Two cases of endocarditis caused by Neisseria elongata subspecies nitroreducens and phenotypic differentiation from Kingella denitrificans

To describe two cases of endocarditis caused by Neisseria elongata subsp. nitroreducens, and present some biochemical characteristics not previously described as useful for differentiating this taxon from Kingella denitrificans. Biochemical and antimicrobial susceptibility tests were carried out on four field strains and three reference strains of N. elongata subsp. nitroreducens, the type strains of N. elongata subsp. elongata and N. elongata subsp. glycolytica, and five reference strains of K. denitrificans. Phenotypic characterization confirmed the identification of N. elongata subsp. nitroreducens in two cases of endocarditis in patients with pre-existing heart damage. Apart from the previously reported differential characteristics between N. elongata subsp. nitroreducens and K. denitrificans, N. elongata subsp. nitroreducens was found to have large mucoid colonies, a positive result of the phenylalanine test, acid production from ethanol and susceptibility to polymyxin, in contrast to K. denitrificans. When blood cultures are positive for N. elongata subsp. nitroreducens, endocarditis should be suspected. Characteristics useful for differentiating N. elongata subsp. nitroreducens from K. denitrificans are larger colonies, reduction of nitrate and nitrite without gas formation, a positive result of the phenylalanine test, production of acid from ethanol and polymyxin susceptibility.