Mitochondria are emblematic structures in cell biology and have traditionally been considered the powerhouse of the cell, as they play an essential role in cellular bioenergetics. In this double-membrane organelle, nutrients are oxidized and removed electrons are used in a process termed oxidative phosphorylation (OXPHOS), which involves the creation and exploitation of a membrane potential across the inner mitochondrial membrane (IMM), resulting in ATP synthesis (Formosa and Ryan, 2018). However, mitochondria are currently recognized as multitask organelles in eukaryotic cell biology as they take part in the regulation of crucial cellular processes. Mitochondria carry out essential steps of heme, iron-sulfur cluster, and amino acid biosynthesis as well as fatty acid oxidation (Lill and Freibert, 2020; Schulz, 2013; Spinelli and Haigis, 2018). They also play an important role in Ca2+ homeostasis and, as a central coordination element for Ca2+-dependent pathways, mitochondria play a key role in the regulation of programmed cell death. For these actions, contact sites of mitochondria with the Ca2+-storing endoplasmic reticulum (ER) are essential (Giorgi et al., 2012). Furthermore, mitochondria can release pro-apoptotic proteins such as cytochrome C into the cytosol, triggering the activation of the intrinsic apoptotic pathway (Obeng, 2021). Mitochondria are also engaged in the initiation of immune signaling and take part in innate immune response by producing mitochondrial reactive oxygen species (mROS), which are secondary messengers involved in the activation of inflammasomes (Herb and Schramm, 2021; West et al., 2011b). Like the ER, mitochondria have a stress response mechanism to deal with the accumulation of unfolded proteins in the matrix (Melber and Haynes, 2018). Along with these features, the highly dynamic morphology of mitochondria is another characteristic of this organelle. Mitochondria are able to undergo fission and fusion events depending on the conditions in the cell. While mitochondrial fission is necessary before cellular division, fusion events take place when there is a low nutritional supply as they might contribute to maintain energy levels (Wai and Langer, 2016).

Considering their central role in multiple cellular processes, mitochondria are an attractive target for intracellular pathogens. Multiple studies have reported how viruses and intracellular bacteria have evolved ways of targeting mitochondria to influence their intracellular survival, to improve the access to nutrients, to delay host apoptosis or to evade host immunity (Marchi et al., 2022; Sorouri et al., 2022). Here we review the different mechanisms that Legionella pneumophila, the intracellular bacteria that causes Legionnaires’ disease in humans, employs to manipulate mitochondrial functions during infection of host cells.

Legionella pneumophila was recognized for the first time during an outbreak of pneumonia in the summer of 1976 during the 58th annual convention of the American Legion in Pennsylvania, United States. An unconventional respiratory disease affected 221 participants and 34 fatal cases were reported (Fraser et al., 1977). In December 1976, Joseph E. McDade and Charles C. Shepard identified and characterized a rod-shaped gram-negative bacterium as the causative agent of the disease. It was named Legionella pneumophila after the American Legion and the new genus was named Legionella, which at that time had only one known species (Brenner et al., 1979; Fraser et al., 1977; McDade et al., 1977). Today, the genus Legionella comprises at least 65 species but not all of them are equally responsible for laboratory-confirmed cases of Legionnaires' disease. L. pneumophila serogroup (Sg) 1 is the most relevant, being responsible for 80–90% of the cases in Europe and the United States (Yu et al., 2002). L. longbeachae accounts for approximately 1% of cases worldwide but is on the rise in Europe and accounts for 50–60% of cases in Australia and New Zealand (Bacigalupe et al., 2017). Other serogroups, such as L. pneumophila Sg3 and Sg6, as well as species such as L. bozemanii or L. micdadei, have also been reported to cause disease in few cases, but they are rare and were isolated mainly from immunocompromised patients (Beauté, 2017; Currie and Beattie, 2015; Yu et al., 2002). Legionella spp. are opportunistic pathogens for humans as they are naturally present in freshwater environments, as well as in moist soil and composted material either as free-living biofilm-associated bacteria or associated with their hosts, primarily aquatic amoebae (Rowbotham, 1980). Human infection most commonly occurs when susceptible individuals inhale Legionella-containing aerosols generated by contaminated manmade water sources, such as showers, hot tubs, plumbing networks, and air-conditioning systems (Blatt et al., 1993) (Fig. 1). Through these routes, Legionella can reach the human lungs, where they can infect alveolar macrophages using the same mechanisms that they utilize to survive within their amoebal hosts. Indeed, L. pneumophila has the capacity to replicate in numerous phagocytic hosts, ranging from different amoeba species to mammalian cells (Boamah et al., 2017; Escoll et al., 2013).

After uptake by host cells, L. pneumophila remodels the phagocytic vesicle into a replicative-permissive-niche known as the Legionella-containing vacuole (LCV) (Fig. 1). Interestingly, during the first steps of infection, physical contacts occur between mitochondria and the LCV (Escoll et al., 2017b; Hoffmann et al., 2014; Horwitz, 1983). Within this sophisticated intracellular compartment, L. pneumophila sequentially evades phagolysosomal degradation, is protected from intracellular defenses and intercepts nutrients to support growth and proliferation. L. pneumophila employs different secretion systems to deliver virulence-associated proteins across the LCV membrane that modulate specific host cell functions to the bacterial advantage. Among them, type 2 and type 4 secretion systems (T2SS and T4SS, respectively) are present in all Legionella species and play essential roles during infection (Mondino et al., 2020). The T2SS translocates around 25 effector proteins that are necessary for environmental persistence and for replication in amoeba, and that play also a role in replication in macrophages (Cianciotto, 2009, Cianciotto, 2013). In. contrast, the T4SS translocates more than 330 effectors which represent about 10% of the bacterial proteome in the host cell and thereby governs all steps of the intracellular life of L. pneumophila in both amoeba and human macrophages (Ensminger, 2016; Lockwood et al., 2022; Mondino et al., 2020). Some of these effectors modulate the cellular metabolism, host cell death and autophagy directly targeting mitochondrial functions (Table 1; Fig. 1). Despite their importance for bacterial virulence, the mechanisms underlying the activity of these effectors on host mitochondria have not been fully understood yet. Here we present an overview of the key findings about the interactions of L. pneumophila with mitochondria during infection, discuss the mechanisms by which T4SS secreted effectors target mitochondrial functions to subvert infected cells, and provide a perspective view on the interplay between Legionella and mitochondria, the extraordinary eukaryotic organelles governing cellular bioenergetics, cell-autonomous immunity and cell death.