Legionella pneumophila бактериясының сипаттамасы

Species of bacterium

Legionella pneumophila is an aerobic, pleomorphic, flagellated, non spore forming, Gram negative bacterium of the genus Legionella. L. pneumophila is the primary human pathogen in the genus Legionella. In nature, L. pneumophila infects soil amoebae of the genus Acanthamoeba and freshwater amoeboflagellates of the genus Naegleria. After initial discovery, it was found that Legionella replicates inside amoeba and amoeboflagellates, which suggests that bacteria replicating in environmental protozoa have the ability to exploit signaling pathways in human macrophages.

L. pneumophila is a coccobacillus. It is a Gram negative, aerobic bacterium unable to hydrolyse gelatin or produce urease. It is also non fermentative. L. pneumophila is neither pigmented nor does it autofluoresce. It is oxidase and catalase positive, and produces beta lactamase. L. pneumophila colony morphology is gray white with a textured, cut glass appearance; it also requires cysteine and iron to thrive. It grows on yeast extract agar as well as in moist environments, such as tap water, in "opal like" colonies.

While L. pneumophila is categorized as a Gram negative organism, it stains poorly due to its unique lipopolysaccharide content in the outer leaflet of the outer cell membrane. The bases for the somatic antigen specificity of this organism are located on the side chains of its cell wall. The chemical composition of these side chains both with respect to components and arrangement of the different sugars, determines the nature of the somatic or O antigenic determinants, which are important means of serologically classifying many Gram negative bacteria. At least 35 different serovars of L. pneumophila have been described, as well as several other species subdivided into a number of serovars.

L. pneumophila is a bacterium that can be found in numerous different environmental conditions. It can preside in temperatures ranging from 0 63 °C, a pH range of 5.0 8.5, and in dissolved oxygen levels of 0.2 15.0 mg/liter. In the environment, it can be found in freshwater environments within this wide range of temperatures. Although it can be found in this wider range, it only multiplies within a temperature of 25 and 42 °C. The main cause of L. pneumophila contamination is the water supply network and this has allowed L. pneumophila to grow and proliferate in places such as cooling towers, water systems of hospitals, hotels, and cruise ships.

Biofilms are specialized, surface attachment communities that can consist of one microbe, or multiple different ones, ranging from bacteria, algae, and protozoa. Biofilms on plumbing systems and in water distribution systems is where a lot of L. pneumophila can survive. Material also plays a role in biofilm proliferation. In water piping, L. pneumophila was more commonly found in plastic pipes at 40 °C, rather than a copper pipe, which actually inhibited growth. This bacterium can infect and survive within protozoa genera such as Acanthamoeba, Vermamoeba and Naegleria. The bacteria are surrounded by an resistant cyst while residing inside the host cell, allowing them to survive harsh environmental conditions such as chlorine, which is commonly used in water treatment systems. Although it is known that protozoa play an important role in the ecology of L. pneumophila, there is little data on how these organisms interact with other microbes in the environment, and how this affects L. pneumophila overall. Roughly 2 per 100,000 people are infected each year in the EU. L. pneumophila often infects individuals through poor quality water sources. Approximately 20% of reported Legionnaires disease cases come from healthcare, senior living, or travel facilities that have been exposed to water contaminated with L. pneumophila. Several large outbreaks of Legionnaire's Disease have come from public hot tubs due to the temperature range of the water being ideal for Legionella growth.

In humans, L. pneumophila invades and replicates inside macrophages. The internalization of the bacteria can be enhanced by the presence of antibody and complement, but is not absolutely required. Internalization of the bacteria appears to occur through phagocytosis. However, L. pneumophila is also capable of infecting non phagocytic cells through an unknown mechanism. A rare form of phagocytosis known as coiling phagocytosis has been described for L. pneumophila, but this is not dependent on the Dot/Icm (defect in organelle trafficking/intracellular multiplication genes) bacterial secretion system and has been observed for other pathogens. Once internalized, the Dot/Icm system begins secreting bacterial effector proteins that recruit host factors to the Legionella containing vacuole (LCV) transforming the membrane into one that is indistinguishable from the ER membrane. This process prevents the LCV from fusing with the lysosomes that would otherwise degrade the bacteria. It's within the LCV that bacterial replication occurs. LCV membrane integrity requires a steady supply of host lipids. Studies have shown that that cellular cholesterol and the cis monounsaturated fatty acid, palmitoleic acid, are important host derived lipids that promote expansion of the LCV membrane.

L. pneumophila exhibits a unique lipopolysaccharide (LPS) structure that is highly hydrophobic due to its being densely packed with branched fatty acids, and elevated levels of O acetyl and N acetyl groups. This structure helps prevent interaction with a common LPS immune system co receptor, CD14. L. pneumophila possesses a singular, polar flagellum that is used for cell motility, adhesion, host invasion, and biofilm formation. which are secreted by the Dot/Icm translocation system to interfere with host cell processes to aid bacterial survival. It has been predicted that the genus Legionella encodes more than 10,000 and possibly up to ~18,000 effectors that have a high probability to be secreted into their host cells. One main way in which L. pneumophila uses its effector proteins is to interfere with fusion of the Legionella containing vacuole with the host's endosomes, and thus protect against lysis. Knock out studies of Dot/Icm translocated effectors indicate that they are vital for the intracellular survival of the bacterium, but many individual effector proteins are thought to function redundantly, in that single effector knock outs rarely impede intracellular survival. This high number of translocated effector proteins and their redundancy is likely a result of the bacterium having evolved in many different protozoan hosts.

For Legionella to survive within macrophages and protozoa, it must create a specialized compartment known as the Legionella containing vacuole (LCV). Through the action of the Dot/Icm secretion system, the bacteria are able to prevent degradation by the normal endosomal trafficking pathway and instead replicate. Shortly after internalization, the bacteria specifically recruit endoplasmic reticulum derived vesicles and mitochondria to the LCV while preventing the recruitment of endosomal markers such as Rab5a and Rab7a. Formation and maintenance of the vacuoles are crucial for pathogenesis; bacteria lacking the Dot/Icm secretion system are not pathogenic and cannot replicate within cells, while deletion of the Dot/Icm effector SdhA results in destabilization of the vacuolar membrane and no bacterial replication.

L. pneumophila uses glycolysis, the Entner Doudoroff (ED) pathway, the pentose phosphate pathway (PP), and the citric acid cycle (TCA). Although L. pneumophila can also perform gluconeogenesis, it does not have the genes to encode for 1,6 biphosphatases. Therefore, other enzymes are used to complete gluconeogenesis. One enzyme used instead is fructose 6 phosphate aldolase. This generates an excess of free amino acids in the cytoplasm of L. pneumophila infected cells that can be used for intravacuolar proliferation of the parasite. The K48 linked polyubiquitination is a marker for proteasomal degradation that releases 2 to 24 amino acid long peptides, which are quickly degraded to amino acids by various oligopeptidases and aminopeptidases present in the cytoplasm. Amino acids are imported into the LCV through various amino acid transporters such as the neutral amino acid transporter B(0). However, promotion of proteasomal degradation for the obtention of amino acids and the hydrolyzation of polysaccharides may not be the only virulence strategies to obtain carbon and energy sources from the host. Type II–secreted degradative enzymes may provide an additional strategy to generate carbon and energy sources. L. pneumophila is the only known intracellular pathogen to have a Type II Secretion System (secretome). In Type II Secretion, proteins are first translocated across the inner membrane into the periplasmic space. This process is mediated by either the Sec or Tat pathway. Soon after, the same proteins are then transported through a specific pore in the outer membrane to the exterior of the cell. This secretome is believed to have as many as 60 proteins incorporated into the system. These four strains resemble a very similar size, with strain Lens around 3.3 Mb and strain Paris and Corby about 3.5 Mb. This larger size reflects a higher number of genes, corresponding with the ability of Legionella to adapt to different hosts and environments. In all four strains of L. pneumophila, there is a relatively high abundance of eukaryotic like proteins (ELPs), suggesting that these ELPs are beneficial for their everyday function. In fact, many of these ELPs are predicted to benefit the pathogen in modulating the host cell. In depth comparative genome analysis using DNA arrays to study the gene content of 180 Legionella strains revealed high genomic plasticity and frequent horizontal gene transfer events. Further insight in the L. pneumophila lifecycle was gained by investigating the gene expression profile of L. pneumophila in Acanthamoeba castellanii, a common host species. L. pneumophila exhibits a biphasic lifecycle and defines transmissive and replicative traits according to gene expression profiles. In order for a bacterium to bind, take up, and recombine exogenous DNA into its chromosome, it must enter a special physiological state referred to as "competence". To determine which molecules may induce competence in L. pneumophila, 64 toxic molecules were tested. Only six of these molecules, all DNA damaging agents, caused strong induction of competence. These were mitomycin C (which introduces DNA inter strand crosslinks), norfloxacin, ofloxacin, and nalidixic acid (inhibitors of DNA gyrase that cause double strand breaks), bicyclomycin (causes double strand breaks), and hydroxyurea (causes oxidation of DNA bases). These results suggest that competence for transformation in L. pneumophila evolved as a response to DNA damage. Further, a cN II class of IMP/GMP specific 5´ nucleotidase which has been extensively characterized kinetically. The tetrameric enzyme shows aspects of positive homotropic cooperativity, substrate activation and presents a unique allosteric site that can be targeted to design effective drugs against the enzyme and thus, the organism. Moreover, the enzyme is distinct than its human counterpart making it an attractive target for drug development.

Antisera have been used both for slide agglutination studies and for direct detection of bacteria in tissues using immunofluorescence via fluorescent labelled antibody. Specific antibody in patients can be determined by the indirect fluorescent antibody test. ELISA and microagglutination tests have also been successfully applied. Legionella stains poorly with Gram stain, stains positive with silver, and is cultured on charcoal yeast extract with iron and cysteine. A consistent method that has been used to detect the disease is the urine antigen test. There has been no significant difference found between using a fluoroquinolone or azithromycin to treat Legionella pneumonia. Levofloxacin and azithromycin have great intracellular activity and are able to penetrate into Legionella infected cells. The Infectious Diseases Society of America recommends 5–10 days of treatment with levofloxacin or 3–5 days of treatment with azithromycin, however patients that are immunocompromised or have a severe disease may require an extended course of treatment.