Listeria monocytogenes, cows raw milk, mastitis and food safety

Abstract

This thesis intends to initially make an overview of the food safety area, and then a review of the main topics about L. monocytogenes, particularly its relationship with milk and the dairy industry, and report the laboratory work done in the scope of a Master of Science (MS) project. The principal aim of the work was the evaluation of the occurrence of L. monocytogenes in cows raw milk, in the Northern Portuguese Coast region. The detection (with VIDAS methodology) and enumeration (direct and according with the Most Probable Number (MPN) technique) of L. monocytogenes was performed in a total of 166 raw milk samples (45 from healthy cows, 58 from cows presenting sub-clinical mastitis, 27 from cows presenting clinical mastitis, and 36 were bulk tank samples), from 39 different dairy farms, from 13 different localities. L. monocytogenes was detected in two of these samples: one from a bulk tank, and the other from a clinical mastitis, both from the same dairy farm. From this dairy, environmental samples, as well as a sample of silage, unifeed, cattle manure and water were also analysed, for detection and enumeration of L. monocytogenes. To our knowledge, this was the first time L. monocytogenes was isolated from a clinical mastitis cow milk in Portugal. Twenty two L. monocytogenes isolates (from the refered milk samples) were identified, and further characterized by multiplex Polymerase Chain Reaction (PCR), antibiotic susceptibility and resistance to arsenic, cadmium and tetracycline. These subtyping techniques had results similar for all the isolates (with the exception of gentamicin Minimum Inhibitory Concentration (MIC)) suggesting that all the isolates belong to the same clone, particularly adapted to the environment of that farm, and that the origin of bulk tank contamination, was an infected cow, excreting L. monocytogenes through the udder. Additional studies using more discriminatory techniques such as pulse field gel electrophoresis (PFGE), however, are needed in order to confirm this hypothesis.

Date of Award	Jul 2007
Original language	English
Awarding Institution	Universidade Católica Portuguesa
Supervisor	Paula Teixeira (Supervisor) & Joana Silva (Co-Supervisor)