The genus Shewanella consists of bacteria primarily inhabiting marine and aquatic environments, including both salt and fresh water. These organisms are frequently isolated from sediments, seawater, and the digestive tracts of marine animals. While the genus contains many species, only a few, such as Shewanella algae and Shewanella putrefaciens, are recognized as human pathogens.
Shewanella species are characterized as opportunistic pathogens, meaning they rarely cause disease in healthy individuals. Infection typically occurs when a host’s natural defenses are compromised, allowing the bacteria to establish themselves. Although historically considered a rare cause of human illness, there has been an increase in reported clinical cases worldwide in recent decades. This rise brings greater attention to the organism’s pathogenesis, the populations at risk, and the challenges associated with effective treatment.
Transmission and Susceptible Hosts
Infection begins when the bacteria gain entry into a host, typically through exposure to contaminated aquatic sources. The most common transmission route is direct contact with water, particularly seawater, which introduces Shewanella to an existing break in the skin barrier. This often involves recreational activities like swimming, fishing, or diving, or occupational exposure in marine environments.
Another route involves the contamination of pre-existing wounds, such as chronic skin ulcers, which provide a vulnerable entry point for the bacteria. Furthermore, the ingestion of raw or undercooked seafood can lead to infection, establishing a route through the gastrointestinal tract. The ability of Shewanella to colonize the hepatobiliary system suggests that exposure may not always be linked to a recent marine-related event.
The development of severe disease is heavily reliant on the host’s underlying health status, making Shewanella a pathogen of the immunocompromised. Chronic liver disease, including cirrhosis and other hepatobiliary disorders, represents a major predisposing factor, found in over 60% of patients in some large case series. The bacteria’s ability to adapt to bile salts may contribute to its survival and subsequent infection of the liver and bile ducts.
Malignancy is another substantial risk factor, present in over a quarter of reported cases, likely due to the resulting immunosuppression. Other debilitating conditions include chronic kidney disease, end-stage renal failure, and diabetes mellitus, which impair immune function and microvascular circulation. Patients receiving immunosuppressive therapy, such as long-term corticosteroid use, are also highly vulnerable to opportunistic infection.
Spectrum of Clinical Manifestations
Soft tissue infections (STIs) are a common presentation, encompassing conditions like cellulitis and wound infections, which often occur at the site of a pre-existing trauma exposed to water. In patients with weakened immune systems, these infections can progress to a severe, rapidly destructive condition known as necrotizing fasciitis.
Bacteremia, or bloodstream infection, is the most severe manifestation and carries the highest risk for poor outcomes. This systemic infection is particularly common in individuals with chronic liver disease or underlying malignancy. The infection can become fulminant, leading to septic shock, especially in highly compromised hosts.
Beyond the skin and blood, Shewanella is capable of causing various focal infections. Hepatobiliary infections, such as cholangitis or liver abscesses, are prominent, reflecting the high prevalence of underlying liver disease in infected patients.
Other Focal Infections
Other reported, though less common, infections include:
- Peritonitis, often in patients undergoing peritoneal dialysis.
- Ocular infections.
- Endocarditis.
- Meningitis.
- Osteomyelitis.
Diagnostic Challenges and Therapeutic Strategies
Identifying Shewanella presents a considerable diagnostic challenge, contributing to potential delays in appropriate treatment. The organism’s biochemical profile can be similar to other Gram-negative bacteria, such as Pseudomonas or Vibrio species, leading to misidentification using standard testing methods. Specialized techniques, such as matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF), are necessary for accurate species-level identification.
The primary difficulty in therapy lies in the organism’s intrinsic and acquired antibiotic resistance mechanisms. Shewanella species are resistant to certain widely used antibiotics, notably ampicillin and first- and second-generation cephalosporins. This intrinsic resistance means that common empirical treatments for Gram-negative infections may fail without targeted therapy.
Effective therapeutic strategies rely on prompt microbiological identification followed by susceptibility testing. Effective antibiotics include fluoroquinolones (e.g., ciprofloxacin) and aminoglycosides (e.g., gentamicin). Certain third and fourth-generation cephalosporins, like ceftazidime, also maintain high rates of susceptibility against most strains.
The emergence of resistance to broad-spectrum agents is a concern, as some strains have acquired genes that confer resistance to carbapenems. Resistance to imipenem has been observed in over 20% of isolates. The presence of resistance genes, including \(bla_{OXA}\) and \(qnr\), further complicates treatment by enabling resistance to carbapenems and quinolones. Treatment protocols must prioritize initial broad-spectrum coverage, followed by a switch to a highly targeted antibiotic regime once susceptibility results are available.