Legionella pneumophila: Growth inhibition by human pharyngeal flora

Several bacteria isolated from human pharyngeal cultures specifically inhibited the growth of Legionella pneumophila. The inhibitory substance from two strains (Streptococcus species 1-3 and Staphylococcus saprophyticus KC) was isolated from a broth supernatant. The inhibitor was active against all strains of L. pneumophila tested, including five strains of L. pneumophila serogroup 1 and one strain each of serogroups 2, 3, and 4. The substance did not inhibit growth of 18 fresh clinical and laboratory pathogens (12 genera). The substance was dialyzable, was resistant to heat and proteolysis, and did not precipitate with ammonium sulfate. Nicotinamide adenine dinucleotide, produced by several upper respiratory tract organisms, did not inhibit L. pneumophila, and L. pneumophila could not be isolated when Streptococcus species 1-3, S. saprophyticus KC, and L. pneumophila were cocultivated. These properties may in part explain the difficulty of isolation and may aid in the identification of L. pneumophila.