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The Occurrence of Staphylococcus aureus on a Farm with Small-Scale Production of Raw Milk Cheese

¹ Department for Feed and Food Hygiene, and
² Department for Animal Health, National Veterinary Institute, Oslo, Norway
³ Department of Food Safety and Infection Biology, Norwegian School of Veterinary Science, Oslo, Norway.

Corresponding author: Hannah Joan Jørgensen; e-mail: hannah.jorgensen{at}vetinst.no.

In recent years, the small-scale production of raw milk products has increased in Norway, and there is some concern that such foods may pose a risk of staphylococcal food poisoning to consumers. The aim of the study was to evaluate potential sources of contamination of raw milk cheese with Staphylococcus aureus on a bovine dairy farm with small-scale production. Samples for bacteriological analyses (n = 144) were collected from the animals, the environment, processing equipments, from humans, and from cheeses at various stages of production. Staphylococcus aureus was isolated from 10 of 11 cows, the farmer, equipment, the environment, and the cheese. Seventy-five Staph. aureus isolates were genotyped by pulsed-field gel electrophoresis, tested for enterotoxin (SE) production by reversed passive latex agglutination, for SE genes by multiplex polymerase chain reaction, and for penicillin resistance by the cloverleaf method. Five different pulsotypes were identified and SE gene fragments were identified in 11 isolates, but no isolates produced SE or were penicillin resistant. Staphylococcus aureus was found throughout the farm, and appeared to be spread with the milk to the environment, equipment, and to products. One pulsotype dominated and was identified from most sample sites on the farm. Raw milk products are vulnerable to contamination with Staph. aureus. Strategies to reduce the occurrence of Staph. aureus in bulk milk are of particular importance on farms where milk is used for raw milk products.

Key Words: Staphylococcus aureus • raw milk cheese • small-scale production • pulsed-field gel electrophoresis

Abbreviation key: BA = blood agar with washed bovine erythrocytes, BP+RPF = Baird-Parker agar with rabbit plasma fibrinogen, PFGE = pulsed-field gel electrophoresis, PT = pulsotype, SE = staphylococcal en-terotoxin

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