Development and use of polymerase chain reaction for the specific detection of Salmonella typhimurium in stool and food samples

Salmonella Typhimurium is one of the most important Salmonella serovars tha t may cause foodborne disease and human salmonellosis infection. Detection of this organism in the clinical samples of persons with gastroenteritis an d the food samples associated with such persons may allow us to trace the c ause of disease. Because malic acid dehydrogenase, an enzyme of the citric acid cycle, is common to organisms, the gene (mdh) coding for this enzyme w as selected for the design of Salmonella Typhimurium-specific polymerase ch ain reaction (PCR) primers. By comparison of the mdh gene sequences of Salm onella Typhimurium and other Salmonella serotypes and of some isolates of o ther genera, two oligonucleotides were designed and used as PCR primers for the specific detection of Salmonella Typhimurium. The molecular weight of the PCR product was 261 bp as expected. Salmonella serovars other than Salm onella Typhimurium and isolates of other genera in the Enterobacteriaceae t hat is closely related to Salmonella did not generate any false-positive re sults. When this primer pair was used for the detection of Salmonella Typhi murium cells artificially inoculated into human stool specimens and food sa mples, such as milk and raw chicken meat, levels as low as 10(0) CFU per 0. 1 g of stool specimen or per mi of milk or food homogenate could be detecte d if an 8- to 12-h preculture step using combined lactose-tetrathionate bro th was performed prior to the PCR.