Comparison of blood culture methods for recovery of Legionella pneumophila from the blood of guinea pigs with experimental infection

Abstract

Blood was cultured from guinea pigs with experimental Legionella pneumophila serogroup 1 pneumonia, using four different methods. A 0.03-ml amount was spread onto each of several plates of buffered charcoal-yeast extract supplemented with alpha-ketoglutarate (BCYE) (direct plate); 1.5 ml each was inoculated into a BCYE agar-yeast extract broth bottle (biphasic), a pediatric Isolator tube (E.I. du Pont de Nemours & Co., Inc., Wilmington, Del.), and a glass tube containing 0.025% sodium polyanethanolsulfonate. Blood processed in the Isolator tube was plated on BCYE, as was the buffy coat blood fraction, which was obtained by centrifugation of the tube containing sodium polyanethanolsulfonate and blood. Observations were made of the number of positive cultures, the time to detection of positive cultures, and the absolute bacterial concentrations. Each system was equally sensitive in detecting bacteremia. The biphasic method required 5 days for cultures to become positive, whereas the other systems required 2 to 3 days to detect all positive cultures (P = 1.3 x 10-5 by Friedman group statistic, and P < 10-5 for comparison of the biphasic and other methods). The direct plating method demonstrated the best quantitative recovery of L. pneumophila in comparison to the other methods tested (P = 2.0 x 10-5 by analysis of variance group statistic and P < 0.05 for comparison between each of the methods). Quantitative recovery by the Isolator method was intermediate between the direct plating buffy coat methods. The biphasic and Isolator blood culture methods performed poorly in comparison to the other methods, indicating the need for caution in choosing blood culture methods for Legionella isolation.