Although closely related to F. tularensis, F. novicida is designated a separate subspecies with a lower prevalence of human and animal infections. F. novicida & F. tularensis have more than 85% DNA-DNA relatedness, demonstrating genetic closeness. F. novicida, on the other hand, is rarely isolated, and there are few papers detailing infections caused by this subspecies.
Infections with F. novicida in humans or animals are uncommon. However, isolated cases have been documented in the United States & Argentina. The precise path of infection for most F. novicida infections is unknown, while contaminated water or ice has been suggested as a probable cause. Tularemia is endemic in many parts of the world, caused by both F. novicida & F. tularensis.
Tularemia is endemic in most states in the United States, notably in the south-central & western regions. Endemic locations in Europe include Scandinavia, Turkey, Russia, & certain Balkan countries. Tularemia is also endemic in Asia, including China, Japan, Mongolia, & certain Central Asian countries. Tularemia prevalence varies by geographic region and population group and is determined by environmental factors & host exposure to F. tularensis.
Tularemia can be transmitted to humans by a variety of mechanisms. It includes tick and deer fly bites, direct contact with infected animals (rodents, hares, rabbits, or cats), consuming contaminated food or drink from infected animals or plants, and inhaling aerosols from infected animals or soil. The specific sources of infection may differ depending on geographical place and season.
The mortality rate linked with severe type A F. tularensis strains has been observed to range between 5% and 15%. Without antibiotic treatment, the death rate in pulmonary or septicemic tularemia cases might be as high as 30% to 60%. Type B strains, including F. novicida, have a reduced mortality rate from 0% to 4%. The mortality rate for F. novicida infection varies between 0% and 33%.
The WHO recorded 10,690 cases of tularemia globally in 2019, most occurring in Europe. However, it is crucial to highlight that this figure may be underestimated due to underreporting or a lack of tularemia surveillance systems in many countries. There were 230 documented cases of tularemia in the United States in 2019, fluctuating over time.
Kingdom: Bacteria
Phylum: Proteobacteria
Class: Gammaproteobacteria
Order: Thiotrichales
Family: Francisellaceae
Genus:Francisella
Species:Francisella novicida
Francisella novicida is a small rod-shaped bacterium measuring approximately 0.2 by 0.2 µm. The bacterium is covered by a capsule-like coat with well-defined borders, providing additional protection.
F. novicida has an outer membrane containing lipid A, a lipopolysaccharide (LPS) component, an essential component of Gram-negative bacteria. The biosynthesis of lipid A in F. novicida involves multiple enzymes, including UDP-N-acetylglucosamine acyltransferase (LpxA), which catalyzes the first step of the pathway.
The crystal structure of F. novicida LpxA (FnLpxA) has been determined at 2.06 Å resolution. It consists of two domains: an N-terminal parallel beta-helix (LβH) domain and a C-terminal alpha-helical domain.
The LβH domain of FnLpxA contains the active site where catalysis occurs. It binds to the substrates UDP-GlcNAc (uridine diphosphate N-acetylglucosamine) and acyl-ACP (acyl carrier protein).
The C-terminal domain of FnLpxA stabilizes the enzyme and forms a homotrimeric structure, contributing to the overall stability of the protein.
The Francisella Pathogenicity Island (FPI) is a genomic region that contains 16 to 19 genes. It encodes a type VI secretion system (T6SS) and other proteins required for intracellular growth, phagosome escape, and host cell death. The FPI contains essential genes such as iglB, iglA, iglC, iglD, pdpB, pdpA, pdpC, pdpE, pdpD, dotU, & vgrG.
FopA is a new surface protein found only in F. novicida. Other Francisella species, like F. tularensis & F. philomiragia, lack it. FopA has been shown in studies to elicit a protective immunological response in mice, implying a function in virulence & immune evasion.
When macrophages are infected, the 23-kDa protein is enormously elevated. It is expressed by a gene close to the FPI & is part of a putative operon that also contains genes for a capsule biosynthesis protein (capB), a lipoprotein (lpp3), & a hypothetical protein (FTN_1138).
FTT0918 (58-kDa lipoprotein): The FTT0918 gene encodes this lipoprotein. Its precise role in F. novicida virulence is unknown. However, lipoproteins have been linked to various pathogenic activities in bacteria.
F. novicida is resistant to the oxidative damage caused by the host immune system. This resistance is related to multiple genes regulating antioxidant enzyme production, DNA repair, and genome integrity. Among these genes are oxyR, uvrB, recB, & ruvC1.
Several strains of F. novicida have been identified with varying degrees of virulence. These include:
Wild type U112– It was isolated from a water sample in Utah, USA.
Mutant XWK4 and Fx1– Obtained from a human wound infection in Texas, USA.
F. novicida3523– A clinical isolate of F. novicida recovered from a human blood culture in Australia.
F. novicidaF6168– An environmental isolate of F. novicida collected from a soil sample in Alaska, USA.
Francisella novicida is an intracellular parasite that can infect various host cell types, such as macrophages, neutrophils, dendritic cells, & epithelial cells. Its ability to infiltrate and proliferate within host cells is critical to its pathogenesis.
When F. novicida enters the host, it can infect cells by phagocytosis or endocytosis. Once within the cell, the bacterium can escape from the phagosome or endosome, allowing it to multiply in the cytosol. The ability of F. novicida to replicate within host cells contributes to its ability to elude the immune system, as the bacterium can hide within the host cell’s protective environment.
F. novicida causes cell death as it multiplies within infected cells, releasing additional germs into the surrounding environment. This mechanism aids the bacterium’s spread to different cells and tissues. F. novicida can quickly spread via the bloodstream, infecting many tissues and organs.
F. novicida‘s intracellular proliferation also contributes to its ability to escape the immune system. F. novicida can avoid identification by immune cells & the ensuing immunological response by living within host cells. Because of this evasion, the bacterium can establish a persistent infection that leads to disease progression.
F. novicida infection can induce systemic inflammation & organ damage, most notably in the liver, spleen, & lungs. The severity of the infection might vary based on factors like the bacterial dose and method of entry, the individual strain of F. novicida, and the host’s immunological condition. F. novicida can cross the blood-brain barrier & cause meningitis or encephalitis in extreme cases.
The innate immune system, which involves diverse molecular and cellular mechanisms to identify & remove intracellular bacteria, is the primary host defense against Francisella novicida infection.
The inflammasome is a multiple proteins complex that identifies cytosolic infections or danger signals & activates caspase-1, causing the pro-inflammatory cytokines IL-1β and IL-18 to be released. Infection with F. novicida activates the inflammasome via receptors like AIM2, NLRP3, & NLRC4. Inflammasome activation is critical for host defense against F. novicida since mice or macrophages lacking inflammasome pathway components (caspase-1, ASC, or IL-1β) are more vulnerable to the bacteria.
Interferons (IFNs) are signaling molecules that promote the expression of several antiviral & antibacterial genes. Infection with F. novicida increases the synthesis of type I interferons (IFN-α and IFN-β) and type II interferons (IFN-γ) by various cell types, namely dendritic cells, macrophages, and T cells.
The interferon signaling pathway is critical in host defense against F. novicida because mice or macrophages lacking IFN receptors (IFN-α/β or IFN-γ receptor) or STAT1 (a key modulator of IFN signaling) are more vulnerable to the bacteria. During F. novicida infection, interferon signaling also influences inflammasome activity.
Autophagy and Lysosomal Biogenesis: Lysosomes are cellular organelles that contain acidic hydrolases that break down intracellular molecules. Autophagy is the process by which cytoplasmic components or pathogens are sent to lysosomes for destruction.
F. novicida infection promotes lysosomal biogenesis and autophagy by inhibiting the activity of the mechanistic target of rapamycin (mTOR) & activating the transcription factors EB (TFEB) & ULK1. Lysosomal biogenesis & autophagy are advantageous for host defense against F. novicida since mice or macrophages lacking TFEB or ULK1 are more sensitive to the bacteria.
Francisella novicida, although considered a rare pathogen, shares similarities with its close relative, Francisella tularensis, which is well-known for causing tularemia. Tularemia is a bacterial infection that can present with various clinical manifestations depending on the disease’s form and the transmission route.
Typhoidal tularemia is a kind of tularemia caused by F. tularensis and, on rare occasions, F. novicida. This type is infrequent and can cause non-specific symptoms like fever, headache, muscle aches, nausea, vomiting, abdominal pain, & exhaustion. With proper laboratory testing, these symptoms may be consistent with those of other infections, making identification easier.
The most prevalent manifestation of tularemia is termed ulceroglandular tularemia. It usually happens because of a tick, insect bite, or bodily contact with an infected animal. The most common symptom is the formation of a skin ulcer at the site of infection, accompanied by swollen & painful lymph nodes. Fever, chills, headache, muscle aches, & weariness are all possible symptoms.
Tularemia can cause pneumonia in severe cases, causing respiratory symptoms like coughing, chest pain, & difficulty breathing. If left untreated, this version of the disease can be fatal.
It’s important to note that cases of F. novicida or F. novicida-like infections are predominantly observed in immunocompromised individuals. These individuals may be more susceptible to severe or atypical manifestations of the infection due to their compromised immune systems.
Culture: Culture is the gold standard for identifying F. novicida and other Francisella species. It involves growing the bacteria on specific media, such as cysteine-glucose-blood agar (CGBA). F. novicida can typically be cultured within 24 hours, while F. tularensis may require several days. F. novicida grows in tiny colonies that are smooth and translucent. Typically, the colonies are non-pigmented or pale white to gray in hue. They are spherical and have a diameter of 1 to 2 millimeters. F. novicida colonies are moist and slightly convex, with a distinct boundary.
Serology: Serological tests detect antibodies against F. novicida and other Francisella species in the serum of infected individuals. These tests are beneficial for retrospective diagnosis or epidemiological studies. Various serological techniques can be employed, such as ELISA, MAT, or TAT. The antigens used for serology include lipopolysaccharide (LPS), capsule antigen (CA), outer membrane protein A (OmpA), or heat shock protein 60 (Hsp60). It is important to note that serology has limitations, including lower sensitivity and specificity during the acute phase of infection.
PCR assay: Polymerase Chain Reaction is a molecular diagnostic method that amplifies specific DNA sequences of F. novicida and other Francisella species. It is a sensitive and rapid technique that can provide results faster than culture. PCR can also differentiate F. novicida from F. tularensis by targeting unique or variable genes between the two species.
Lateral Flow Assay: These are rapid and simple diagnostic tests that utilize CRISPR technology. FnCas9, an enzyme associated with CRISPR, can recognize and cleave specific DNA sequences. Lateral flow assays use FnCas9 and guide RNA to target specific regions of the Francisella genome and generate a colorimetric signal that can be visualized on a paper strip. This method offers quick and point-of-care detection of F. novicida and other Francisella species.
Avoid contact with potentially contaminated water, ice, soil, or animals, especially rodents, rabbits, or hares. It can help minimize the risk of acquiring F. novicida infection. Wear protective clothing, gloves, and insect repellents when working or traveling in areas where F. novicida or other Francisella species may be endemic.
Practice good personal hygiene, like regular handwashing with soap and water, especially after handling animals or environments where F. novicida may be present.
Although closely related to F. tularensis, F. novicida is designated a separate subspecies with a lower prevalence of human and animal infections. F. novicida & F. tularensis have more than 85% DNA-DNA relatedness, demonstrating genetic closeness. F. novicida, on the other hand, is rarely isolated, and there are few papers detailing infections caused by this subspecies.
Infections with F. novicida in humans or animals are uncommon. However, isolated cases have been documented in the United States & Argentina. The precise path of infection for most F. novicida infections is unknown, while contaminated water or ice has been suggested as a probable cause. Tularemia is endemic in many parts of the world, caused by both F. novicida & F. tularensis.
Tularemia is endemic in most states in the United States, notably in the south-central & western regions. Endemic locations in Europe include Scandinavia, Turkey, Russia, & certain Balkan countries. Tularemia is also endemic in Asia, including China, Japan, Mongolia, & certain Central Asian countries. Tularemia prevalence varies by geographic region and population group and is determined by environmental factors & host exposure to F. tularensis.
Tularemia can be transmitted to humans by a variety of mechanisms. It includes tick and deer fly bites, direct contact with infected animals (rodents, hares, rabbits, or cats), consuming contaminated food or drink from infected animals or plants, and inhaling aerosols from infected animals or soil. The specific sources of infection may differ depending on geographical place and season.
The mortality rate linked with severe type A F. tularensis strains has been observed to range between 5% and 15%. Without antibiotic treatment, the death rate in pulmonary or septicemic tularemia cases might be as high as 30% to 60%. Type B strains, including F. novicida, have a reduced mortality rate from 0% to 4%. The mortality rate for F. novicida infection varies between 0% and 33%.
The WHO recorded 10,690 cases of tularemia globally in 2019, most occurring in Europe. However, it is crucial to highlight that this figure may be underestimated due to underreporting or a lack of tularemia surveillance systems in many countries. There were 230 documented cases of tularemia in the United States in 2019, fluctuating over time.
Kingdom: Bacteria
Phylum: Proteobacteria
Class: Gammaproteobacteria
Order: Thiotrichales
Family: Francisellaceae
Genus:Francisella
Species:Francisella novicida
Francisella novicida is a small rod-shaped bacterium measuring approximately 0.2 by 0.2 µm. The bacterium is covered by a capsule-like coat with well-defined borders, providing additional protection.
F. novicida has an outer membrane containing lipid A, a lipopolysaccharide (LPS) component, an essential component of Gram-negative bacteria. The biosynthesis of lipid A in F. novicida involves multiple enzymes, including UDP-N-acetylglucosamine acyltransferase (LpxA), which catalyzes the first step of the pathway.
The crystal structure of F. novicida LpxA (FnLpxA) has been determined at 2.06 Å resolution. It consists of two domains: an N-terminal parallel beta-helix (LβH) domain and a C-terminal alpha-helical domain.
The LβH domain of FnLpxA contains the active site where catalysis occurs. It binds to the substrates UDP-GlcNAc (uridine diphosphate N-acetylglucosamine) and acyl-ACP (acyl carrier protein).
The C-terminal domain of FnLpxA stabilizes the enzyme and forms a homotrimeric structure, contributing to the overall stability of the protein.
The Francisella Pathogenicity Island (FPI) is a genomic region that contains 16 to 19 genes. It encodes a type VI secretion system (T6SS) and other proteins required for intracellular growth, phagosome escape, and host cell death. The FPI contains essential genes such as iglB, iglA, iglC, iglD, pdpB, pdpA, pdpC, pdpE, pdpD, dotU, & vgrG.
FopA is a new surface protein found only in F. novicida. Other Francisella species, like F. tularensis & F. philomiragia, lack it. FopA has been shown in studies to elicit a protective immunological response in mice, implying a function in virulence & immune evasion.
When macrophages are infected, the 23-kDa protein is enormously elevated. It is expressed by a gene close to the FPI & is part of a putative operon that also contains genes for a capsule biosynthesis protein (capB), a lipoprotein (lpp3), & a hypothetical protein (FTN_1138).
FTT0918 (58-kDa lipoprotein): The FTT0918 gene encodes this lipoprotein. Its precise role in F. novicida virulence is unknown. However, lipoproteins have been linked to various pathogenic activities in bacteria.
F. novicida is resistant to the oxidative damage caused by the host immune system. This resistance is related to multiple genes regulating antioxidant enzyme production, DNA repair, and genome integrity. Among these genes are oxyR, uvrB, recB, & ruvC1.
Several strains of F. novicida have been identified with varying degrees of virulence. These include:
Wild type U112– It was isolated from a water sample in Utah, USA.
Mutant XWK4 and Fx1– Obtained from a human wound infection in Texas, USA.
F. novicida3523– A clinical isolate of F. novicida recovered from a human blood culture in Australia.
F. novicidaF6168– An environmental isolate of F. novicida collected from a soil sample in Alaska, USA.
Francisella novicida is an intracellular parasite that can infect various host cell types, such as macrophages, neutrophils, dendritic cells, & epithelial cells. Its ability to infiltrate and proliferate within host cells is critical to its pathogenesis.
When F. novicida enters the host, it can infect cells by phagocytosis or endocytosis. Once within the cell, the bacterium can escape from the phagosome or endosome, allowing it to multiply in the cytosol. The ability of F. novicida to replicate within host cells contributes to its ability to elude the immune system, as the bacterium can hide within the host cell’s protective environment.
F. novicida causes cell death as it multiplies within infected cells, releasing additional germs into the surrounding environment. This mechanism aids the bacterium’s spread to different cells and tissues. F. novicida can quickly spread via the bloodstream, infecting many tissues and organs.
F. novicida‘s intracellular proliferation also contributes to its ability to escape the immune system. F. novicida can avoid identification by immune cells & the ensuing immunological response by living within host cells. Because of this evasion, the bacterium can establish a persistent infection that leads to disease progression.
F. novicida infection can induce systemic inflammation & organ damage, most notably in the liver, spleen, & lungs. The severity of the infection might vary based on factors like the bacterial dose and method of entry, the individual strain of F. novicida, and the host’s immunological condition. F. novicida can cross the blood-brain barrier & cause meningitis or encephalitis in extreme cases.
The innate immune system, which involves diverse molecular and cellular mechanisms to identify & remove intracellular bacteria, is the primary host defense against Francisella novicida infection.
The inflammasome is a multiple proteins complex that identifies cytosolic infections or danger signals & activates caspase-1, causing the pro-inflammatory cytokines IL-1β and IL-18 to be released. Infection with F. novicida activates the inflammasome via receptors like AIM2, NLRP3, & NLRC4. Inflammasome activation is critical for host defense against F. novicida since mice or macrophages lacking inflammasome pathway components (caspase-1, ASC, or IL-1β) are more vulnerable to the bacteria.
Interferons (IFNs) are signaling molecules that promote the expression of several antiviral & antibacterial genes. Infection with F. novicida increases the synthesis of type I interferons (IFN-α and IFN-β) and type II interferons (IFN-γ) by various cell types, namely dendritic cells, macrophages, and T cells.
The interferon signaling pathway is critical in host defense against F. novicida because mice or macrophages lacking IFN receptors (IFN-α/β or IFN-γ receptor) or STAT1 (a key modulator of IFN signaling) are more vulnerable to the bacteria. During F. novicida infection, interferon signaling also influences inflammasome activity.
Autophagy and Lysosomal Biogenesis: Lysosomes are cellular organelles that contain acidic hydrolases that break down intracellular molecules. Autophagy is the process by which cytoplasmic components or pathogens are sent to lysosomes for destruction.
F. novicida infection promotes lysosomal biogenesis and autophagy by inhibiting the activity of the mechanistic target of rapamycin (mTOR) & activating the transcription factors EB (TFEB) & ULK1. Lysosomal biogenesis & autophagy are advantageous for host defense against F. novicida since mice or macrophages lacking TFEB or ULK1 are more sensitive to the bacteria.
Francisella novicida, although considered a rare pathogen, shares similarities with its close relative, Francisella tularensis, which is well-known for causing tularemia. Tularemia is a bacterial infection that can present with various clinical manifestations depending on the disease’s form and the transmission route.
Typhoidal tularemia is a kind of tularemia caused by F. tularensis and, on rare occasions, F. novicida. This type is infrequent and can cause non-specific symptoms like fever, headache, muscle aches, nausea, vomiting, abdominal pain, & exhaustion. With proper laboratory testing, these symptoms may be consistent with those of other infections, making identification easier.
The most prevalent manifestation of tularemia is termed ulceroglandular tularemia. It usually happens because of a tick, insect bite, or bodily contact with an infected animal. The most common symptom is the formation of a skin ulcer at the site of infection, accompanied by swollen & painful lymph nodes. Fever, chills, headache, muscle aches, & weariness are all possible symptoms.
Tularemia can cause pneumonia in severe cases, causing respiratory symptoms like coughing, chest pain, & difficulty breathing. If left untreated, this version of the disease can be fatal.
It’s important to note that cases of F. novicida or F. novicida-like infections are predominantly observed in immunocompromised individuals. These individuals may be more susceptible to severe or atypical manifestations of the infection due to their compromised immune systems.
Culture: Culture is the gold standard for identifying F. novicida and other Francisella species. It involves growing the bacteria on specific media, such as cysteine-glucose-blood agar (CGBA). F. novicida can typically be cultured within 24 hours, while F. tularensis may require several days. F. novicida grows in tiny colonies that are smooth and translucent. Typically, the colonies are non-pigmented or pale white to gray in hue. They are spherical and have a diameter of 1 to 2 millimeters. F. novicida colonies are moist and slightly convex, with a distinct boundary.
Serology: Serological tests detect antibodies against F. novicida and other Francisella species in the serum of infected individuals. These tests are beneficial for retrospective diagnosis or epidemiological studies. Various serological techniques can be employed, such as ELISA, MAT, or TAT. The antigens used for serology include lipopolysaccharide (LPS), capsule antigen (CA), outer membrane protein A (OmpA), or heat shock protein 60 (Hsp60). It is important to note that serology has limitations, including lower sensitivity and specificity during the acute phase of infection.
PCR assay: Polymerase Chain Reaction is a molecular diagnostic method that amplifies specific DNA sequences of F. novicida and other Francisella species. It is a sensitive and rapid technique that can provide results faster than culture. PCR can also differentiate F. novicida from F. tularensis by targeting unique or variable genes between the two species.
Lateral Flow Assay: These are rapid and simple diagnostic tests that utilize CRISPR technology. FnCas9, an enzyme associated with CRISPR, can recognize and cleave specific DNA sequences. Lateral flow assays use FnCas9 and guide RNA to target specific regions of the Francisella genome and generate a colorimetric signal that can be visualized on a paper strip. This method offers quick and point-of-care detection of F. novicida and other Francisella species.
Avoid contact with potentially contaminated water, ice, soil, or animals, especially rodents, rabbits, or hares. It can help minimize the risk of acquiring F. novicida infection. Wear protective clothing, gloves, and insect repellents when working or traveling in areas where F. novicida or other Francisella species may be endemic.
Practice good personal hygiene, like regular handwashing with soap and water, especially after handling animals or environments where F. novicida may be present.
Although closely related to F. tularensis, F. novicida is designated a separate subspecies with a lower prevalence of human and animal infections. F. novicida & F. tularensis have more than 85% DNA-DNA relatedness, demonstrating genetic closeness. F. novicida, on the other hand, is rarely isolated, and there are few papers detailing infections caused by this subspecies.
Infections with F. novicida in humans or animals are uncommon. However, isolated cases have been documented in the United States & Argentina. The precise path of infection for most F. novicida infections is unknown, while contaminated water or ice has been suggested as a probable cause. Tularemia is endemic in many parts of the world, caused by both F. novicida & F. tularensis.
Tularemia is endemic in most states in the United States, notably in the south-central & western regions. Endemic locations in Europe include Scandinavia, Turkey, Russia, & certain Balkan countries. Tularemia is also endemic in Asia, including China, Japan, Mongolia, & certain Central Asian countries. Tularemia prevalence varies by geographic region and population group and is determined by environmental factors & host exposure to F. tularensis.
Tularemia can be transmitted to humans by a variety of mechanisms. It includes tick and deer fly bites, direct contact with infected animals (rodents, hares, rabbits, or cats), consuming contaminated food or drink from infected animals or plants, and inhaling aerosols from infected animals or soil. The specific sources of infection may differ depending on geographical place and season.
The mortality rate linked with severe type A F. tularensis strains has been observed to range between 5% and 15%. Without antibiotic treatment, the death rate in pulmonary or septicemic tularemia cases might be as high as 30% to 60%. Type B strains, including F. novicida, have a reduced mortality rate from 0% to 4%. The mortality rate for F. novicida infection varies between 0% and 33%.
The WHO recorded 10,690 cases of tularemia globally in 2019, most occurring in Europe. However, it is crucial to highlight that this figure may be underestimated due to underreporting or a lack of tularemia surveillance systems in many countries. There were 230 documented cases of tularemia in the United States in 2019, fluctuating over time.
Kingdom: Bacteria
Phylum: Proteobacteria
Class: Gammaproteobacteria
Order: Thiotrichales
Family: Francisellaceae
Genus:Francisella
Species:Francisella novicida
Francisella novicida is a small rod-shaped bacterium measuring approximately 0.2 by 0.2 µm. The bacterium is covered by a capsule-like coat with well-defined borders, providing additional protection.
F. novicida has an outer membrane containing lipid A, a lipopolysaccharide (LPS) component, an essential component of Gram-negative bacteria. The biosynthesis of lipid A in F. novicida involves multiple enzymes, including UDP-N-acetylglucosamine acyltransferase (LpxA), which catalyzes the first step of the pathway.
The crystal structure of F. novicida LpxA (FnLpxA) has been determined at 2.06 Å resolution. It consists of two domains: an N-terminal parallel beta-helix (LβH) domain and a C-terminal alpha-helical domain.
The LβH domain of FnLpxA contains the active site where catalysis occurs. It binds to the substrates UDP-GlcNAc (uridine diphosphate N-acetylglucosamine) and acyl-ACP (acyl carrier protein).
The C-terminal domain of FnLpxA stabilizes the enzyme and forms a homotrimeric structure, contributing to the overall stability of the protein.
The Francisella Pathogenicity Island (FPI) is a genomic region that contains 16 to 19 genes. It encodes a type VI secretion system (T6SS) and other proteins required for intracellular growth, phagosome escape, and host cell death. The FPI contains essential genes such as iglB, iglA, iglC, iglD, pdpB, pdpA, pdpC, pdpE, pdpD, dotU, & vgrG.
FopA is a new surface protein found only in F. novicida. Other Francisella species, like F. tularensis & F. philomiragia, lack it. FopA has been shown in studies to elicit a protective immunological response in mice, implying a function in virulence & immune evasion.
When macrophages are infected, the 23-kDa protein is enormously elevated. It is expressed by a gene close to the FPI & is part of a putative operon that also contains genes for a capsule biosynthesis protein (capB), a lipoprotein (lpp3), & a hypothetical protein (FTN_1138).
FTT0918 (58-kDa lipoprotein): The FTT0918 gene encodes this lipoprotein. Its precise role in F. novicida virulence is unknown. However, lipoproteins have been linked to various pathogenic activities in bacteria.
F. novicida is resistant to the oxidative damage caused by the host immune system. This resistance is related to multiple genes regulating antioxidant enzyme production, DNA repair, and genome integrity. Among these genes are oxyR, uvrB, recB, & ruvC1.
Several strains of F. novicida have been identified with varying degrees of virulence. These include:
Wild type U112– It was isolated from a water sample in Utah, USA.
Mutant XWK4 and Fx1– Obtained from a human wound infection in Texas, USA.
F. novicida3523– A clinical isolate of F. novicida recovered from a human blood culture in Australia.
F. novicidaF6168– An environmental isolate of F. novicida collected from a soil sample in Alaska, USA.
Francisella novicida is an intracellular parasite that can infect various host cell types, such as macrophages, neutrophils, dendritic cells, & epithelial cells. Its ability to infiltrate and proliferate within host cells is critical to its pathogenesis.
When F. novicida enters the host, it can infect cells by phagocytosis or endocytosis. Once within the cell, the bacterium can escape from the phagosome or endosome, allowing it to multiply in the cytosol. The ability of F. novicida to replicate within host cells contributes to its ability to elude the immune system, as the bacterium can hide within the host cell’s protective environment.
F. novicida causes cell death as it multiplies within infected cells, releasing additional germs into the surrounding environment. This mechanism aids the bacterium’s spread to different cells and tissues. F. novicida can quickly spread via the bloodstream, infecting many tissues and organs.
F. novicida‘s intracellular proliferation also contributes to its ability to escape the immune system. F. novicida can avoid identification by immune cells & the ensuing immunological response by living within host cells. Because of this evasion, the bacterium can establish a persistent infection that leads to disease progression.
F. novicida infection can induce systemic inflammation & organ damage, most notably in the liver, spleen, & lungs. The severity of the infection might vary based on factors like the bacterial dose and method of entry, the individual strain of F. novicida, and the host’s immunological condition. F. novicida can cross the blood-brain barrier & cause meningitis or encephalitis in extreme cases.
The innate immune system, which involves diverse molecular and cellular mechanisms to identify & remove intracellular bacteria, is the primary host defense against Francisella novicida infection.
The inflammasome is a multiple proteins complex that identifies cytosolic infections or danger signals & activates caspase-1, causing the pro-inflammatory cytokines IL-1β and IL-18 to be released. Infection with F. novicida activates the inflammasome via receptors like AIM2, NLRP3, & NLRC4. Inflammasome activation is critical for host defense against F. novicida since mice or macrophages lacking inflammasome pathway components (caspase-1, ASC, or IL-1β) are more vulnerable to the bacteria.
Interferons (IFNs) are signaling molecules that promote the expression of several antiviral & antibacterial genes. Infection with F. novicida increases the synthesis of type I interferons (IFN-α and IFN-β) and type II interferons (IFN-γ) by various cell types, namely dendritic cells, macrophages, and T cells.
The interferon signaling pathway is critical in host defense against F. novicida because mice or macrophages lacking IFN receptors (IFN-α/β or IFN-γ receptor) or STAT1 (a key modulator of IFN signaling) are more vulnerable to the bacteria. During F. novicida infection, interferon signaling also influences inflammasome activity.
Autophagy and Lysosomal Biogenesis: Lysosomes are cellular organelles that contain acidic hydrolases that break down intracellular molecules. Autophagy is the process by which cytoplasmic components or pathogens are sent to lysosomes for destruction.
F. novicida infection promotes lysosomal biogenesis and autophagy by inhibiting the activity of the mechanistic target of rapamycin (mTOR) & activating the transcription factors EB (TFEB) & ULK1. Lysosomal biogenesis & autophagy are advantageous for host defense against F. novicida since mice or macrophages lacking TFEB or ULK1 are more sensitive to the bacteria.
Francisella novicida, although considered a rare pathogen, shares similarities with its close relative, Francisella tularensis, which is well-known for causing tularemia. Tularemia is a bacterial infection that can present with various clinical manifestations depending on the disease’s form and the transmission route.
Typhoidal tularemia is a kind of tularemia caused by F. tularensis and, on rare occasions, F. novicida. This type is infrequent and can cause non-specific symptoms like fever, headache, muscle aches, nausea, vomiting, abdominal pain, & exhaustion. With proper laboratory testing, these symptoms may be consistent with those of other infections, making identification easier.
The most prevalent manifestation of tularemia is termed ulceroglandular tularemia. It usually happens because of a tick, insect bite, or bodily contact with an infected animal. The most common symptom is the formation of a skin ulcer at the site of infection, accompanied by swollen & painful lymph nodes. Fever, chills, headache, muscle aches, & weariness are all possible symptoms.
Tularemia can cause pneumonia in severe cases, causing respiratory symptoms like coughing, chest pain, & difficulty breathing. If left untreated, this version of the disease can be fatal.
It’s important to note that cases of F. novicida or F. novicida-like infections are predominantly observed in immunocompromised individuals. These individuals may be more susceptible to severe or atypical manifestations of the infection due to their compromised immune systems.
Culture: Culture is the gold standard for identifying F. novicida and other Francisella species. It involves growing the bacteria on specific media, such as cysteine-glucose-blood agar (CGBA). F. novicida can typically be cultured within 24 hours, while F. tularensis may require several days. F. novicida grows in tiny colonies that are smooth and translucent. Typically, the colonies are non-pigmented or pale white to gray in hue. They are spherical and have a diameter of 1 to 2 millimeters. F. novicida colonies are moist and slightly convex, with a distinct boundary.
Serology: Serological tests detect antibodies against F. novicida and other Francisella species in the serum of infected individuals. These tests are beneficial for retrospective diagnosis or epidemiological studies. Various serological techniques can be employed, such as ELISA, MAT, or TAT. The antigens used for serology include lipopolysaccharide (LPS), capsule antigen (CA), outer membrane protein A (OmpA), or heat shock protein 60 (Hsp60). It is important to note that serology has limitations, including lower sensitivity and specificity during the acute phase of infection.
PCR assay: Polymerase Chain Reaction is a molecular diagnostic method that amplifies specific DNA sequences of F. novicida and other Francisella species. It is a sensitive and rapid technique that can provide results faster than culture. PCR can also differentiate F. novicida from F. tularensis by targeting unique or variable genes between the two species.
Lateral Flow Assay: These are rapid and simple diagnostic tests that utilize CRISPR technology. FnCas9, an enzyme associated with CRISPR, can recognize and cleave specific DNA sequences. Lateral flow assays use FnCas9 and guide RNA to target specific regions of the Francisella genome and generate a colorimetric signal that can be visualized on a paper strip. This method offers quick and point-of-care detection of F. novicida and other Francisella species.
Avoid contact with potentially contaminated water, ice, soil, or animals, especially rodents, rabbits, or hares. It can help minimize the risk of acquiring F. novicida infection. Wear protective clothing, gloves, and insect repellents when working or traveling in areas where F. novicida or other Francisella species may be endemic.
Practice good personal hygiene, like regular handwashing with soap and water, especially after handling animals or environments where F. novicida may be present.
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