An in vitro evaluation of the interactions of Legionella pneumophila serogroups 2 to 14 strains with other bacteria in the same habitat

Abstract

The aim of the present study is to determine in vitro inhibitory and/or stimulatory effects of different bacteria on concomitant Legionella pneumophila. The interactions between Legionella and other bacteria were investigated by using bacteria culture and cell-free supernatants (CFSs) on Buffered Charcoal Yeast Extract agar (BCYEA) and/or in BCYEA lacking L-cysteine. Additionally, CFSs of non-Legionella bacteria that possess inhibitory effect on L. pneumophila were characterised using enzyme and heat treatments. The inhibition ratio of the CFSs and the cultures of Gram-negative rod bacteria (GNRB) and Gram-positive rod bacteria (GPRB) on the growth of L. pneumophila strains were 26-47% and 33-67%, respectively, on BCYEA. It was detected that the cultures of Aeromonas hydrophila, Aeromonas spp. strain and Bacillus pumilus stimulated the growth of one of the investigated L. pneumophila strains, but their CFSs did not show any stimulatory effect. The results indicated that growth and multiplication of legionellae could be affected by different bacteria sharing the same habitat and the level of this effect varies among the species. To our knowledge, this is the first study which determined the inhibitory effects of B. pumilus and Brevibacillus brevis against Legionella. The biologically active substances produced by the bacteria could play an important role in the control of L. pneumophila. This phenomenon may be used an alternative approach for controlling legionellae in man-made environments.