Raoultella ornithinolytica: Introduction, Morphology, Pathogenicity

Introduction

Table of Contents

Raoultella ornithinolytica is a Gram-negative, encapsulated, non-motile, rod-shaped bacterium belonging to the family Enterobacteriaceae. Originally classified under the genus Klebsiella, it has been reclassified due to its distinct biochemical properties, including the ability to metabolize ornithine. This bacterium is commonly found in water, soil, and aquatic environments, indicating its widespread presence in nature. While it often exists harmlessly in these environments, It can cause opportunistic infections in humans, especially in individuals with compromised immune systems.

Raoultella ornithinolytica on CLED agar of older patient urine culture after 24 hours of incubation — Fig. *Raoultella ornithinolytica* on CLED agar of older patient urine culture after 24 hours of incubation

Such infections may include bacteremia, urinary tract infections, and pneumonia, among others. Its identification in clinical settings necessitates specific diagnostic techniques, given its resistance to multiple antibiotics, which poses challenges in treatment. The study of R. ornithinolytica is significant for understanding antibiotic resistance and developing effective treatments for its infections.

Morphology

Raoultella ornithinolytica, a Gram-negative bacterium, showcases a rod-shaped morphology typical of the Enterobacteriaceae family. This encapsulated organism does not exhibit motility, setting it apart from some of its relatives.

Fig. Raoultella ornithinolytica in wet mount

Under microscopic examination, particularly after Gram staining, it appears pink due to its inability to retain the crystal violet dye, instead absorbing the counterstain safranin. Often observed in clusters or pairs, these bacteria reveal their reproductive behavior and cellular interactions. The capsule surrounding each cell is significant, contributing to its ability to resist certain immune responses and aiding in its pathogenic potential. Recognizing these morphological traits is essential for microbiologists in identifying it in clinical specimens, thereby facilitating timely and accurate diagnosis of infections caused by this bacterium.

Pathogenicity

R. ornithinolytica , primarily known for residing in aquatic and soil environments, emerges as an opportunistic pathogen capable of causing a range of infections in humans. Its pathogenicity stems from its ability to produce a unique enzyme, ornithine decarboxylase, leading to conditions such as bacteremia, urinary tract infections (UTIs), pneumonia, and wound infections, particularly in individuals with compromised immune systems. The bacterium’s encapsulated structure enhances its virulence by evading host immune responses.

Raoultella ornithinolytica in wet mount of culture microscopy at a magnification of 1600X — Fig. *R. ornithinolytica* in wet mount of culture microscopy at a magnification of 1600X

Moreover, it is notable for its resistance to multiple antibiotics, complicating treatment strategies. This resistance, coupled with its enzymatic production, underscores the need for accurate identification and targeted therapy in clinical settings. Understanding its pathogenic mechanisms is crucial for developing effective interventions against infections caused by this increasingly relevant bacterium.

Lab Diagnosis

Diagnosing Raoultella ornithinolytica infections in the laboratory hinges on a combination of culture, biochemical identification, and molecular methods. Initially, specimens from suspected infection sites undergo culture on standard bacteriological media. It’s growth manifests as distinctive colonies, which technicians then subject to Gram staining, revealing its characteristic Gram-negative, rod-shaped morphology. Biochemical tests play a pivotal role, especially the ability to de-carboxylate ornithine, a trait distinguishing it from similar pathogens.

Routella ornithinolytica lactose fermenting colony on MacConkey agar — Fig. Routella ornithinolytica mucoid lactose fermenting ( MLF) colony on MacConkey agar

Automated systems like VITEK 2 enhance identification accuracy, providing rapid and reliable results. Molecular diagnostics, including PCR (Polymerase Chain Reaction), offer specificity in detecting R. ornithinolytica‘s DNA, confirming its presence. Antimicrobial susceptibility testing (AST) is crucial due to its inherent antibiotic resistance, guiding effective treatment options. This multifaceted diagnostic approach ensures precise identification, enabling clinicians to tailor appropriate treatment strategies for infections caused by this opportunistic pathogen.

Treatment

Treating infections caused by R. ornithinolytica necessitates a tailored approach due to its intrinsic antibiotic resistance. Upon confirming diagnosis through laboratory testing, clinicians assess the bacterium’s antibiotic susceptibility testing (AST) to identify effective treatment options. Typically, carbapenems are considered reliable against this bacterium infections, given the organism’s resistance to first-line antibiotics like ampicillin. In cases of severe infections, such as bacteremia or pneumonia, combination antibiotic therapy may be employed to enhance treatment efficacy.

Routella ornithinolytica on CLED medium — Fig. *Routella ornithinolytica* on CLED medium

The selection of antibiotics is critically informed by susceptibility test results, ensuring the chosen regimen targets the specific resistance mechanisms of the strain involved. Supportive care, addressing hydration and electrolyte balance, complements antibiotic treatment, promoting recovery. Timely, appropriate antibiotic therapy is essential to mitigate the risk of complications from theses infections.

Prevention

Preventing R. ornithinolytica infections involves a comprehensive approach centered on hygiene, water and food safety, and infection control measures. Ensuring clean, safe drinking water and proper sewage treatment can significantly reduce environmental exposure to this bacterium. In healthcare settings, strict adherence to hand hygiene and sterilization protocols minimizes the risk of nosocomial infections. Food safety practices, including thorough washing and cooking of seafood and vegetables, are crucial to prevent foodborne illnesses caused by R. ornithinolytica. Additionally, educating healthcare providers and the public about the potential risks and transmission routes of this organism enhances awareness and encourages preventive behaviors. Implementing these measures effectively reduces the incidence of this bacterium infections, protecting vulnerable populations and promoting public health.

Keynotes

Raoultella ornithinolytica, previously classified under the genus Klebsiella, is a Gram-negative, encapsulated, non-motile, and rod-shaped bacterium. It is an opportunistic pathogen found in the environment, especiallyin water and soil, and can also be isolated from a variety of foods. When in its active form, meaning it is causing an infection, this oragnisms has several key features and implications:

Infections and Clinical Significance: It is known to cause a range of infections, particularly in individuals with compromised immune systems. The bacterium can cause pneumonia, bacteremia, cholangitis, and urinary tract infections. Its presence in clinical settings is of concern due to its resistance to multiple antibiotics and its potential to cause severe infections.

Raoultella ornithinolytica on prolonged incubation on CLED Agar — Fig. *Raoultella ornithinolytica* on prolonged incubation on CLED agar after 5 days of incubation losing lactose fermenting (LF) colonies to non-lactose fermenting colonies (NLF)

Antibiotic Resistance: One of the hallmark features of R. ornithinolytica is its inherent and acquired resistance to various antibiotics. This includes resistance to commonly used antibiotics such as ampicillin. The bacterium produces beta-lactamase enzymes, which contribute to its antibiotic resistance, complicating treatment options for infections.

Histamine Production: R. ornithinolytica is noteworthy for its ability to produce histamine, a compound involved in allergic reactions, in significant quantities when it contaminates fish and seafood. Consumption of food products contaminated with high levels of histamine produced by this bacterium can lead to scombroid poisoning, characterized by symptoms such as rash, nausea, vomiting, and headache.

Detection and Identification: In a clinical microbiology laboratory, R. ornithinolytica can be identifiable through its biochemical characteristics, such as its ability to decarboxylate ornithine. Advanced molecular techniques, including PCR and sequencing of specific genes, provide more accurate identification and characterization of the bacterium.

Treatment Challenges: The treatment of infections caused by R. ornithinolytica is complicated by its resistance to multiple antibiotics. However, the bacterium generally remains susceptible to carbapenems, making them a treatment option for severe infections and the selection of appropriate antibiotics requires susceptibility testing to ensure efficacy.

Epidemiology: R. ornithinolytica is increasingly recognized as an emerging pathogen, particularly in healthcare settings and its ability to colonize medical devices and form biofilms contributes to its transmission and persistence in these environments.

Raoultella ornithinolytica: Introduction, Morphology, Pathogenicity, Lab Diagnosis, Treatment, Prevention, and Keynotes