Raoultella planticola: Profile, Distribution, and Infections

Raoultella planticola is a gram-negative, rod-shaped bacterium that lives primarily in water, soil, and the guts of fish, and only occasionally causes infections in humans. When it does infect people, it tends to target those whose immune systems are already weakened by cancer, liver disease, or kidney failure. The organism spent decades classified as a species of Klebsiella, and that misidentification still muddles diagnostics in many hospitals today, making the true scope of R. planticola infections difficult to pin down.

From Klebsiella to Raoultella

Until 2001, R. planticola was known as Klebsiella planticola. A phylogenetic study that year compared 16S rRNA and rpoB gene sequences across nine Klebsiella species and twenty species from eleven other genera in the Enterobacteriaceae family. The genetic data, combined with earlier biochemical and DNA-hybridization work, showed that the genus Klebsiella actually contained two distinct evolutionary clusters. The researchers proposed a new genus name, Raoultella, for the second cluster, reclassifying three species: R. planticola, R. ornithinolytica, and R. terrigena.¹ The name honors Didier Raoult, a French microbiologist.

This reclassification was not just academic housekeeping. Because many clinical laboratories still used older biochemical identification kits at the time, and some still do, R. planticola has been routinely misidentified as Klebsiella pneumoniae. That matters because treatment decisions, epidemiological tracking, and outbreak investigations all depend on knowing which organism you are dealing with.

Where R. planticola Lives

R. planticola is fundamentally an environmental organism. It thrives in water, soil, and on the surfaces and in the intestines of freshwater fish.² Its environmental versatility extends to some surprising niches. One study identified the organism in nasal swabs from cows with respiratory problems, suggesting it can colonize the upper airways of livestock.³ Genomic analysis of a strain isolated from industrial wastewater revealed about 47 genes conferring resistance to a range of heavy metals including lead, arsenic, zinc, mercury, nickel, silver, and chromium, which helps explain how the bacterium persists in polluted environments that would kill many other microbes.⁴

The organism is not part of the normal human gut flora in most people. It reaches humans mainly through environmental exposure: contaminated water, food (particularly seafood), or contact with soil. In hospital settings, R. planticola has been found contaminating liquid soap dispensers, which is a particularly troubling route because healthcare workers use those dispensers constantly.

The Identification Problem

One of the most persistent issues with R. planticola is that standard clinical lab equipment frequently mistakes it for Klebsiella pneumoniae. A retrospective study of bacteria collected from horse necropsies found that misidentification was common when using the Api 20E biochemical identification system or PCR methods designed to detect K. pneumoniae. The misidentifications primarily affected Raoultella species (R. terrigena and R. planticola) rather than closely related Klebsiella species. More advanced technology, specifically MALDI-TOF mass spectrometry, proved sufficient to distinguish the organisms correctly, though the researchers noted that commercial reference databases still need to be expanded with better R. terrigena and R. planticola spectra.⁵

The same misidentification problem has turned up in hospitals with human patients. When a batch of liquid soap for hospital use was found to be contaminated with R. planticola, the organism was initially identified as K. pneumoniae because the automated system in use could not tell the two apart.⁶ This kind of error means we are probably underestimating how often R. planticola causes infections. Many cases that were recorded in hospital databases as K. pneumoniae infections may have actually been caused by its reclassified relative.

Who Gets Infected

R. planticola infections are uncommon in otherwise healthy people. The published literature consistently points to immunocompromised patients as the primary risk group. People with active cancers, advanced liver disease, and chronic kidney disease seem especially vulnerable.⁷ In one study of twenty patients with R. planticola bloodstream infections, 85% had an underlying malignancy. The most common cancers were adenocarcinoma of the gallbladder or bile duct (seven patients) and blood cancers like leukemia and lymphoma (six patients).⁸

Liver cirrhosis is another strong risk factor. A case report described a 66-year-old man with alcoholic liver cirrhosis who developed R. planticola bloodstream infection and sepsis. He had been eating seafood at local restaurants in the two weeks before admission. The reporting clinicians also flagged his use of a proton pump inhibitor (a common acid-suppressing medication) as a possible contributor, since reducing stomach acid may allow ingested bacteria to survive passage through the stomach more easily.⁹

That said, healthy individuals are not completely immune. A case of R. planticola cholecystitis (gallbladder infection) was reported in a patient with no trauma history and no significant comorbidities, which the authors noted as unusual given the pattern seen elsewhere in the literature.¹⁰

Types of Infection

R. planticola does not have a single signature infection the way, say, Helicobacter pylori is associated with stomach ulcers. Instead, it shows up across a range of body sites, though certain patterns stand out.

Bloodstream Infections and Sepsis

Bacteremia, where the organism enters the bloodstream, is the most commonly reported presentation in the literature. The prognosis is generally good when the infection is caught early and the right antibiotics are started promptly.⁷ The danger rises sharply in patients who are already severely ill from other conditions, particularly those with compromised immune systems or organ failure.

Biliary and Abdominal Infections

R. planticola has a notable affinity for the biliary tract, the system of ducts connecting the liver, gallbladder, and intestine. A retrospective analysis of abdominal infections caused by Raoultella species found that biliary tract infections were the most common type, accounting for 20 out of 40 total cases (which included both R. planticola and R. ornithinolytica). Liver infections and peritonitis made up most of the remaining cases.¹¹ One case involved chronic cholecystitis with an unusual tissue change called adenomyomatous hyperplasia, where the gallbladder wall thickens abnormally. Bile cultures from the surgical removal of the gallbladder grew R. planticola, which matched the organism found in the patient’s blood cultures.¹²

Urinary Tract Infections

R. planticola can also cause urinary tract infections, though these seem to cluster among patients with underlying structural problems in the urinary system or a recent history of urological procedures. A five-year retrospective review found that structural abnormalities of the urinary tract were present in nine of the patients studied, while four had a recent history of urological procedures or instrumentation.¹³ Catheterization, kidney stones, and anatomical abnormalities all seem to create conditions that favor colonization.

Respiratory and Wound Infections

Pneumonia caused by R. planticola has been reported but appears rare.¹⁴ Wound infections also appear in the literature, generally in patients with open wounds exposed to contaminated soil or water. These less common presentations reinforce the picture of an opportunistic organism that takes advantage of whatever breach in the body’s defenses it can find.

Antibiotic Resistance

This is where R. planticola becomes genuinely worrying. While many strains remain susceptible to common antibiotics, researchers have documented multidrug-resistant (MDR) strains that are resistant to nearly everything in the medicine cabinet. One strain isolated from a clinical setting carried 30 antibiotic resistance genes spread across its chromosome and nine plasmids. It was resistant to every antibiotic class tested except colistin, a drug considered a last resort because of its toxicity. The strain carried two copies of the blaNDM-1 gene and one copy of blaKPC-2 on three different plasmids, explaining its resistance to all cephalosporins and carbapenems.¹⁵

Another carbapenem-resistant strain harbored blaIMP-4 and blaSHV-12, conferring resistance to third-generation cephalosporins and carbapenems, along with resistance to fluoroquinolones, fosfomycin, and tigecycline.¹⁶ Extended-spectrum beta-lactamase (ESBL) production has also been reported, including a nosocomial wound infection from sub-Saharan Africa involving a CTX-M-9 group ESBL-producing R. planticola strain.¹⁷

The resistance genes in R. planticola frequently sit on plasmids, which are small, mobile pieces of DNA that bacteria can swap with each other like trading cards. This is the same mechanism that has driven the global spread of antibiotic resistance in better-known pathogens. Because R. planticola shares an ecological and clinical niche with Klebsiella pneumoniae, there is a real concern that it could serve as a reservoir for resistance genes that eventually transfer to more virulent species. Clinicians reviewing cases of worsening infection despite apparently adequate antibiotic therapy should consider resistant strains of R. planticola as a possible cause.¹⁰

Virulence and Biofilm

R. planticola has traditionally been considered a low-virulence organism, meaning it is generally not aggressive enough to cause disease in healthy people. But “low virulence” does not mean harmless. The same extensively drug-resistant strain described above was shown to produce biofilm, a sticky matrix of sugars and proteins that bacteria secrete to anchor themselves to surfaces and shield themselves from antibiotics and the immune system. When tested in a Galleria mellonella (wax moth larva) infection model, the strain proved lethal, confirming that at least some R. planticola isolates carry meaningful virulence potential.¹⁵

Biofilm production is relevant beyond the laboratory. Biofilm-forming bacteria are notoriously harder to clear from catheters, surgical implants, and wound surfaces. If R. planticola can form biofilms in clinical settings, it may explain why some infections persist despite antibiotics that should work based on susceptibility testing alone.

Histamine Production and Scombroid-Like Poisoning

One of R. planticola’s more unusual traits is its ability to convert the amino acid histidine into histamine. This matters because histamine is the compound responsible for the flushing, headache, cramping, and diarrhea associated with scombroid fish poisoning, the illness you can get from eating improperly stored fish. When fish containing high levels of histidine (tuna, mackerel, and similar species) are not refrigerated properly, bacteria like R. planticola can multiply on the flesh and convert histidine to histamine in large quantities.¹⁸

The connection to seafood comes up repeatedly in case reports. The cirrhosis patient described earlier had been eating seafood at restaurants for two weeks before becoming septic.⁹ Whether the seafood itself was the vehicle of infection or whether the patient’s GI tract was already colonized remains hard to prove in individual cases, but the organism’s affinity for fish and its histamine-producing ability create a plausible exposure route.

Hospital Soap Contamination

R. planticola has an uncomfortable habit of surviving in hospital hygiene products. Two separate investigations have documented its presence in liquid soap. In one case, a batch of hospital soap was found to be contaminated with the organism, and the initial lab identification called it K. pneumoniae due to the limitations of the automated system in use. The investigators recommended that high-risk hospitalized patients use antimicrobial soaps for personal hygiene and called for stronger inspection of cosmetic and hygiene products entering the hospital supply chain.⁶

A separate investigation found R. planticola growing in refillable antimicrobial soap dispensers in a dental clinic. The bacterial loads were high enough to raise infection control concerns for both healthcare workers and patients. The study supported the CDC’s existing recommendation that soap dispensers should never be “topped off” (refilled before being empty), because topping off introduces fresh nutrients into an established bacterial colony, helping it grow rather than eliminating it.¹⁹ Disposable soap cartridges that are replaced entirely when empty are safer in clinical settings.

Beneficial Uses in Agriculture and Bioremediation

Not every encounter with R. planticola is a problem. Certain strains have attracted interest for their potential benefits in agriculture and environmental cleanup. The strain R. planticola Rs-2 has been studied as a plant growth-promoting bacterium, a type of soil microbe that helps plants absorb nutrients and resist disease. Researchers developed a controlled-release biofertilizer by encapsulating Rs-2 in a blend of alginate, starch, and bentonite, creating pellets that slowly release the bacteria into soil under field conditions. The formulation was considered a viable option for sustainable agriculture.²⁰

The organism’s tolerance for heavy metals also opens up possibilities in bioremediation, the use of living organisms to clean up contaminated sites. The strain isolated from industrial wastewater that carried dozens of heavy-metal resistance genes could potentially be deployed to help break down or sequester toxic metals in polluted soil and water.⁴ These applications represent the flip side of R. planticola’s environmental toughness: the same traits that let it persist in difficult habitats and resist chemicals can be harnessed for human purposes, so long as the strains used are carefully selected and do not carry the drug-resistance genes that make their clinical cousins dangerous.