Survival of Brucella abortus in ripened soft cheese made from naturally infected cow's milk

Abstract

Milk from eigh heifers experimentally inoculated with a virulent strain of Brucella abortus, and excreting large numbers ofthese bacteria at calving, was used to make three successive batches of a soft, camembert-type, cheese. Lactic fermentation was induced by a Streptococcus lactis and S. cremoris starter that caused the pH to fall to 4.5-4.6 after 20 hours. Surface inoculation of the curd with Penicillium caseicolum was performed before ripening at 12 oc. Brucella were enumerated by plating milk or liquid suspensions of chee se on a selective medium. Initially, Brucellae in the milk became concentrated in the curd and counts remained high up to 20 hours, subsequently declining regularly. No Brucella were found after eighteen days. Two groups of 20 mice were each fed 600-670 g of 22-25-day-old cheeses. No Brucella were isolated from these mice autopsied 15 days later. The scientific literature reports long survival times for Brucellae in cheeses of different types prepared from milk to which laboratory-grown cells had been added. It appears that Brucella cells which have multiplied in vivo and been excreted in the milk along with antibodies and inflammatory cells, may be more susceptible to inactivation during the cheesemaking process than laboratory-grown Brucella cells. Résumé Survie de Brucella abortus dans un fromage à pâte molle fabriqué avec du lait de vaches infectées