Influence of milk product type and its initial contamination on the efficiency of different methods for detection of Salmonella Enteritidis, Listeria monocytogenes and Escherichia coli O157:H7

Abstract

This paper investigates differences in efficacy of isolating pathogenic bacteria Salmonella Enteritidis, Listeria monocytogenes and Escherichia coli O157:H7 between conventional cultivation (ISO method) and immunomagnetic separation (IMS method) related to the types of dairy products and initial numbers of bacteria. Different milk products (dairy pudding-vanilla or chocolate; a mixture of yoghurt and pudding; solid, liquid and fruit yoghurt; AB culture -with or without fruit; cheese spread) were intentionally contaminated with different numbers (≈10 and ≈30) of live cells of the observed bacteria per mL. The obtained results showed that the classical ISO procedure still represents an equally adequate method for the detection of S. Enteritidis and L. monocytogenes in dairy products as well as the IMS method. However, the ISO method was found to be inefficient for determination of E. coli O157:H7 when the initial contamination was low (≈10 live cells per mL). In such cases, even the IMS method appeared to be inefficient when used for fermented dairy products analysis. Fermented dairy products in contrast to the non-fermented ones, still represent a challenge for the development of routine detection methods, especially for S. Enteritidis, whilst the detection of L. monocytogenes and E. coli O157:H7 has improved by introducing the IMS method. The largest difference in the ability to detect bacteria in dairy product samples with reference to the initial number of bacteria by both methods was in the detection of E. coli O157:H7. The choice of broth (non-selective fluid broth vs. selective fluid broth) did not matter in the in the detection of S. Enteritidis and L. monocytogenes by applying the IMS procedure. However, for the detection of E. coli O157:H7 the application of modified tripton-soya broth with novobiocin (mTSB+Nb) has proved to be superior when compared to using the buffered peptone water (BPW). The presented results may be of importance as a scientific basis for future determination of standard methods related to laboratory detection of pathogens in dairy products.