Virulence and genotype stability ofSalmonella entericaserovar Berta during a natural outbreak

Abstract

SUMMARYStrains ofSalmonella entericaserotype Berta, collected over a period of 6 years from a well documented natural outbreak in Denmark, have been characterized in order to assess the stability of chromosomal typing systems and virulence properties. Outbreak strains were identical inPvuII andPstI IS200profiles, all but two strains showed the sameSmaI ribotype, and all but one strain showed the sameNotI pulsed field gel electrophoretic pattern, indicating that these molecular markers remained almost constant during the outbreak. In general, strains ofS. Berta were found to be of moderate to low virulence; log VC10values were found to vary between 3·0 and 4·4 after i.p. challenge of mice, and maximum CFU in internal organs of day-old chicks varied between 2 and 4 log10units following oral challenge. The minor differences observed between strainsin vivodid not correlate with differences inin vitroinvasion into cultured MDCK cells, nor within vitrogrowth characteristics. A succession of different plasmid profile types was observed during the outbreak but a hierarchical selection of clones based on differences in virulence was unlikely to have caused the succession of types ofS. Berta during this outbreak.