Legionella is a class of aerobic Gram-negative bacilli, widely distributed in natural fresh water, artificial water supply systems (such as bathers, fountains, cooling water towers), air conditioning refrigeration units, vehicle wiper tanks and unicellular protozoan cells. At present, more than 60 strains of this genus have been found, and more than half of them can be pathogenic to humans, among which Legionella pneumophila is the most virulent. Human Legionnaires’ disease, also known as Legionnaires’ disease, is caused by inhalation of aerosols containing Legionella pneumophila in immunocompromised people.
Legionnaires’ disease is a fatal acute febrile respiratory infectious disease, named after the first outbreak in 1976 during a veterans convention held in Philadelphia, USA. Since then, there have been several small-scale outbreaks around the world. According to the clinical manifestations of patients, Legionnaires’ disease can be divided into two types: Pontiac fever and Legionella pneumonia. Pontiac fever is a non-pneumonic type of Legionnaires’ disease, mainly manifested as acute influenza syndrome, without obvious pulmonary lesions, and usually resolves spontaneously within 3 to 5 days. Legionella pneumonia is an acute systemic disease characterized by lung lesions (such as suppurative bronchitis, lobar pneumonia with small abscess formation), which can be spread by lung migration into the blood, resulting in skin, muscle, digestive, nerve and other multi-system damage. The rapid onset and high fatality rate of Legionella pneumonia have attracted widespread attention in the medical community. Intracellular survival mechanism of Legionella pneumophila Legionella pneumophila can survive for several months or even a year in sewage with less nutrients, and it is easy to interact with other microorganisms in the water to form a mixed bacterial biofilm. At this time, Legionella pneumophila has a strong viability, but a relatively weak ability to infect host cells, and is in a “viable but non-culturable” (VBNC) state. Free-living amoeba (FLA) can feed on bacteria and is called the “Trojan horse” of the microbial world; while anti-amebic bacteria can survive and proliferate in FLA cells, causing the spread and spread of human pathogenic bacteria. Legionella pneumophila is a typical facultative intracellular bacteria and anti-amebic bacteria, which can survive and proliferate in its cells through the phagocytosis of FLA (such as Acanthamoeba, Neglia flexneri). In addition to the intracellular parasitism and proliferation of unicellular primitive animals such as amoeba, Legionella can also infect mammalian alveolar epithelial cells (epithelial cells) and mononuclear macrophages (macrophages), and proliferate in them. After being preyed on by amoeba, Legionella pneumophila is quickly enclosed in phagocytic vesicles surrounded by mitochondria and cellular vesicles. This structure containing Legionella pneumophila vesicle bodies is called Legionella-containing vacuole (LCV). The effector proteins secreted by Legionella pneumophila can regulate the intracellular transport pathways and the internal and external environment of the host. At the initial stage of phagocytosis, the surface of LCV will not be acidified because of the host mitochondrial and ribosomal components, and at the same time, proteins transported by endoplasmic reticulum vesicles are recruited and linked to it. In the final stage of phagocytosis, the LCV membrane merges into the rough endoplasmic reticulum and is surrounded by multiple layers of rough endoplasmic reticulum membranes, forming an endosome blocking phagosome (EMB). The formation of EMB further inhibits the fusion of LCV with lysosomes and avoids being degraded and digested. In the early stage of infection, under the action of effector proteins, Legionella pneumophila interferes with the cell cycle of the host, inhibits the apoptosis of host cells, and creates an ideal intracellular proliferation environment, which helps Legionella to escape and proliferate from host cells. In the later stage of infection, as Legionella pneumophila continuously consumes host nutrients and proliferates in EMB, the phagocytic vesicle eventually bursts. Certain effector proteins secreted by Legionella pneumophila can induce host cell apoptosis, causing the host cell to lyse and die, thereby being successfully released from the host cell.
The Pathogenic Substance of Legionella Pneumophila
Enzymes (peptidyl prolyl isomerase, phosphatase, DNase, proteolytic enzyme, etc.), toxins (endotoxins, cytotoxins, etc.) and hemolysins produced by Legionella pneumophila can cause host cell damage. Among them, peptidyl prolyl isomerase can enhance the ability to infect macrophages; phosphatase can inhibit the production of superoxide anion in neutrophils, and the latter can participate in physiological functions such as immune response as a second messenger; Cytotoxins can affect the oxidative metabolism of neutrophils; lipopolysaccharides can promote the adhesion of L. pneumophila to host cells and have pro-inflammatory effects, causing lung infections. These pathogenic substances produced by Legionella can also antagonize the host’s immune killing, allowing it to survive and proliferate in phagocytic vesicles, ultimately leading to host cell death. In addition to causing the same physiological and pathological reactions as Gram-negative endotoxins such as high fever, hypotension, disseminated intravascular coagulation, and endotoxin shock, the endotoxin of Legionella pneumophila can also activate the complement system to promote the entry of Legionella pneumophila into the mononuclear giant. In addition, the adhesion of fimbriae and the anti-phagocytosis of microcapsules are also involved in its pathogenesis. The survival of L. pneumophila in host cells mainly depends on its unique virulence secretion system to inject secreted effector proteins into the host cells. Legionella pneumophila has I, II, IVA, IVB and V secretion systems, among which the IVB secretion system (T4BSS) plays a crucial role in the process of infecting the host. The T4BSS secretion system, also known as the Dot/Icm secretion system, is mainly composed of two pathogenic regions (a total of 7 genes and 18 Icm). Dot stands for defective organelle trafficking gene, and Icm stands for intracellular reproduction gene. The Dot/Icm IVB secretion system of Legionella pneumophila is its main virulence system, which can inject more than 300 effector proteins into the host cells, and these proteins play important biological functions in the process of infecting the host.