Polymerase chain reaction (PCR) for detection of pathogenic microorganisms in bacteriological monitoring of dairy products

Abstract

The presence of pathogenic bacteria poses a serious problem in sustaining the safety of dairy products. Microbiological routine controls of these products make use of selective culture techniques. To detect pathogenic species, isolated colonies are characterized by specific metabolic activities and by serotyping. We present an alternative biochemical approach that does not require culture of bacteria. The total bacterial populations of food samples were isolated by centrifugation and analysed by PCRs specific for pathogenic species. A total of 90 raw milk samples and dairy products made from raw milk were screened by this method for the presence of Listeria monocytogenes, Escherichia coli, enterotoxigenic E. coli, Campylobacter jejuni and C. coli. Detection rates were 12/90 (13%) for L. monocytogenes, 41/90 (46%) for E. coli, 18/90 (20%) for enterotoxigenic E. coli producing heat-labile toxin type I or heat-stable toxin type I, and 6/90 (7%) for C. jejuni or C. coli. Except for the use of different amplification primers, this approach is identical for any bacterial species to be detected. Direct PCR analysis of food samples offers rapid screening for the presence of specific bacteria and enables selection of critical samples prior to culture.