Legionella pneumophila: The Paradox of a Highly Sensitive Opportunistic Waterborne Pathogen Able to Persist in the Environment

Abstract

Legionella pneumophila, the major causative agent of Legionnaires' disease, is found in freshwater environments in close association with free-living amoebae and multispecies biofilms, leading to persistence, spread, biocide resistance, and elevated virulence of the bacterium. Indeed, legionellosis outbreaks are mainly due to the ability of this bacterium to colonize and persist in water facilities, despite harsh physical and chemical treatments. However, these treatments are not totally efficient and, after a lag period, L. pneumophila may be able to quickly re-colonize these systems. Several natural compounds (biosurfactants, antimicrobial peptides…) with anti-Legionella properties have recently been described in the literature, highlighting their specific activities against this pathogen. In this review, we first consider this hallmark of Legionella to resist killing, in regard to its biofilm or host-associated life style. Then, we focus more accurately on natural anti-Legionella molecules described so far, which could provide new eco-friendly and alternative ways to struggle against this important pathogen in plumbing.

Keywords: Legionella pneumophila; amoebae; antimicrobial peptides; biocides; biofilms; biosurfactants; essential oils; natural compounds.

FIGURE 1

Ultrastructural changes in Legionella dumoffii cells after treatment with Galleria mellonella apolipophorin III. Transmission electron micrographs of (A) untreated bacteria and (B) treated bacteria with 0.4 mg/ml ApoLp-III. The presence of vacuoles is indicated by arrowhead. IM, Inner membrane; OM, Outer membrane (Source: Palusinska-Szysz et al., 2012).

FIGURE 2

Structural model of warnericin RK in a membrane-like environment. Green: hydrophobic residues; Purple: hydrophilic and neutral residues; Blue: negatively charged residues; Red: positively charged residues (Source: Adapted from Verdon et al., 2009a).

FIGURE 3

Activity of Ci-MAM-A24 against intra-amoebic L. pneumophila observed by confocal microscopy. A. castellanii cells infected with GFP expressing L. pneumophila Lens were incubated 6 h post-infection with (A) 12.5 μM of Ci-MAM-A24 or (B) peptide solvent during 42 h (48 h post-infection) (Source: Schlusselhuber et al., 2015).

FIGURE 4

Anti-Legionella activity of Thymus vulgaris EO observed by transmission electron microscopy. Micrographs of (A) untreated control cells of L. pneumophila Lens strain and (B) treated L. pneumophila Lens with 70 μg/ml Thymus vulgaris EO (Source: Chaftar et al., 2015b).

FIGURE 5

Apotome imaging of surfactin-treated 6-day-old biofilms formed by L. pneumophila. Biofilms were treated 2 h either with (A) ethanol as control or (B) 66 μg/ml surfactin (Source: Loiseau et al., 2015).