Biofilms: the stronghold of Legionella pneumophila

Abstract

Legionellosis is mostly caused by Legionella pneumophila and is defined as a severe respiratory illness with a case fatality rate ranging from 5% to 80%. L. pneumophila is ubiquitous in natural and anthropogenic water systems. L. pneumophila is transmitted by inhalation of contaminated aerosols produced by a variety of devices. While L. pneumophila replicates within environmental protozoa, colonization and persistence in its natural environment are also mediated by biofilm formation and colonization within multispecies microbial communities. There is now evidence that some legionellosis outbreaks are correlated with the presence of biofilms. Thus, preventing biofilm formation appears as one of the strategies to reduce water system contamination. However, we lack information about the chemical and biophysical conditions, as well as the molecular mechanisms that allow the production of biofilms by L. pneumophila. Here, we discuss the molecular basis of biofilm formation by L. pneumophila and the roles of other microbial species in L. pneumophila biofilm colonization. In addition, we discuss the protective roles of biofilms against current L. pneumophila sanitation strategies along with the initial data available on the regulation of L. pneumophila biofilm formation.

Figure 1

Monospecies biofilm of L. pneumophila labelled with the DNA stain Syto62. Scale bar represents 100 μm.

Figure 2

A. castellanii infected with L. pneumophila expressing green fluorescent protein. Scale bar represents 10 μm.

Figure 3

L. pneumophila (denoted by Lpn and shown in orange) replicate within environmental protozoa. Uptake within protozoan hosts is promoted by the presence of other amoebae and bacterial species such as P. aeruginosa (top left) and by environmental cues such as quorum sensing (green hexagons, top right). Environmental cues can also influence changes in L. pneumophila cell metabolism that favour biofilm production and colonization, which may occur following replication within protozoa or independently of protozoa infection (middle). Other microbial species such as P. aeruginosa (Lpn antagonistic microbial species, brown) can inhibit L. pneumophila colonization (bottom left). The presence of other microorganisms such as K. pneumoniae alleviates the inhibitory effect of P. aeruginosa (Lpn permissive microbial species, blue) and allows L. pneumophila to be incorporated within biofilms. L. pneumophila produces a surfactant (brown circles), which is toxic to other Legionella species (red), and may therefore prevent incorporation of these bacteria within biofilms. Physio-chemical parameters such as divalent cations (grey circles) can favour L. pneumophila biofilm colonization while other factors such as the presence of nanoparticles and copper (blue circles) can hinder L. pneumophila colonization.