Legionella: From Protozoa to Humans

Abstract

Pathogens that are able to enter and multiply within human cells are responsible for multiple diseases and millions of deaths worldwide. Thus, the challenge is to elucidate these pathogen-specific and cell biological mechanisms involved in intracellular growth and spread. Bacteria from the genus Legionella belong to this group of pathogens. They are environmental bacteria and ubiquitous in nature, where they parasitize protozoa. Strikingly, the capacity to grow intracellularly in protozoa like Acanthamoeba castellanii, Hartmannella sp., or Naegleria sp., has generated a pool of virulence traits during evolution, which allow Legionella to infect also human cells. Thus important human pathogens are present within the genus Legionella, the most prominent are L. pneumophila (Fraser et al., 1977; Mcdade et al., 1977) and L. longbeachae (Mckinney et al., 1981). These bacteria are the causative agents of Legionnaires’ disease, a severe pneumonia diagnosed mainly in people whose immune defenses are weakened. Legionella is transmitted through breathing infected aerosols present in many artificial water systems like air conditioning systems, cooling towers, showers, and other aerosolizing devices. When reaching the alveolar parts of the lungs Legionella is engulfed by macrophages where it is able to multiply resulting in a severe, often fatal pneumonia. Intracellular infection is a consequence of the bacterium's capacity to manipulate host cellular processes using bacterial proteins that are delivered into the host cell by specialized secretion systems (Isberg et al., 2009; Hubber and Roy, 2010). In this Research Topic, we present a collection of Review, Opinion, Perspective, and Primary research articles that present both the well-established and the newly discovered strategies used by Legionella to achieve this intracellular lifestyle while escaping from the host immune response.