Raoultella Planticola: Its Role in Human Infections

Raoultella planticola is a gram-negative bacterium that was long considered harmless, living quietly in soil, water, and on plant surfaces. Over the past two decades, though, it has been turning up in clinical labs with increasing frequency as the cause of urinary tract infections, bloodstream infections, biliary disease, wound infections, and neonatal sepsis. Whether this represents a genuine rise in infections or simply better diagnostic tools catching what older systems missed is an open question, but either way, the organism now commands real clinical attention.

From Klebsiella to Raoultella

For most of its known history, R. planticola was classified as a species of Klebsiella, the genus responsible for a large share of hospital-acquired infections. In 2001, gene sequencing of 16S rRNA and rpoB regions revealed enough genetic divergence to justify splitting it into a new genus, Raoultella.¹ That reclassification matters beyond taxonomy. Many automated laboratory identification systems were designed to detect Klebsiella species, and they continued to misidentify Raoultella for years after the split. A study comparing common lab platforms found that MicroScan and API20E systematically failed to correctly identify Raoultella isolates, labeling R. planticola as Klebsiella pneumoniae every time.² This means an unknown number of past Klebsiella infections were actually caused by Raoultella, and the organism’s true clinical footprint has been undercounted for decades.

Why Case Reports Are Multiplying

The recent wave of published R. planticola infections looks dramatic, but the trend is driven at least partly by improvements in laboratory identification. Older biochemical panels like the ID32E system correctly identified the species about 81% of the time, while newer platforms like VITEK2 reached roughly 93%, and MALDI-TOF mass spectrometry hit about 97%.³ MALDI-TOF in particular has spread rapidly through clinical microbiology labs over the past decade, and with it the ability to distinguish Raoultella from Klebsiella reliably. A comprehensive review noted that the introduction of more precise identification techniques is the most likely explanation for the recent surge in case reports, and that the true epidemiology and clinical significance of Raoultella infections remain uncertain.⁴

The upshot is that R. planticola is not necessarily becoming more dangerous or more common. It may have always been causing a share of infections that were chalked up to Klebsiella. Even 16S rRNA gene sequencing, one of the gold standards for bacterial identification, cannot reliably separate Raoultella from Klebsiella; rpoB sequencing or MALDI-TOF are needed for a clean distinction.²

Who Gets Infected and What the Risk Factors Are

R. planticola predominantly affects people who are already medically compromised. In one five-year retrospective study of 37 urinary R. planticola isolates, the average patient age was 77, and diabetes was the most common underlying condition, present in 16 of the patients.⁵ A literature review of UTI cases found that four out of six patients were immunosuppressed, with risk factors including invasive procedures and contaminated medical equipment.¹ A review of broader case literature noted that infection frequently follows foreign-body introduction (catheters, surgical instruments), and that ICU stays and indwelling catheters are recurring risk factors.⁶

Still, not every case fits the immunocompromised profile. One reported case involved a young, otherwise healthy man who developed a serious soft-tissue infection after a crush injury to his thumb in a soiled environment.⁷ When a healthy person is exposed to heavy environmental contamination, particularly through a break in the skin, R. planticola can become an opportunistic pathogen on its own terms.

Urinary Tract Infections

The urinary tract is one of the most commonly reported sites for R. planticola infection. In the five-year retrospective study mentioned earlier, the spectrum ranged from simple cystitis in 15 patients to urosepsis in two patients and septic shock in two others.⁵ The most common presenting complaint was altered mental status, followed by fever, which can make the infection tricky to recognize in elderly patients who may not show classic urinary symptoms. A separate study of UTI treatment outcomes found that ceftriaxone and ciprofloxacin were the most frequently used antibiotics, and the median hospital stay was five days, though two patients with bacteremia and severe underlying disease died during hospitalization.⁸

Pediatric UTI cases also occur. A two-year-old girl with dysuria and foul-smelling urine was found to be co-infected with both R. planticola and E. coli, and was treated with a ten-day course of antibiotics followed by imaging to rule out kidney abnormalities.⁹ Children with congenital anomalies of the kidney and urinary tract may be at heightened risk.¹⁰

Bloodstream Infections and Sepsis

When R. planticola reaches the bloodstream, outcomes can be severe. A striking theme in case reports is a link to seafood consumption. One case described a 56-year-old woman who developed bacteremia and symptoms consistent with cholangitis four days after eating a seafood salad containing squid and octopus.¹¹ Another involved a 66-year-old man with alcoholic liver cirrhosis who developed sepsis after two weeks of eating seafood at local restaurants; his use of a proton pump inhibitor, which reduces stomach acid that would normally kill ingested bacteria, may have facilitated translocation from the gut to the bloodstream.¹²

R. planticola bacteremia has also been fatal. A burn patient developed septic shock on the tenth day of hospitalization, and blood cultures identified R. planticola.¹³ When compared head-to-head with Klebsiella oxytoca, a close relative, patients with Raoultella-positive cultures were significantly more likely to be in the ICU at the time of or within 48 hours of specimen collection (about 44% versus 25%), and a higher proportion had bloodstream infections rather than urinary isolates.¹⁴ That comparison raises the possibility that Raoultella may be underestimated as a pathogen, though selection bias in who gets tested could also play a role.

Biliary and Abdominal Infections

The biliary system, which includes the gallbladder and bile ducts, is another notable target. As of a 2017 literature review, only five cases of biliary R. planticola infections had been reported, including cholecystitis, cholangitis, and pancreatitis with retroperitoneal abscess.¹⁵ More recent data from a larger retrospective study of 40 abdominal Raoultella infections found that half were biliary tract infections, with the remainder including liver infections and peritonitis.¹⁶

One illustrative case involved a 67-year-old woman whose blood cultures grew R. planticola and whose bile cultures after cholecystectomy grew the same organism. Pathology of the removed gallbladder revealed chronic cholecystitis and adenomyomatous hyperplasia.¹⁷ The connection between R. planticola and the biliary system makes biological sense given the organism’s environmental presence in food and water, its likely route through the gastrointestinal tract, and the biliary system’s anatomical connection to the gut.

Wound and Soft-Tissue Infections

Skin and soft-tissue infections caused by R. planticola tend to follow direct environmental contamination. The young tiler with the crush injury needed hospital admission, surgical washout, and debridement after oral antibiotics failed to control the infection.⁷ Another case involved a 73-year-old woman who developed a wound infection of her lower leg after a dog scratch.¹⁸ Postoperative wound infections have also been reported, including one following Mohs surgery for skin cancer.¹⁹ Nosocomial wound infections are especially concerning. One hospital-acquired case from sub-Saharan Africa involved a strain carrying CTX-M-9 group extended-spectrum beta-lactamase resistance genes, meaning many first-line antibiotics were useless against it.²⁰

Infections in Newborns and Children

Some of the most sobering cases involve neonates. A report of four neonatal sepsis cases found that all the infants had been hospitalized for days to weeks before infection and had received broad-spectrum antibiotics beforehand. Symptoms included poor responsiveness, recurrent cyanosis (a bluish discoloration from low oxygen), apnea, and jaundice. All four R. planticola strains were resistant to cephalosporins and penicillins, and some had developed resistance to carbapenems, leaving only amikacin as an effective option. All four infants ultimately recovered.²¹

In another case, a premature infant initially developed purulent eye discharge on day 34 of life. Conjunctival cultures grew R. planticola, and she was treated with topical netilmicin eye drops. Eleven days later, however, the same organism appeared in her blood cultures, accompanied by fever, desaturation, apnea, and severely abnormal inflammatory markers.²² These neonatal cases underscore a pattern: prolonged hospitalization and prior antibiotic exposure may clear competing bacteria and create space for R. planticola to establish itself and develop resistance.

The Antibiotic Resistance Problem

R. planticola carries a chromosomal class A beta-lactamase called PLA-1, which provides an innate ability to break down penicillins and some cephalosporins. Laboratory characterization has shown that PLA-1 also has measurable activity against cefepime and, to a lesser extent, cefotaxime, though it does not attack ceftazidime.²³ This built-in resistance to penicillins is why ampicillin, for instance, consistently shows up as ineffective in susceptibility reports across case studies.

Beyond its innate defenses, R. planticola can acquire additional resistance genes on plasmids, the small circular DNA molecules that bacteria trade with each other. One genomic analysis identified a multidrug-resistant strain carrying both IMP-4 and SHV-12 genes on a single plasmid, conferring resistance to carbapenems as well as extended-spectrum cephalosporins.²⁴ Even more alarming, a pair of near-identical clinical isolates were found to carry three separate resistance plasmids simultaneously, including two different NDM-1 plasmids and a KPC-2 plasmid, which together confer resistance to virtually all beta-lactam antibiotics including the carbapenems that are typically reserved as last-resort drugs.²⁵

Because R. planticola is a close relative of Klebsiella pneumoniae, one of the most clinically significant drug-resistant organisms worldwide, the ability of Raoultella to harbor and potentially transfer these resistance plasmids is a concern that extends beyond the organism itself. If Raoultella acts as a reservoir for resistance genes in hospital environments, it could accelerate the spread of carbapenem resistance to other members of the Enterobacteriaceae family.

How R. planticola Stacks Up Against Klebsiella

The close genetic relationship between Raoultella and Klebsiella invites comparison. A study directly comparing clinical outcomes between Raoultella species and Klebsiella oxytoca found some notable differences. Patients with Raoultella were significantly more likely to be in the ICU at or near the time their cultures were collected (about 44% versus about 25%), and bloodstream infections were proportionally more common among Raoultella patients, while urinary isolates were more common among K. oxytoca patients.¹⁴ Whether this reflects greater virulence in Raoultella, or simply that Raoultella gets identified more often in sicker patients who receive the advanced diagnostic workup needed to detect it, is not yet settled.

Treatment Approaches

Most R. planticola infections respond to standard antibiotic therapy once the organism and its susceptibility profile are identified. In urinary tract cases, ceftriaxone and ciprofloxacin are the most commonly used agents, and outcomes are generally good for patients without severe comorbidities.⁸ Biliary infections frequently require surgical intervention alongside antibiotics, as seen in cholecystectomy cases.¹⁷ Wound infections may need debridement. In each case, the antibiotic choice hinges on susceptibility testing, because innate and acquired resistance patterns vary enough that empirical treatment can miss.

The neonatal cases are a particularly instructive cautionary tale. All four strains in one report were resistant to cephalosporins and penicillins, and some had developed carbapenem resistance, leaving amikacin as the sole effective option.²¹ In resource-limited settings where amikacin or other aminoglycosides may not be readily available, a multidrug-resistant R. planticola infection could become very difficult to manage.

The Seafood Connection and Other Environmental Routes

R. planticola was originally described as a bacterium of soil, water, and plant surfaces.¹⁰ Several bloodstream infection cases have pointed to seafood as a plausible route of entry, particularly raw or undercooked preparations.¹¹ The organism can survive in aquatic environments and colonize marine organisms, so the path from contaminated food to the gastrointestinal tract and then to the biliary system or bloodstream is biologically plausible. Factors that reduce stomach acid, such as proton pump inhibitors, may lower the barrier to gut translocation.¹²

Soil exposure is another clear route. The soft-tissue cases following a crush injury in a dirty work environment and a dog scratch both involved direct inoculation of environmental bacteria into broken skin.⁷ Hospital environments present a third route, where contaminated equipment, catheters, or surfaces can introduce the organism to vulnerable patients.

Beyond Human Disease

R. planticola has a rich life outside the clinic. Genomic analysis of a strain isolated from industrial wastewater identified roughly 47 genes related to resistance to heavy metals including lead, arsenic, zinc, mercury, and chromium. The same genome contained plant growth-promoting genes involved in producing the plant hormone indole acetic acid and in solubilizing phosphate, a key nutrient for crops.²⁶ These traits make R. planticola a candidate for bioremediation of contaminated sites and for agricultural applications as a plant-growth promoter. The dual identity of this organism, both a potential human pathogen and a beneficial environmental microbe, complicates any blanket characterization of it as simply dangerous.

On the veterinary side, a study of local rabbits with respiratory distress found that R. planticola caused mild to moderate emphysema in the animals’ lungs.²⁷ Whether animals serve as meaningful intermediaries for human infection remains unclear, but the finding confirms that R. planticola is not limited to colonizing plant and aquatic surfaces; it can actively cause disease in mammals outside laboratory conditions.

Pneumonia and Other Rare Presentations

Although the urinary tract, bloodstream, and biliary system account for the majority of reported R. planticola infections, the organism has also been documented as a cause of pneumonia.²⁸ A case of infective endocarditis in a four-year-old child has been reported as well.⁶ As an early case report noted, only 29 total human infections and just two urinary tract cases had been published at that point, underscoring how recently R. planticola entered the clinical conversation.²⁹ The list of documented infection sites will almost certainly keep growing as diagnostic identification continues to improve and more clinicians think to look for it.

The definite pathogenic mechanism by which R. planticola causes disease in humans is still not well understood. Unlike better-studied relatives such as K. pneumoniae, which have well-characterized capsule and siderophore systems, R. planticola’s virulence toolkit has not been comprehensively mapped. The organism’s ability to cause disease in otherwise healthy individuals, albeit rarely, suggests that it possesses genuine virulence factors beyond simple opportunism, but defining those factors will require further research.