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

A series of experiments was conducted using faecal samples collected from c ommercial swine farms to evaluate the effects of variation in methods used for the detection of Salmonella bacteria. The primary objective of the stud ies was to compare the protocols routinely used in two laboratories in the USA. The studies included five experiments comparing the enrichment protoco ls used routinely in the respective laboratories (Method 1:10 g faeces-buff ered peptone water (BPW) pre-enrichment-selective enrichment in Rappaport/V assiliadis (RV) broth; Method 2: similar to 1g faeces-primary enrichments i n tetrathionate and Hajna GN broths-secondary enrichment in RV broth). The effects of enrichment temperatures (37 vs 42 degrees 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 ba cteria than the Method 1 protocol. Enrichment in RV at 42 degrees C was sup erior to 37 degrees C, particularly for samples that were pre-enriched in B PW. Delayed secondary enrichment increased detection of Salmonella bacteria in swine faeces. These results highlight the imperfect sensitivity of cult ure methods, and the need for researchers to consider the sensitivity of ba cteriological methods in the design and interpretation of the results of ep idemiologic studies based on faecal culture.