained with ethidium bromide, and visualized under UV light. Samples showing fragments of 307 and/or 225 base pairs were considered positive for Salmonella Dublin.

Concluding Thoughts: The Future of Salmonella Detection

The results indicated that traditional bacteriological isolation detected Salmonella Dublin in 44 samples (57.9%) and 42 samples (55.3%) using SC and TMK broths, respectively. PCR detected 41 samples (53.9%) and 35 samples (46.1%) as positive, also with SC and TMK. The study found that traditional methods were statistically superior to PCR in detecting positive samples. Despite this, PCR identified positive samples missed by traditional isolation, offering detection in a shorter time frame. While traditional bacteriological isolation showed superior detection rates for Salmonella Dublin in calf fecal samples, the PCR method offers quicker results and can identify cases missed by traditional methods. The study also highlighted that selenite cystine broth (SC) is preferable for both techniques. These advances promise more effective strategies for managing Salmonella in livestock, thereby safeguarding both animal and human health.

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This article is based on research published under:

DOI-LINK: 10.1590/s0102-09352010000300035, Alternate LINK

Title: Avaliação Da Reação Em Cadeia Da Polimerase E Do Isolamento Bacteriológico Convencional Na Detecção De Salmonella Dublin Em Amostras De Fezes De Bezerros Infectados Experimentalmente

Subject: General Veterinary

Journal: Arquivo Brasileiro de Medicina Veterinária e Zootecnia

Publisher: FapUNIFESP (SciELO)

Authors: D.G. Silva, D.R. Silva, P.R.L. Silva, E.A.S. Cícero, A.L.J. Ferraz, M.V.F. Lemos, J.J. Fagliari

Published: 2010-06-01

Everything You Need To Know

What are the traditional methods for detecting Salmonella, and what are their limitations?

Traditional methods for detecting Salmonella, like bacterial culture, involve selective media and biochemical tests. These methods usually take four to seven days to produce results. While reliable, the time delay can be a significant drawback, especially when dealing with outbreaks requiring rapid response. The labor and cost involved in these methods are also important considerations for large-scale applications.

How does PCR (Polymerase Chain Reaction) improve Salmonella detection, and how is it used effectively in fecal samples?

PCR, or Polymerase Chain Reaction, offers a quicker and more sensitive method for detecting Salmonella. It works by amplifying specific DNA segments, allowing even small amounts of the bacteria to be detected. However, inhibitory substances in fecal samples can interfere with PCR. To counteract this, a selective enrichment step is often used, where the sample is cultured in a selective medium before DNA extraction. This increases the number of target bacteria and dilutes the inhibitory substances, leading to more reliable PCR results.

In the study comparing PCR and traditional methods, what were the detection rates for Salmonella Dublin using each method?

In the study, the traditional bacteriological isolation method detected Salmonella Dublin in 57.9% and 55.3% of samples using selenite cystine (SC) and tetrathionate Muller-Kauffman (TKM) broths, respectively. The PCR method detected Salmonella Dublin in 53.9% and 46.1% of samples using SC and TKM broths, respectively. These results indicate that the traditional methods showed superior detection rates in calf fecal samples compared to PCR in this study.

How was the PCR method specifically implemented in the study for detecting Salmonella Dublin, including the targeted genes and reaction parameters?

The study used a multiplex PCR assay targeting the rfbE and fliC segments present in Salmonella serogroup D strains to detect Salmonella Dublin. DNA was extracted from selective enrichment broths using the salting-out technique. The PCR reaction involved denaturation at 95°C, annealing at 56°C, and extension at 72°C for 30 seconds over 35 cycles. Amplified DNA products were then separated by gel electrophoresis and visualized under UV light to confirm the presence of Salmonella Dublin. This multiplex approach increases the assay's efficiency by simultaneously detecting multiple target sequences.

What are the implications of using PCR for Salmonella detection in livestock management, considering both its advantages and disadvantages compared to traditional methods?

While traditional bacteriological isolation showed superior detection rates for Salmonella Dublin in calf fecal samples, the PCR method offers quicker results and can identify cases missed by traditional methods. The study also highlighted that selenite cystine broth (SC) is preferable for both techniques. These advances promise more effective strategies for managing Salmonella in livestock, thereby safeguarding both animal and human health. PCR's speed can significantly reduce the time to diagnosis which helps improve intervention strategies.