Usefulness of Selection of Legionella by Colony Appearance

Abstract

  An acid or heat preprocessing step or a selective isolation agar is required for the Legionella culture test, since the test sample may contain various other bacteria. However, miscellaneous bacterial growth is sometimes not possible to control even with such precautions. Many Legionella-test manuals indicate that isolated Legionella colonies are characterized by different-sized grayish-white moist look with the slight acidic odor peculiar to Legionella. However, in the actual test, Legionella colonies are often difficult to distinguish from other bacteria in presence of many similar-looking colonies. Therefore, we suggest a colony-observation method which is more precise and simple. The characteristic outward structures (cut-glass like or mosaic like appearance) of Legionella spp. were observed under a stereo microscope with oblique illumination over the growing culture on the isolation agar. Legionella spp. could be distinguished from other bacteria and efficiently obtained using this observational method. In addition, bacteria count was easy and extremely accurate with this method. In this study, 13 serogroups of L. pneumophila and 17 types of other Legionella spp. were detected efficiently by this method from environmental samples. Since the same features were observed in the L. pneumophila serogroup 1, which was detected most frequently in Legionella pneumonia patients, this observational method is one of the most effective test methods for regular monitoring of environmental water or investigations of infection or contamination sources.