Comparison of methods for isolating Salmonella bacteria from faeces of naturally infected pigs

Abstract

A series of experiments was conducted using faecal samples collected from commercial swine farms to evaluate the effects of variation in methods used for the detection of Salmonella bacteria. The primary objective of the studies was to compare the protocols routinely used in two laboratories in the USA. The studies included five experiments comparing the enrichment protocols used routinely in the respective laboratories (Method 1: 10 g faeces—buffered peptone water (BPW) pre‐enrichment—selective enrichment in Rappaport/Vassiliadis (RV) broth; Method 2: ∼1g faeces—primary enrichments in tetrathionate and Hajna GN broths—secondary enrichment in RV broth). The effects of enrichment temperatures (37 vs 42 °C) using RV broth (two experiments) and delayed secondary enrichment (four experiments) were also evaluated. Direct comparison of Method 1 and Method 2 indicated comparable results. However, when compared using faecal samples of equal weight, the Method 2 enrichment protocol was more sensitive for detecting Salmonella bacteria than the Method 1 protocol. Enrichment in RV at 42 °C was superior to 37 °C, particularly for samples that were pre‐enriched in BPW. Delayed secondary enrichment increased detection of Salmonella bacteria in swine faeces. These results highlight the imperfect sensitivity of culture methods, and the need for researchers to consider the sensitivity of bacteriological methods in the design and interpretation of the results of epidemiologic studies based on faecal culture

Use of Pooled Samples for the Detection of Salmonella in Feces by Polymerase Chain Reaction

“…The majority of epidemiological studies of Salmonella still rely on conventional bacteriological culture methods to detect Salmonella in fecal samples. 3,5 The primary reason for using culture-based methods in these investigations is to provide isolates that can be further analyzed.…”

Sensitivity of the ISO 6579:2002/Amd 1:2007 Standard Method for Detection of Salmonella spp. on Mesenteric Lymph Nodes from Slaughter Pigs

“…It is well recognized that the Se of bacteriological culture varies with regard to factors such as the sample of choice (feces, lymph nodes, or tonsils), the type of sample (single or pooled), the amount of sample processed, or the combination of culture media used for isolation of the bacterium (7)(8)(9)(10)(11)(12). For instance, the Se of bacteriology on fecal samples has been reported to be as low as 9% (10) and higher than 90% (11,12).…”

“…In fact, most studies estimate the Se of a given bacteriological technique relative to a combination of different but imperfect bacteriological methods (7,9,11). To overcome this problem, latent-class methods with Bayesian approaches have been used to obtain unbiased estimates of the Se's of different culture protocols for the diagnosis of pig salmonellosis on fecal samples under field conditions (12,14).…”

Estimation of the Salmonella spp. prevalence in pig farms with dry and wet feeding

“…The amount of faeces, secondary and delayed enrichment, may affect the sensitivity of bacterial culture (Šišak et al, 2011;Funk et al, 2000). In this study, we used 25 g of faeces for faecal sample and applied two enrichment steps in order to increase the sensitivity of the test.…”

“…From each farm, 30 faecal samples and 10 feed samples were collected. Samples were cultured using 25 g of feces and a double enrichment step (Davies et al, 2000). Twenty-five grams of feces were added to 225 ml of buffered peptone water (BPW) and incubated for 24 h at 37°C.…”