Raoultella planticola: Profile, Distribution, and Infections

Raoultella planticola is a gram-negative, facultatively anaerobic bacterium historically considered an environmental organism. However, in recent years, it has gained attention for its potential to cause opportunistic infections, particularly in immunocompromised individuals. Though relatively rare, documented infections highlight the need for greater awareness among clinicians and microbiologists.

Understanding its characteristics, distribution, and clinical significance can help improve detection and treatment strategies.

Taxonomy And Classification

Raoultella planticola belongs to the Enterobacteriaceae family, a diverse group of gram-negative bacteria that includes both commensal and pathogenic species. Initially classified within the genus Klebsiella due to genetic and phenotypic similarities, it was reclassified in 2001 when Haeggman et al. used 16S rRNA and rpoB gene sequencing to establish the Raoultella genus, named after microbiologist Didier Raoult.

Within this genus, R. planticola is closely related to Raoultella ornithinolytica and Raoultella terrigena. Despite shared biochemical traits, genetic analyses reveal distinct evolutionary pathways, particularly in metabolism and environmental adaptations. Whole-genome sequencing has further refined its classification, confirming its divergence from Klebsiella while highlighting conserved genetic elements that aid its survival.

This taxonomic distinction has clinical implications. Traditional biochemical identification methods often misclassify R. planticola as Klebsiella pneumoniae or Klebsiella oxytoca, leading to diagnostic errors that can affect antimicrobial susceptibility interpretations. Molecular techniques such as MALDI-TOF mass spectrometry and PCR-based assays are essential for accurate classification.

Natural Habitats And Distribution

Raoultella planticola thrives in freshwater, soil, and plant-associated ecosystems, metabolizing diverse organic compounds to persist in various environments. It has been identified in river sediments, coastal waters, and estuaries, where it contributes to organic matter decomposition and nutrient cycling. Its association with plant roots suggests interactions with plant microbiomes, though further research is needed.

Beyond natural ecosystems, R. planticola has been detected in seafood, dairy, and vegetables, likely due to contamination from water or soil during agricultural and processing stages. A study in the International Journal of Food Microbiology reported its presence in raw shellfish, raising concerns about its potential as a foodborne microorganism. While not a primary foodborne pathogen, its ability to survive refrigeration and resist certain preservatives suggests it can persist in food supply chains.

Municipal wastewater and hospital effluents also serve as reservoirs, reflecting its adaptability to human-impacted environments. Its presence in sewage treatment plants suggests a role in wastewater bioremediation, though the potential for horizontal gene transfer with other bacteria warrants further investigation.

Morphological And Biochemical Traits

Raoultella planticola is a short, rod-shaped gram-negative bacillus measuring approximately 0.3–1.0 µm in width and 1.0–3.0 µm in length. It is non-spore-forming and encapsulated, with a polysaccharide capsule that contributes to its mucoid colony morphology. On MacConkey agar, it forms pink, lactose-fermenting colonies.

The bacterium is facultatively anaerobic, capable of both fermentation and aerobic respiration. It is catalase-positive and oxidase-negative, differentiating it from many non-enteric gram-negative bacteria. It hydrolyzes urea, a trait shared with some Klebsiella species, and produces indole variably.

Carbohydrate metabolism highlights its versatility. It ferments glucose, sucrose, maltose, and mannitol but does not utilize dulcitol. It also uses citrate as a sole carbon source, a common diagnostic feature. Its enzymatic profile includes beta-galactosidase for lactose breakdown and ornithine decarboxylase for amino acid metabolism. These traits enable it to persist in nutrient-variable environments.

Identified Virulence Factors

Once considered an environmental bacterium with limited pathogenic potential, Raoultella planticola has been found to possess virulence factors that contribute to opportunistic infections. Its polysaccharide capsule enhances resistance to desiccation and antimicrobial agents while aiding adherence to host tissues. The mucoid phenotype in some strains suggests increased ability to evade phagocytosis.

The bacterium also produces siderophores, iron-chelating molecules essential for survival in iron-limited environments like the human body. Siderophore production enhances bacterial proliferation by sequestering iron from host proteins. Comparative genomic analyses reveal that R. planticola shares siderophore biosynthesis genes with Klebsiella pneumoniae, suggesting a conserved mechanism of iron acquisition.

Documented Infections In Humans

Although primarily environmental, Raoultella planticola has been increasingly reported in human infections, particularly in individuals with underlying conditions such as diabetes, malignancies, or immunosuppression. Infections arise from exposure to contaminated water, food, or medical devices, with clinical manifestations varying by site. Bloodstream infections, urinary tract infections, pneumonia, and soft tissue infections have all been documented.

Bacteremia, particularly in patients with central venous catheters or undergoing invasive procedures, is among the most concerning presentations. A study in BMC Infectious Diseases described a fatal case of septic shock in an immunocompromised patient, underscoring its potential severity. Gastrointestinal infections, including spontaneous bacterial peritonitis in cirrhotic patients, have also been reported. While community-acquired infections remain rare, nosocomial outbreaks highlight its persistence in healthcare settings.

Laboratory Identification Methods

Accurate identification of Raoultella planticola is crucial for differentiating it from closely related pathogens like Klebsiella pneumoniae. Traditional biochemical tests frequently lead to misclassification due to overlapping metabolic traits. Automated systems such as VITEK 2 and API 20E often misidentify R. planticola as Klebsiella species, necessitating more precise molecular techniques.

MALDI-TOF mass spectrometry has significantly improved species-level identification, providing rapid and reliable results. PCR-based assays targeting genetic markers such as the 16S rRNA and rpoB genes offer additional specificity. Whole-genome sequencing further refines classification, distinguishing R. planticola from other Enterobacteriaceae. Chromogenic media and selective culture conditions can aid in preliminary differentiation, particularly in mixed infections with multiple gram-negative bacilli. Enhancing diagnostic capabilities is essential for guiding appropriate antimicrobial therapy and epidemiological tracking.

Antibiotic Susceptibility Profiles

Treatment of Raoultella planticola infections depends on its antibiotic susceptibility profile, which varies among clinical isolates. While many strains remain susceptible to broad-spectrum beta-lactams, cephalosporins, and carbapenems, reports of multidrug-resistant (MDR) variants are increasing. Resistance mechanisms, including extended-spectrum beta-lactamase (ESBL) production and carbapenemase expression, have been identified in hospital-associated isolates.

A study in Antimicrobial Agents and Chemotherapy documented R. planticola strains harboring the blaKPC gene, conferring carbapenem resistance and significantly limiting treatment options. Aminoglycosides, fluoroquinolones, and trimethoprim-sulfamethoxazole generally show good activity against susceptible strains, though resistance patterns vary regionally.

Colistin resistance, though rare, has been observed in some isolates, complicating treatment in MDR cases. Empirical therapy should be guided by local antibiograms, with definitive treatment based on susceptibility testing. Given its potential for acquiring resistance genes through horizontal gene transfer, continuous surveillance of antimicrobial resistance trends is necessary to inform effective treatment strategies.