Pantoea stewartii: A new pathogen as a cause of bacteremia?

The genus Pantoea currently comprises 31 species and two subspecies of Gram-negative bacilli (List of Prokaryotic names with Standing in Nomenclature; http://lpsn.dsmz.de). These micro-organisms are rarely considered pathogenic; the most significant species in humans is Pantoea agglomerans (P. agglomerans), previously called Enterobacter agglomerans^1,2. In 1993, Pantoea stewartii (P. stewartii) was transferred from the genus Erwinia, thus forming a new species within the genus Pantoea³. To our knowledge, this is the first report of bacteraemia caused by P. stewartii in a patient with a stroke.

A 57-year-old woman with hypertension and a basilar aneurysm was admitted to the intensive care unit after an arteriogram, placement of a stent and embolisation of the aneurysm. Upon completion of this procedure, she presented nausea, diplopia and a decreased level of consciousness. Another arteriogram showed complete thrombosis of the implanted stent, whereupon a new stent was placed. A computed tomography scan of the head showed multiple infarctions in the posterior region with bulbar involvement. Clinically, the patient was tetraplegic with pathological extensor motor response. Following 48 days of admission, the patient presented abdominal pain and fever (38.5 °C). Before antibiotic therapy was started, two blood samples and one urine sample were collected for culture (which was negative). In addition, empirical treatment was started with intravenous levofloxacin (500 mg/12 h). The blood cultures were incubated in the BACTEC FX monitoring system (Becton Dickinson, Frankin Lakes, NY, United States). After 11 h of incubation, the two samples tested positive, and subculture was performed on agar, with incubation at 37 °C. Gram staining revealed Gram-negative bacilli and, after 18 h of incubation, growth of abundant circular, greyish, shiny colonies was observed in pure culture on blood agar (Fig. 1). There was also growth of colonies with bluish pigmentation on UriSelect^TM 4 Medium (Bio-Rad Laboratories Inc., Marnes-la-Coquette, France) (Fig. 2). Matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) mass spectrometry (MS) version 9 (8.468 msp) (Bruker Biotyper, Billerica, MA, United States) was used to identify Pantoea septica (score 2.30), with a moderately consistent range of identification (up to range 5). However, the strain was sent to the Centro de Genómica e Investigación Oncológica [Centre for Genomics and Oncology Research] (GENYO) in Granada, Spain, for analysis of the 16S rRNA gene by means of sequencing⁴. A fragment of 1,212 base pairs was amplified, yielding 99.69% similarity to P. stewartii, strain 08BF 11 TN (access number KX 146472.1). The MicroScan WalkAway system (Beckman Coulter, Inc., CA, United States), panel NC82 for enterobacteria, was used to determine sensitivity to antimicrobial agents, revealing sensitivity to all antibiotics tested, except for ampicillin (minimum inhibitory concentration [MIC] >8 μg/ml). The interpretation was performed according to the criteria established by the European Committee on Antimicrobial Susceptibility Testing (EUCAST)⁵. The antibiotic therapy was replaced with ciprofloxacin (400 mg/12 h/IV) and maintained for 10 days. After the fourth day of treatment, the patient’s fever disappeared and, from a neurological point of view, she followed a favourable course with partial recovery of mobility. She was discharged after four months of admission.

Fig. 1.

Greyish-white, round, shiny colonies were observed on blood agar and ultimately identified as Pantoea stewartii (growth after 18 h).

Fig. 2.

Bluish colonies of Pantoea stewartii were seen on UriSelect^TM 4 Medium (growth after 48 h).

Pantoea spp. are Gram-negative, non-encapsulated, non–spore-forming micro-organisms that can be isolated from plants, seeds, environmental samples and human faeces^1,2. The most commonly pathogenic species in humans is P. agglomerans⁶, which can cause infection in various locations, including hospital outbreaks^7,8. In general, there are few reported cases of infection with this genus, although bacteraemia due to Pantoea dispersa has been reported⁹.

These micro-organisms are generally considered to be of low pathogenicity and most cases of infection occur in immunosuppressed patients. They can also cause infections in immunocompetent individuals, especially cholecystitis, and in recent years they have been reported to cause neonatal sepsis with a growing frequency¹⁰. It should be noted that the cases reported responded favourably to antibiotic therapy. They tend to be highly sensitive to antimicrobial agents, but most P. agglomerans strains are resistant to fosfomycin. The introduction of MALDI-TOF MS into the diagnostic routine may lead to the identification of uncommon new pathogenic species. However, under certain circumstances, such as a low identification score, an inconsistent diagnosis or the presence of unusual micro-organisms, definitive identification should be performed using molecular techniques, such as 16S rRNA sequencing. When MALDI-TOF MS is used for identification, some species of Enterobacterales, such as Klebsiella ozaenae, may be mistaken for micro-organisms belonging to the genus Pantoea⁹. Notable among all the biochemical characteristics of P. stewartii, along with Pantoea ananatis, are a positive indole test result (only the indologenes subspecies) and gelatin hydrolysis which could help to distinguish it from other species. However, due to genetic heterogeneity, this genus is difficult to identify by means of biochemical tests.

In conclusion, to our knowledge, this is the first case of bacteraemia caused by P. stewartii in pure culture. This case emphasises the need to confirm results under the above-mentioned circumstances.

Funding

No funding was received for this study.

Conflicts of interest

The authors declare that they have no conflicts of interest.