Enhancement of recovery of Legionella pneumophila from contaminated respiratory tract specimens by heat

Abstract

Heating of Legionella pneumophila and other Legionella spp. was studied to determine whether this technique could be used as a selective technique with contaminated clinical specimens. Studies of 13 different strains of Legionella spp. showed heterogeneous heat survival; heating at 60°C for 1 to 2 min did not affect the survival of the majority of strains. Heating of four Pseudomonas aeruginosa strains at 60°C for 2 min reduced bacterial counts by 98% or greater. Enterococci were heat tolerant, with virtually no inhibition under the same conditions. No inoculum effect was noted for any of the organisms tested. Heating of eight contaminated clinical specimens before plating on buffered charcoal-yeast extract medium reduced the numbers of contaminants on most plates but increased by only one the number of specimens yielding L. pneumophila. Plating the same specimens on selective media with or without heat pretreatment yielded L. pneumophila in every case. Heating of clinical specimens at 60°C for 1 to 2 min before plating may occasionally increase the recovery of L. pneumophila from contaminated specimens, but this technique should not be generally used.