Serratia liquefaciens is an uncommon species of Serratia that can cause human infections. According to a study by Mahlen in 2011, S. liquefaciens accounted for only 3.8% of all Serratia isolates from clinical specimens in the United States between 1998 and 2008.
Serratia bacteremia, involving S. liquefaciens, has been shown to be 1.03 per 100,000 individuals yearly, having 47% of episodes having their onset in the community. While the prevalence of Serratia species as a cause of nosocomial infections is decreasing, these bacteria can still lead to hospital outbreaks, particularly in intensive care units.
The most common sources of Serratia liquefaciens infections are urinary tract infections (UTIs), respiratory tract infections, and wound infections. These infections can be mild to severe, with potential complications including endocarditis, osteomyelitis, meningitis, and septic arthritis, although they are less common.
The mortality rate associated with Serratia liquefaciens infection varies depending on the severity and infection type, the underlying health conditions of the patient, and the antibiotic resistance of the bacterial strain. In general, the mortality rate ranges from 10% to 50%, with higher rates observed in cases of bacteremia and endocarditis.
Kingdom: Bacteria
Phylum: Pseudomonadota
Class: Gammaproteobacteria
Order: Enterobacterales
Family: Yersiniaceae
Genus:Serratia
Species:Serratia liquefaciens
Serratia liquefaciens is a straight rod-shaped, facultative anaerobe bacterium with an elongated and cylindrical morphology. The cells appear as bacilli, meaning they are elongated and cylindrical.
They are typically around 0.5-0.8 μm in diameter and 0.9-2.0 μm in length.It has a thin peptidoglycan layer surrounded by an inner & an outer membrane. The outer membrane comprises lipopolysaccharides (LPS).Serratia liquefaciens is usually motile, and peritrichous flagella facilitate this motility.
Serratia liquefaciens strains harbor several virulence genes that encode various enzymes and proteins responsible for pathogenicity. These genes include:
chiA: The gene encoding chitinase, an enzyme involved in the breakdown of chitin, a component of the cell walls of certain fungi and insects. Chitinase aids in the degradation of host tissues and may play a role in tissue invasion.
lipA: The gene encoding lipase, an enzyme that breaks down lipids. Lipase can contribute to the hydrolysis of host cell membranes, facilitating the bacterium’s access to nutrients and invasion of host tissues.
prtA: The gene encoding protease, an enzyme that breaks down proteins. Proteases can disrupt host immune responses and contribute to tissue damage, promoting the spread of infection.
Serratia liquefaciens produces several virulence proteins that are essential to its pathogenicity. By dissolving down extracellular DNA traps established by the host to capture infections, the DNase enzyme allows the bacteria to elude the host’s immune systems. Gelatinase is an enzyme that dissolves gelatin, produced from collagen, and helps in tissue invasion & host tissue disintegration.
Hemolysin is a protein that can lyse red blood cells, causing hemoglobin to be released and potentially damaging host cells & tissues. Serratia liquefaciens also produces numerous isozymes of alkaline phosphatase, enzymes involved in phosphate metabolism that may aid the bacterium’s survival and adaption in the host environment. These virulence proteins, when combined, increase the bacterium’s potential to cause infections & pose a substantial threat.
Several strains of Serratia liquefaciens have been identified and characterized. DSM 4487, the type strain, was isolated from milk. ATCC 27592 is a strain obtained from W. H. Ewing at the CDC. MQ-4 is a novel lytic phage that infects S. liquefaciens and is isolated from sewage. Other strains of Serratia liquefaciens, like strain HUMV-3250, have also been isolated.
The pathogenesis of Serratia liquefaciens involves a combination of factors and mechanisms that contribute to its ability to cause opportunistic infections and evade host defenses. One of the key challenges in treating Serratia infections is its ability to develop multiple resistance mechanisms against various classes of antibiotics, including penicillins, cephalosporins, carbapenems, aminoglycosides, and fluoroquinolones.
These resistance mechanisms, such as extended-spectrum beta-lactamases (ESBLs), AmpC beta-lactamases, carbapenemases, and efflux pumps, make the bacterium highly resistant to common antimicrobial agents, leading to treatment challenges and an increased risk of treatment failure and mortality.
Furthermore, Serratia liquefaciens can produce siderophores, molecules that bind and transport iron, an essential nutrient for bacterial growth. Serratia can enhance its virulence and competitiveness by scavenging iron from the host or the environment, promoting its survival and growth in the host’s tissues.
The bacterium also produces various virulence factors that contribute to its pathogenicity. Hemolysin, for instance, is a protein that can lyse red blood cells, causing hemolysis and anemia. Serratia can also form biofilms on surfaces, such as catheters and prosthetic valves, as well as in different sites of infection, such as the urinary and respiratory tract. Biofilms protect the bacterium, making it more resistant to antibiotics and immune responses and promoting persistent infections.
As an opportunistic pathogen, Serratia liquefaciens primarily targets individuals with underlying health conditions and weakened immune systems. It is often associated with nosocomial infections, particularly in healthcare settings like intensive care units, where it can spread between patients through contaminated medical equipment or healthcare personnel.
As critical players in host defense, Macrophages are crucial in recognizing & responding to Serratia liquefaciens. Upon infection with the clinical isolate HUMV-3250, macrophages trigger a rapid and potent cytotoxic effect. The cytotoxicity process depends on live bacteria, adherence, and protein synthesis but not on phagocytosis or bacterial internalization.
It suggests that Serratia liquefaciens can induce cytotoxicity in macrophages without being internalized, which may represent a unique evasion tactic used by the bacterium.Macrophage viability decreases rapidly upon infection with Serratia liquefaciens, and the degree of cytotoxicity depends on the Multiplicity of Infection (MOI) used.
The infection outcome is not affected by caspase-1 inhibitors or specific phagocytosis inhibitors, indicating that the bacterium’s cytotoxic effect is mediated through alternative mechanisms.In addition to macrophages, other host immune system components also play critical roles in defense against Serratia liquefaciens.
Studies have shown that mice infected with this bacterium develop antibodies against bacterial flagellin, a protein that forms the structure of the bacterial flagella. These antibodies activate the complement system, leading to increased production of anaphylatoxins (C5a and C3a).
Anaphylatoxins induce inflammation and attract immune cells, which enhances the immune response against the pathogen. However, the increased inflammation caused by anaphylatoxins can also worsen lung injury and increase the mortality rate in infected mice, highlighting the delicate balance between immune defense and immunopathology.
Natural killer cells, a type of lymphocyte, are also involved in host defense against Serratia liquefaciens. NK cells can recognize & engulf infected or abnormal cells, contributing to eliminating the bacterium.
Clinical manifestations of Serratia liquefaciens infections can vary depending on the site of infection. In respiratory tract infections, common symptoms include body aches, malaise, green sputum production, fever, and chills, indicative of pneumonia or bronchitis.
Urinary tract infections caused by S. liquefaciens may be asymptomatic or present with itching, burning, discharge, and kidney pain. The bacterium can invade the urinary tract through catheters, surgical procedures, or sexual contact, leading to UTIs. Additionally, S. liquefaciens can cause rare but serious complications such as meningitis or cerebral abscess, affecting the brain and spinal cord.
Intra-abdominal infections may result in biliary drainage, liver abscess, pancreatic abscess, or peritoneal fluid secretion, with contamination during surgery or translocation from the intestinal tract being common routes of infection.S. liquefaciens can cause osteomyelitis & arthritis by spreading via the bloodstream or direct inoculation from trauma or surgery.
The bacterium is also linked to endocarditis, which affects heart valves and causes fever, heart murmurs, embolism, or heart failure. S. liquefaciens ocular infections can manifest as conjunctive inflammation, keratitis, endophthalmitis, or blindness, with contamination through contact lens use, eye surgery, or eye trauma being the most common form of infection.
Serratia species commonly colonize the respiratory and urinary tracts, accounting for approximately 2% of nosocomial infections in various sites such as the lower respiratory tract, bloodstream, surgical wounds, urinary system, & skin and soft tissues.
Culture method:Serratia liquefaciens culture assays use selective media like MacConkey agar, Eosin Methylene Blue (EMB) agar, & Hektoen enteric (HE) agar. Serratia liquefaciens produces pink colonies on MacConkey agar due to its capacity to digest lactose. Non-lactose fermenters, on the other hand, will create colorless colonies.
Serratia liquefaciens colonies on EMB agar look dark purple with a dark core, indicating considerable lactose fermentation. Strong lactose fermenters will have colonies with a green metallic sheen, whereas non-lactose fermenters will have colorless or light pink colonies. And, due to its ability to ferment sucrose and lactose, Serratia liquefaciens colonies on HE agar will appear salmon colored. Colonies of non-fermenters will be blue-green with black cores.
Under microscopic examination, Serratia liquefaciens cells appear as elongated and cylindrical bacilli.
Imaging Studies:
Chest radiography: Performed in patients with suspected pneumonia or respiratory distress to assess lung involvement.
CT scanning or Abdominal ultrasonography: Used to rule out obstructive hydronephrois or intra-abdominal abscesses, which may occur in severe infections.
Transthoracic or transesophageal echocardiography: Useful in detecting valvular vegetations and regurgitation in cases of suspected endocarditis.
Laboratory studies for Serratia liquefaciens infection include a complete blood count (CBC) with differential, which may show leukocytosis with an increased number of neutrophils (neutrophilia) and the presence of more than 10% immature neutrophils (bands), suggesting an ongoing inflammatory response. Additionally, serum biochemistry tests assess glucose, urea, and creatinine levels, providing valuable information about the patient’s overall health and kidney function. These diagnostic tests are crucial in evaluating the severity of the infection and guiding appropriate management and treatment strategies.
Proper care and maintenance of catheters, including urinary and central venous catheters, can help reduce the risk of catheter-associated S. liquefaciens infections.
Cleaning and disinfecting patient care spaces, medical equipment, & high-touch surfaces regularly and thoroughly can help minimize the spread of S. liquefaciens. It is critical to ensure that disinfectants have efficacy against Gram-negative bacteria.
To avoid the development of antibiotic resistance, promote the appropriate and prudent use of antibiotics. Antimicrobial stewardship plans should be in place at healthcare facilities to guide the appropriate use of antibiotics & prevent the chance of treatment failure.
Serratia liquefaciens is an uncommon species of Serratia that can cause human infections. According to a study by Mahlen in 2011, S. liquefaciens accounted for only 3.8% of all Serratia isolates from clinical specimens in the United States between 1998 and 2008.
Serratia bacteremia, involving S. liquefaciens, has been shown to be 1.03 per 100,000 individuals yearly, having 47% of episodes having their onset in the community. While the prevalence of Serratia species as a cause of nosocomial infections is decreasing, these bacteria can still lead to hospital outbreaks, particularly in intensive care units.
The most common sources of Serratia liquefaciens infections are urinary tract infections (UTIs), respiratory tract infections, and wound infections. These infections can be mild to severe, with potential complications including endocarditis, osteomyelitis, meningitis, and septic arthritis, although they are less common.
The mortality rate associated with Serratia liquefaciens infection varies depending on the severity and infection type, the underlying health conditions of the patient, and the antibiotic resistance of the bacterial strain. In general, the mortality rate ranges from 10% to 50%, with higher rates observed in cases of bacteremia and endocarditis.
Kingdom: Bacteria
Phylum: Pseudomonadota
Class: Gammaproteobacteria
Order: Enterobacterales
Family: Yersiniaceae
Genus:Serratia
Species:Serratia liquefaciens
Serratia liquefaciens is a straight rod-shaped, facultative anaerobe bacterium with an elongated and cylindrical morphology. The cells appear as bacilli, meaning they are elongated and cylindrical.
They are typically around 0.5-0.8 μm in diameter and 0.9-2.0 μm in length.It has a thin peptidoglycan layer surrounded by an inner & an outer membrane. The outer membrane comprises lipopolysaccharides (LPS).Serratia liquefaciens is usually motile, and peritrichous flagella facilitate this motility.
Serratia liquefaciens strains harbor several virulence genes that encode various enzymes and proteins responsible for pathogenicity. These genes include:
chiA: The gene encoding chitinase, an enzyme involved in the breakdown of chitin, a component of the cell walls of certain fungi and insects. Chitinase aids in the degradation of host tissues and may play a role in tissue invasion.
lipA: The gene encoding lipase, an enzyme that breaks down lipids. Lipase can contribute to the hydrolysis of host cell membranes, facilitating the bacterium’s access to nutrients and invasion of host tissues.
prtA: The gene encoding protease, an enzyme that breaks down proteins. Proteases can disrupt host immune responses and contribute to tissue damage, promoting the spread of infection.
Serratia liquefaciens produces several virulence proteins that are essential to its pathogenicity. By dissolving down extracellular DNA traps established by the host to capture infections, the DNase enzyme allows the bacteria to elude the host’s immune systems. Gelatinase is an enzyme that dissolves gelatin, produced from collagen, and helps in tissue invasion & host tissue disintegration.
Hemolysin is a protein that can lyse red blood cells, causing hemoglobin to be released and potentially damaging host cells & tissues. Serratia liquefaciens also produces numerous isozymes of alkaline phosphatase, enzymes involved in phosphate metabolism that may aid the bacterium’s survival and adaption in the host environment. These virulence proteins, when combined, increase the bacterium’s potential to cause infections & pose a substantial threat.
Several strains of Serratia liquefaciens have been identified and characterized. DSM 4487, the type strain, was isolated from milk. ATCC 27592 is a strain obtained from W. H. Ewing at the CDC. MQ-4 is a novel lytic phage that infects S. liquefaciens and is isolated from sewage. Other strains of Serratia liquefaciens, like strain HUMV-3250, have also been isolated.
The pathogenesis of Serratia liquefaciens involves a combination of factors and mechanisms that contribute to its ability to cause opportunistic infections and evade host defenses. One of the key challenges in treating Serratia infections is its ability to develop multiple resistance mechanisms against various classes of antibiotics, including penicillins, cephalosporins, carbapenems, aminoglycosides, and fluoroquinolones.
These resistance mechanisms, such as extended-spectrum beta-lactamases (ESBLs), AmpC beta-lactamases, carbapenemases, and efflux pumps, make the bacterium highly resistant to common antimicrobial agents, leading to treatment challenges and an increased risk of treatment failure and mortality.
Furthermore, Serratia liquefaciens can produce siderophores, molecules that bind and transport iron, an essential nutrient for bacterial growth. Serratia can enhance its virulence and competitiveness by scavenging iron from the host or the environment, promoting its survival and growth in the host’s tissues.
The bacterium also produces various virulence factors that contribute to its pathogenicity. Hemolysin, for instance, is a protein that can lyse red blood cells, causing hemolysis and anemia. Serratia can also form biofilms on surfaces, such as catheters and prosthetic valves, as well as in different sites of infection, such as the urinary and respiratory tract. Biofilms protect the bacterium, making it more resistant to antibiotics and immune responses and promoting persistent infections.
As an opportunistic pathogen, Serratia liquefaciens primarily targets individuals with underlying health conditions and weakened immune systems. It is often associated with nosocomial infections, particularly in healthcare settings like intensive care units, where it can spread between patients through contaminated medical equipment or healthcare personnel.
As critical players in host defense, Macrophages are crucial in recognizing & responding to Serratia liquefaciens. Upon infection with the clinical isolate HUMV-3250, macrophages trigger a rapid and potent cytotoxic effect. The cytotoxicity process depends on live bacteria, adherence, and protein synthesis but not on phagocytosis or bacterial internalization.
It suggests that Serratia liquefaciens can induce cytotoxicity in macrophages without being internalized, which may represent a unique evasion tactic used by the bacterium.Macrophage viability decreases rapidly upon infection with Serratia liquefaciens, and the degree of cytotoxicity depends on the Multiplicity of Infection (MOI) used.
The infection outcome is not affected by caspase-1 inhibitors or specific phagocytosis inhibitors, indicating that the bacterium’s cytotoxic effect is mediated through alternative mechanisms.In addition to macrophages, other host immune system components also play critical roles in defense against Serratia liquefaciens.
Studies have shown that mice infected with this bacterium develop antibodies against bacterial flagellin, a protein that forms the structure of the bacterial flagella. These antibodies activate the complement system, leading to increased production of anaphylatoxins (C5a and C3a).
Anaphylatoxins induce inflammation and attract immune cells, which enhances the immune response against the pathogen. However, the increased inflammation caused by anaphylatoxins can also worsen lung injury and increase the mortality rate in infected mice, highlighting the delicate balance between immune defense and immunopathology.
Natural killer cells, a type of lymphocyte, are also involved in host defense against Serratia liquefaciens. NK cells can recognize & engulf infected or abnormal cells, contributing to eliminating the bacterium.
Clinical manifestations of Serratia liquefaciens infections can vary depending on the site of infection. In respiratory tract infections, common symptoms include body aches, malaise, green sputum production, fever, and chills, indicative of pneumonia or bronchitis.
Urinary tract infections caused by S. liquefaciens may be asymptomatic or present with itching, burning, discharge, and kidney pain. The bacterium can invade the urinary tract through catheters, surgical procedures, or sexual contact, leading to UTIs. Additionally, S. liquefaciens can cause rare but serious complications such as meningitis or cerebral abscess, affecting the brain and spinal cord.
Intra-abdominal infections may result in biliary drainage, liver abscess, pancreatic abscess, or peritoneal fluid secretion, with contamination during surgery or translocation from the intestinal tract being common routes of infection.S. liquefaciens can cause osteomyelitis & arthritis by spreading via the bloodstream or direct inoculation from trauma or surgery.
The bacterium is also linked to endocarditis, which affects heart valves and causes fever, heart murmurs, embolism, or heart failure. S. liquefaciens ocular infections can manifest as conjunctive inflammation, keratitis, endophthalmitis, or blindness, with contamination through contact lens use, eye surgery, or eye trauma being the most common form of infection.
Serratia species commonly colonize the respiratory and urinary tracts, accounting for approximately 2% of nosocomial infections in various sites such as the lower respiratory tract, bloodstream, surgical wounds, urinary system, & skin and soft tissues.
Culture method:Serratia liquefaciens culture assays use selective media like MacConkey agar, Eosin Methylene Blue (EMB) agar, & Hektoen enteric (HE) agar. Serratia liquefaciens produces pink colonies on MacConkey agar due to its capacity to digest lactose. Non-lactose fermenters, on the other hand, will create colorless colonies.
Serratia liquefaciens colonies on EMB agar look dark purple with a dark core, indicating considerable lactose fermentation. Strong lactose fermenters will have colonies with a green metallic sheen, whereas non-lactose fermenters will have colorless or light pink colonies. And, due to its ability to ferment sucrose and lactose, Serratia liquefaciens colonies on HE agar will appear salmon colored. Colonies of non-fermenters will be blue-green with black cores.
Under microscopic examination, Serratia liquefaciens cells appear as elongated and cylindrical bacilli.
Imaging Studies:
Chest radiography: Performed in patients with suspected pneumonia or respiratory distress to assess lung involvement.
CT scanning or Abdominal ultrasonography: Used to rule out obstructive hydronephrois or intra-abdominal abscesses, which may occur in severe infections.
Transthoracic or transesophageal echocardiography: Useful in detecting valvular vegetations and regurgitation in cases of suspected endocarditis.
Laboratory studies for Serratia liquefaciens infection include a complete blood count (CBC) with differential, which may show leukocytosis with an increased number of neutrophils (neutrophilia) and the presence of more than 10% immature neutrophils (bands), suggesting an ongoing inflammatory response. Additionally, serum biochemistry tests assess glucose, urea, and creatinine levels, providing valuable information about the patient’s overall health and kidney function. These diagnostic tests are crucial in evaluating the severity of the infection and guiding appropriate management and treatment strategies.
Proper care and maintenance of catheters, including urinary and central venous catheters, can help reduce the risk of catheter-associated S. liquefaciens infections.
Cleaning and disinfecting patient care spaces, medical equipment, & high-touch surfaces regularly and thoroughly can help minimize the spread of S. liquefaciens. It is critical to ensure that disinfectants have efficacy against Gram-negative bacteria.
To avoid the development of antibiotic resistance, promote the appropriate and prudent use of antibiotics. Antimicrobial stewardship plans should be in place at healthcare facilities to guide the appropriate use of antibiotics & prevent the chance of treatment failure.
Serratia liquefaciens is an uncommon species of Serratia that can cause human infections. According to a study by Mahlen in 2011, S. liquefaciens accounted for only 3.8% of all Serratia isolates from clinical specimens in the United States between 1998 and 2008.
Serratia bacteremia, involving S. liquefaciens, has been shown to be 1.03 per 100,000 individuals yearly, having 47% of episodes having their onset in the community. While the prevalence of Serratia species as a cause of nosocomial infections is decreasing, these bacteria can still lead to hospital outbreaks, particularly in intensive care units.
The most common sources of Serratia liquefaciens infections are urinary tract infections (UTIs), respiratory tract infections, and wound infections. These infections can be mild to severe, with potential complications including endocarditis, osteomyelitis, meningitis, and septic arthritis, although they are less common.
The mortality rate associated with Serratia liquefaciens infection varies depending on the severity and infection type, the underlying health conditions of the patient, and the antibiotic resistance of the bacterial strain. In general, the mortality rate ranges from 10% to 50%, with higher rates observed in cases of bacteremia and endocarditis.
Kingdom: Bacteria
Phylum: Pseudomonadota
Class: Gammaproteobacteria
Order: Enterobacterales
Family: Yersiniaceae
Genus:Serratia
Species:Serratia liquefaciens
Serratia liquefaciens is a straight rod-shaped, facultative anaerobe bacterium with an elongated and cylindrical morphology. The cells appear as bacilli, meaning they are elongated and cylindrical.
They are typically around 0.5-0.8 μm in diameter and 0.9-2.0 μm in length.It has a thin peptidoglycan layer surrounded by an inner & an outer membrane. The outer membrane comprises lipopolysaccharides (LPS).Serratia liquefaciens is usually motile, and peritrichous flagella facilitate this motility.
Serratia liquefaciens strains harbor several virulence genes that encode various enzymes and proteins responsible for pathogenicity. These genes include:
chiA: The gene encoding chitinase, an enzyme involved in the breakdown of chitin, a component of the cell walls of certain fungi and insects. Chitinase aids in the degradation of host tissues and may play a role in tissue invasion.
lipA: The gene encoding lipase, an enzyme that breaks down lipids. Lipase can contribute to the hydrolysis of host cell membranes, facilitating the bacterium’s access to nutrients and invasion of host tissues.
prtA: The gene encoding protease, an enzyme that breaks down proteins. Proteases can disrupt host immune responses and contribute to tissue damage, promoting the spread of infection.
Serratia liquefaciens produces several virulence proteins that are essential to its pathogenicity. By dissolving down extracellular DNA traps established by the host to capture infections, the DNase enzyme allows the bacteria to elude the host’s immune systems. Gelatinase is an enzyme that dissolves gelatin, produced from collagen, and helps in tissue invasion & host tissue disintegration.
Hemolysin is a protein that can lyse red blood cells, causing hemoglobin to be released and potentially damaging host cells & tissues. Serratia liquefaciens also produces numerous isozymes of alkaline phosphatase, enzymes involved in phosphate metabolism that may aid the bacterium’s survival and adaption in the host environment. These virulence proteins, when combined, increase the bacterium’s potential to cause infections & pose a substantial threat.
Several strains of Serratia liquefaciens have been identified and characterized. DSM 4487, the type strain, was isolated from milk. ATCC 27592 is a strain obtained from W. H. Ewing at the CDC. MQ-4 is a novel lytic phage that infects S. liquefaciens and is isolated from sewage. Other strains of Serratia liquefaciens, like strain HUMV-3250, have also been isolated.
The pathogenesis of Serratia liquefaciens involves a combination of factors and mechanisms that contribute to its ability to cause opportunistic infections and evade host defenses. One of the key challenges in treating Serratia infections is its ability to develop multiple resistance mechanisms against various classes of antibiotics, including penicillins, cephalosporins, carbapenems, aminoglycosides, and fluoroquinolones.
These resistance mechanisms, such as extended-spectrum beta-lactamases (ESBLs), AmpC beta-lactamases, carbapenemases, and efflux pumps, make the bacterium highly resistant to common antimicrobial agents, leading to treatment challenges and an increased risk of treatment failure and mortality.
Furthermore, Serratia liquefaciens can produce siderophores, molecules that bind and transport iron, an essential nutrient for bacterial growth. Serratia can enhance its virulence and competitiveness by scavenging iron from the host or the environment, promoting its survival and growth in the host’s tissues.
The bacterium also produces various virulence factors that contribute to its pathogenicity. Hemolysin, for instance, is a protein that can lyse red blood cells, causing hemolysis and anemia. Serratia can also form biofilms on surfaces, such as catheters and prosthetic valves, as well as in different sites of infection, such as the urinary and respiratory tract. Biofilms protect the bacterium, making it more resistant to antibiotics and immune responses and promoting persistent infections.
As an opportunistic pathogen, Serratia liquefaciens primarily targets individuals with underlying health conditions and weakened immune systems. It is often associated with nosocomial infections, particularly in healthcare settings like intensive care units, where it can spread between patients through contaminated medical equipment or healthcare personnel.
As critical players in host defense, Macrophages are crucial in recognizing & responding to Serratia liquefaciens. Upon infection with the clinical isolate HUMV-3250, macrophages trigger a rapid and potent cytotoxic effect. The cytotoxicity process depends on live bacteria, adherence, and protein synthesis but not on phagocytosis or bacterial internalization.
It suggests that Serratia liquefaciens can induce cytotoxicity in macrophages without being internalized, which may represent a unique evasion tactic used by the bacterium.Macrophage viability decreases rapidly upon infection with Serratia liquefaciens, and the degree of cytotoxicity depends on the Multiplicity of Infection (MOI) used.
The infection outcome is not affected by caspase-1 inhibitors or specific phagocytosis inhibitors, indicating that the bacterium’s cytotoxic effect is mediated through alternative mechanisms.In addition to macrophages, other host immune system components also play critical roles in defense against Serratia liquefaciens.
Studies have shown that mice infected with this bacterium develop antibodies against bacterial flagellin, a protein that forms the structure of the bacterial flagella. These antibodies activate the complement system, leading to increased production of anaphylatoxins (C5a and C3a).
Anaphylatoxins induce inflammation and attract immune cells, which enhances the immune response against the pathogen. However, the increased inflammation caused by anaphylatoxins can also worsen lung injury and increase the mortality rate in infected mice, highlighting the delicate balance between immune defense and immunopathology.
Natural killer cells, a type of lymphocyte, are also involved in host defense against Serratia liquefaciens. NK cells can recognize & engulf infected or abnormal cells, contributing to eliminating the bacterium.
Clinical manifestations of Serratia liquefaciens infections can vary depending on the site of infection. In respiratory tract infections, common symptoms include body aches, malaise, green sputum production, fever, and chills, indicative of pneumonia or bronchitis.
Urinary tract infections caused by S. liquefaciens may be asymptomatic or present with itching, burning, discharge, and kidney pain. The bacterium can invade the urinary tract through catheters, surgical procedures, or sexual contact, leading to UTIs. Additionally, S. liquefaciens can cause rare but serious complications such as meningitis or cerebral abscess, affecting the brain and spinal cord.
Intra-abdominal infections may result in biliary drainage, liver abscess, pancreatic abscess, or peritoneal fluid secretion, with contamination during surgery or translocation from the intestinal tract being common routes of infection.S. liquefaciens can cause osteomyelitis & arthritis by spreading via the bloodstream or direct inoculation from trauma or surgery.
The bacterium is also linked to endocarditis, which affects heart valves and causes fever, heart murmurs, embolism, or heart failure. S. liquefaciens ocular infections can manifest as conjunctive inflammation, keratitis, endophthalmitis, or blindness, with contamination through contact lens use, eye surgery, or eye trauma being the most common form of infection.
Serratia species commonly colonize the respiratory and urinary tracts, accounting for approximately 2% of nosocomial infections in various sites such as the lower respiratory tract, bloodstream, surgical wounds, urinary system, & skin and soft tissues.
Culture method:Serratia liquefaciens culture assays use selective media like MacConkey agar, Eosin Methylene Blue (EMB) agar, & Hektoen enteric (HE) agar. Serratia liquefaciens produces pink colonies on MacConkey agar due to its capacity to digest lactose. Non-lactose fermenters, on the other hand, will create colorless colonies.
Serratia liquefaciens colonies on EMB agar look dark purple with a dark core, indicating considerable lactose fermentation. Strong lactose fermenters will have colonies with a green metallic sheen, whereas non-lactose fermenters will have colorless or light pink colonies. And, due to its ability to ferment sucrose and lactose, Serratia liquefaciens colonies on HE agar will appear salmon colored. Colonies of non-fermenters will be blue-green with black cores.
Under microscopic examination, Serratia liquefaciens cells appear as elongated and cylindrical bacilli.
Imaging Studies:
Chest radiography: Performed in patients with suspected pneumonia or respiratory distress to assess lung involvement.
CT scanning or Abdominal ultrasonography: Used to rule out obstructive hydronephrois or intra-abdominal abscesses, which may occur in severe infections.
Transthoracic or transesophageal echocardiography: Useful in detecting valvular vegetations and regurgitation in cases of suspected endocarditis.
Laboratory studies for Serratia liquefaciens infection include a complete blood count (CBC) with differential, which may show leukocytosis with an increased number of neutrophils (neutrophilia) and the presence of more than 10% immature neutrophils (bands), suggesting an ongoing inflammatory response. Additionally, serum biochemistry tests assess glucose, urea, and creatinine levels, providing valuable information about the patient’s overall health and kidney function. These diagnostic tests are crucial in evaluating the severity of the infection and guiding appropriate management and treatment strategies.
Proper care and maintenance of catheters, including urinary and central venous catheters, can help reduce the risk of catheter-associated S. liquefaciens infections.
Cleaning and disinfecting patient care spaces, medical equipment, & high-touch surfaces regularly and thoroughly can help minimize the spread of S. liquefaciens. It is critical to ensure that disinfectants have efficacy against Gram-negative bacteria.
To avoid the development of antibiotic resistance, promote the appropriate and prudent use of antibiotics. Antimicrobial stewardship plans should be in place at healthcare facilities to guide the appropriate use of antibiotics & prevent the chance of treatment failure.
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