Characterization of Deoxyribonuclease-Positive Enterococci Isolated from Milk and Milk Products

Abstract

Five hundred and ten isolates of enterococci recovered from milk and milk products were screened for deoxyribonuclease (DNase) production. Of the 166 (32.5%) DNase-positive cultures, 29 (5.7%) produced thermonuclease that resisted boiling for 15 min. Although the incidence of thermolabile DNase-positive enterococci was 50.5% in Cheddar cheese, the majority of thermonuclease-producing enterococci was recovered from dried milks and infant foods. On the basis of biochemical, physiological and serological tests, all DNase-producing enterococci were characterized as S. faecalis var. faecalis (21), S. faecalis var. zymogenes (9), S. faecalis var. liquefaciens (90), S. faecium (23) and S. durans (16). The predominant thermonuclease-positive type was S. faecalis var. faecalis followed by S. faecalis var. zymogenes. Six strains of enterococci were found to be toxigenic when tested in ligated rabbit ileal loops, infant mice and rabbit skin.