Shewanella: Introduction, Morphology, Pathogenicity, Lab

Introduction

Table of Contents

Shewanella is a genus of Gram-negative, facultative anaerobic bacteria that belongs to the family Shewanellaceae. These bacteria are known for their remarkable versatility and adaptability to a wide range of environmental conditions. They were first discovered in 1980 by researchers studying microbial fuel cells, where they play a crucial role in electron transfer processes.

One of the most intriguing characteristics of Shewanella is their ability to respire using a diverse range of electron acceptors, including various metal ions, which makes them important players in biogeochemical cycles. They are particularly notable for their capacity to reduce metals, such as iron, manganese, and uranium, through a process called dissimilatory metal reduction. This ability has drawn significant interest in fields like bioremediation and environmental cleanup, where Shewanella can potentially help in detoxifying heavy metal-contaminated environments.

Shewanella species (they) are typically found in various aquatic environments, such as marine sediments, freshwater systems, and even subsurface environments. They have also been isolated from other habitats, including the gastrointestinal tracts of animals.

These bacteria have a wide range of metabolic capabilities and are known to degrade various organic compounds, including hydrocarbons, making them relevant in biotechnology and bioenergy research. Some Shewanella strains have also shown promise in producing bioelectricity and biohydrogen through their electrogenic and hydrogenogenic abilities.

Aside from their ecological importance, Shewanella has garnered interest in medical research due to its association with human health. Some strains have been isolated from clinical settings and have been associated with infections, particularly in immunocompromised individuals. Understanding the mechanisms behind Shewanella infections is essential for appropriate clinical management.

Morphology

The morphology of Shewanella refers to their physical appearance and cellular structure. As Gram-negative bacteria, they have distinct characteristics that set them apart from Gram-positive bacteria. Here are some key features of the morphology of Shewanella:

Cell Shape: They are typically rod-shaped (bacilli) and can vary in size, ranging from about 0.5 to 2.5 micrometers in length.
Gram Staining: Shewanella cells stain Gram-negative, which means they have a thin peptidoglycan layer in their cell wall, surrounded by an outer membrane. During the Gram staining process, they appear pink or red under a microscope.
Motility: Most species of Shewanella are motile, possessing one or more flagella that allow them to move through liquid environments. This motility helps them explore and colonize various habitats, including sediments and aquatic environments.
Capsule and Biofilm Formation: Some Shewanella strains can produce capsules and form biofilms, which are protective structures that help them adhere to surfaces and survive in challenging conditions.
Polar Pili: They can produce polar pili, which are hair-like appendages on the cell surface that aid in adherence to surfaces and possibly facilitate intercellular communication.
Cell Arrangement: Shewanella cells can occur as single cells or form chains (streptobacilli) or clusters (staphylobacilli) depending on the species and growth conditions.
Color and Pigmentation: Some them are known to produce pigments, giving them distinct colors. For example, Shewanella oneidensis MR-1 is known for its unique purple color due to the production of a cytochrome-based pigment.

Gram negative rods (GNRs) of Shewanella species in Gram staining of culture microscopy at a magnification of 4000X — Fig. Gram negative rods (GNRs) of *Shewanella* species in Gram staining of culture microscopy at a magnification of 4000X

Pathogenicity

They are primarily considered environmental microorganisms and are not usually associated with causing diseases in healthy individuals. They are commonly found in various aquatic environments, including marine and freshwater systems, where they play important roles in biogeochemical cycles.

However, in certain situations, they can be opportunistic pathogens and cause infections, especially in individuals with compromised immune systems or underlying health conditions. The most commonly reported infections associated with Shewanella include skin and soft tissue infections, wound infections, and bacteremia (presence of bacteria in the bloodstream).

Shewanella growth on blood agar — Fig. *Shewanella* growth on blood agar

The pathogenicity of Shewanella is thought to be facilitated by factors such as their ability to form biofilms, produce various enzymes and toxins, and adapt to different environmental conditions. Infections are often linked to exposure to aquatic environments, marine-related injuries, or contaminated medical devices. Additionally, patients with pre-existing conditions such as diabetes, cancer, or immunosuppression are at higher risk of developing Shewanella infections.

Lab Diagnosis

The laboratory diagnosis of Shewanella infections involves several steps to accurately identify the presence of the bacteria in clinical samples. As they are relatively rare as a human pathogen, their detection require specific testing and identification methods. Below are the general steps involved in the lab diagnosis-

Shewanella species colony characteristics on Muller-Hinton agar (MHA)

Sample Collection: Clinical samples are collected from the site of infection, which may include wound swabs, tissue biopsies, blood cultures, or other relevant specimens based on the suspected source of infection.
Gram Stain: A Gram stain is performed on the clinical sample to assess the bacterial morphology and Gram reaction. They will appear as Gram-negative rods.
Culture: The clinical sample is inoculated onto appropriate culture media. Blood samples are usually inoculated into blood culture bottles. On solid media, Shewanella may grow as smooth, translucent colonies with a bluish-green or brownish coloration due to pigment production (e.g., Shewanella algae may produce a green pigment).
Biochemical Tests: Several biochemical tests are performed to differentiate Shewanella from other bacteria and to confirm its identity. These tests may include oxidase test (positive for Shewanella), catalase test (positive), citrate utilization test (negative), and others as required.
Molecular Identification: Due to the rare nature of Shewanella infections and its similarity to other bacterial species, molecular methods such as Polymerase Chain Reaction (PCR) and DNA sequencing are often employed for definitive identification. Targeting specific genes can help confirm the presence of Shewanella and distinguish it from other related bacteria.
Antimicrobial Susceptibility Testing: Once the bacteria are identified as Shewanella, antimicrobial susceptibility testing is performed to determine the most appropriate antibiotics for treatment. This is essential because they may show variable susceptibility patterns to different antibiotics.

Treatment

The treatment of Shewanella infections typically involves antibiotic therapy. However, due to the relative rarity of these infections and limited clinical data, there are no standardized guidelines for treating these infections. The choice of antibiotics may vary depending on the susceptibility of the specific Shewanella strain isolated from the patient’s clinical samples.

Shewanella reaction in TSI agar showing no carbohydrate fermented or non-glucose fermenter, H2S production without gas formation — Fig. *Shewanella* reaction in TSI agar showing no carbohydrate fermented or non-glucose fermenter, H2S production without gas formation

Some antibiotics that have been reported to be effective against Shewanella infections include:

Third-generation cephalosporins (e.g., ceftriaxone, cefotaxime): These are often considered as initial empiric therapy for Shewanella infections.
Fluoroquinolones (e.g., ciprofloxacin): These antibiotics have shown activity against some Shewanella strains.
Aminoglycosides (e.g., gentamicin, amikacin): They can be used in combination with other antibiotics for severe infections.
Carbapenems (e.g., imipenem, meropenem): These are broad-spectrum antibiotics and may be considered in severe or complicated cases.

Shewanella species in Motility Indole Urea (MIU) agar showing motile, indole and urease negative — Fig. *Shewanella* species in Motility Indole Urea (MIU) agar showing motile, indole and urease negative

The choice of antibiotics and treatment duration should be guided by susceptibility testing results, if available. Since Shewanella infections are relatively uncommon, consultation with infectious disease specialists or reference laboratories with experience in dealing with rare pathogens may be beneficial in determining the most appropriate treatment regimen.

It is crucial to monitor the patient’s response to treatment closely and adjust the antibiotic regimen if needed. Additionally, wound care and infection control measures are essential to prevent the spread of infection and promote healing in patients with localized Shewanella infections.

Prevention

Preventing Shewanella infections involves taking appropriate measures to reduce exposure to the bacteria, especially in high-risk environments. While theses infections are relatively rare, they can occur, particularly in individuals with compromised immune systems or those exposed to aquatic environments. Here are some preventive measures to reduce the risk of Shewanella infections:

Citrate utilization test of Shewanella negative — Fig. Citrate utilization test of *Shewanella* negative

Personal Hygiene: Practicing good personal hygiene is essential to prevent infections. Thoroughly wash hands with soap and water after coming into contact with potentially contaminated surfaces or after handling aquatic specimens.
Wound Care: Proper wound care is crucial in preventing infections. Keep wounds clean, covered, and protected to reduce the risk of bacterial entry.
Avoiding Contaminated Water: If you have open wounds or cuts, avoid swimming or engaging in water activities in potentially contaminated waters, such as stagnant ponds, polluted lakes, or areas with reported algal blooms.
Protective Clothing: When working or engaging in activities that may expose you to water environments, consider using appropriate protective clothing, such as gloves and waterproof footwear, to reduce the risk of skin exposure to potential pathogens.
Infection Control in Healthcare Settings: In healthcare settings, strict infection control practices should be followed to prevent the spread of infections. This includes appropriate sterilization of medical equipment and adherence to standard precautions when handling patients.
Early Diagnosis and Treatment: If you have underlying health conditions or a compromised immune system, seek medical attention promptly for any signs of infection, especially if you suspect exposure to aquatic environments.
Disinfection: Regularly disinfect and clean surfaces that may come into contact with potentially contaminated water or aquatic specimens.
Public Health Measures: Public health authorities should monitor and address any potential outbreaks related to Shewanella infections to identify the source of contamination and implement appropriate control measures.

Keynotes

Here are some keynotes about Shewanella:

Genus of Bacteria: Shewanella is a genus of Gram-negative, facultative anaerobic bacteria belonging to the family Shewanellaceae.
Environmental Bacteria: They are primarily environmental bacteria and are commonly found in various aquatic environments, such as marine sediments and freshwater systems.
Versatile Metabolism: Shewanella is known for its remarkable metabolic versatility, including the ability to respire using a wide range of electron acceptors, such as metal ions, and its involvement in biogeochemical cycles.
Metal Reduction: Some Shewanella strains are capable of dissimilatory metal reduction, which involves reducing metal ions like iron, manganese, and uranium as part of their energy metabolism.
Bioelectricity and Biohydrogen Production: Certain species have been studied for their ability to generate bioelectricity and biohydrogen through electrogenic and hydrogenogenic processes.
Rare Human Pathogen: While it is generally considered an environmental microorganism, it can cause infections in humans, especially in immunocompromised individuals or those with underlying health conditions.
Skin and Soft Tissue Infections: Shewanella infections in humans are most commonly associated with skin and soft tissue infections, wound infections, and bacteremia.
Diagnosis: Laboratory diagnosis of Shewanella infections involves Gram staining, culture, biochemical tests, and molecular identification methods like PCR and DNA sequencing.
Treatment: Treatment of thses infections typically involves antibiotic therapy, and the choice of antibiotics depends on susceptibility testing results and the severity of the infection.
Prevention: Preventive measures to reduce the risk of Shewanella infections include practicing good personal hygiene, proper wound care, avoiding contact with contaminated water, and following infection control practices in healthcare settings.

Shewanella: Introduction, Morphology, Pathogenicity, Lab Diagnosis, Treatment, Prevention, and Keynotes