Bloodstream Infections by Pantoea Species: Clinical and Microbiological Findings from a Retrospective Study, Italy, 2018–2023

(1) Background: The widespread use of MALDI-TOF coupled to mass spectrometry has improved diagnostic accuracy by identifying uncommon bacteria. Among Enterobacterales, Pantoea species have been seen to be implicated in several human infections, but their clinical and microbiological framework is currently based on a few anecdotal reports. (2) Methods: We conducted this five-year (2018–2023) single-center study aimed at investigating the prevalence and clinical and microbiological findings of Pantoea species bloodstream infections. (3) Results: Among the 4996 bloodstream infection Gram-negative isolates collected during the study period, Pantoea species accounted for 0.4% (n = 19) of isolates from 19 different patients, 5 of them being pediatric cases. Among Pantoea species isolates, P. agglomerans was the most frequently detected (45%; n = 9) followed by P. eucrina (30%; n = 6) and P. septica (15%; n = 3). Malignancy (35.7%) in adults and malignancy (40%) and cerebrovascular disease following meconium aspiration (40%) in pediatric patients as comorbidities and shivering and/or fever following parenteral infusion (36.8%) as a symptom/sign of Pantoea species bloodstream infection onset were the most frequently observed clinical features. Among adults, primary bloodstream infection was the most frequent (50%), whereas among pediatric patients, the most commonly identified sources of infection were catheter-related (40%) and the respiratory tract (40%). Overall, Pantoea species bloodstream infection isolates displayed high susceptibility to all the antibiotics except for ampicillin (63.2%), fosfomycin (73.7%), and piperacillin/tazobactam (84.2%). Targeted antibiotic treatment was prescribed as monotherapy for adults (71.4%) and combination therapy for pediatric patients (60%). The most prescribed antibiotic regimens were piperacillin/tazobactam (21.4%) in adults and meropenem- (40%) and aminoglycoside-containing (40%) antibiotics in pediatric patients. The overall 28-day all-cause mortality rate was 5.3% (n = 1). (4) Conclusions: The prevalence and 28-day mortality rate of Pantoea species bloodstream infections were low. The prescription of targeted therapy including broad-spectrum antibiotics could indicate an underestimation of the specific involvement of the Pantoea species in the onset of the disease, warranting further studies defining their pathogenic potential.


Introduction
Bloodstream infection is a serious clinical condition associated with increased mortality despite the development of new antibiotics and supportive therapies.Gram-negative bacilli are frequently involved in this kind of infection with their effects, which are strongly influenced by the type of pathogen and burden of drug resistance [1], patient conditions and rapidity and appropriateness of diagnosis and treatment [2,3].Among Enterobacterales, carbapenem-resistant Escherichia coli and Klebsiella pneumoniae are often the target of surveillance programs and studies, and the literature provides numerous elements on which to base the clinical and diagnostic management of patients with infections sustained by these pathogens [1].
However, the widespread use of MALDI-TOF coupled to mass spectrometry and 16S rRNA gene sequencing have improved diagnostic accuracy in microbiological laboratories, even identifying uncommon bacteria whose clinical relevance and antimicrobial susceptibility are sometimes difficult to determine [4,5], hindering the introduction of early and optimal antibiotic therapy.Furthermore, for many of these bacteria, EUCAST does not provide specific breakpoints, and the results of antimicrobial susceptibility tests must be interpreted using PK-PD (non-species-related) breakpoints.
Pantoea species' widespread ecological niche [6][7][8][9] and ability to spread in healthcare settings [18][19][20][21]49,[57][58][59][60][61] as well as the progressive increase in the number of immunocompromised and chronically ill patients could represent some favorable conditions for the likelihood of an increasing number of both sporadic and nosocomial infections sustained by these species in the years to come.
Given that our knowledge of infections caused by Pantoea species is based on case reports and case series and to inform current antibiotic treatments against these species, we conducted this five-year single-center study aimed at investigating the prevalence and clinical and microbiological findings of Pantoea species bloodstream infections.
Regarding targeted antibiotic treatment, adults were predominantly treated with monotherapy (71.4%), whereas combination therapy (60%) was preferred for pediatric patients.The most frequently prescribed antibiotic regimens were piperacillin/tazobactam (20%) in adults and meropenem-(40%) and aminoglycoside-containing (40%) antibiotics in pediatric patients.Patients with S. epidermidis or Enterococcus species polymicrobial bloodstream infection were also treated with vancomycin or daptomycin according to the antibiotic susceptibility testing results.One patient was found to have a Sphingomonas paucimobilis infection and was treated with piperacillin/tazobactam, while Lactococcus garvieae was detected in the same patient with detection of P. eucrina, and no treatment was started.

Discussion
Over the past decade, the widespread use of MALDI-TOF coupled to mass spectrometry has revolutionized clinical laboratory routines, allowing for the implementation of diagnostic protocols aimed at the rapid identification of species and resistance mechanisms [69][70][71][72][73][74][75][76] for early and optimal antibiotic prescription [77].The increase in speed and diagnostic accuracy has also led to the identification of rare bacterial species whose pathogenic potential has not yet been fully defined.This study offered a contemporary insight into the epidemiology and clinical-microbiological features of Pantoea species bloodstream infections, also benefiting from 16S rRNA gene sequencing to avoid misidentification [78].Its findings reveal Pantoea species predominantly caused healthcare-associated bloodstream infections, with a very low prevalence among Gram-negative bacteria.Malignancy in adults and malignancy and cerebrovascular disease following meconium aspiration in pediatric patients as comorbidities and shivering and/or fever following parenteral infusion as a symptom/sign of Pantoea species bloodstream infection onset were the most commonly observed clinical features.Microbiologically, despite the multisusceptibility profile, Pantoea species bloodstream infections were predominantly treated with broad-spectrum antibiotics.From a prognostic point of view, the 28-day mortality rate was low, delineating the features of an infection with favorable outcomes.
Immunodeficiency seems to be a dominant clinical feature of patients with Pantoea species bloodstream infection, with malignant neoplasms being the main comorbidity in reported series [11,37,58,59,61].Although completely non-specific, the present study also highlighted the importance of shivering and/or fever following parenteral infusion as a symptom/sign of Pantoea species bloodstream infection onset, adding a factor to be evaluated for antibiotic prescription.
Despite the potential for de-escalation therapy, our study also showed that Pantoea species bloodstream infection patients were treated mainly with broad-spectrum antibiotics.On the one hand, this could indicate an underestimation of the specific involvement of Pantoea species in the onset of the disease.On the other, it could have been due to both the high Charlson comorbidity index and ampicillin resistance rate.Comparing the main reported series, the mortality of patients with Pantoea species bloodstream infection seems to vary quite significantly.In pediatric patients, it ranges from 0% [18] to 12.5% [79], and in adults, from 13% [11] to over 50% [60].Our study showed a lower all-cause mortality rate among adults (7.1%) and no death among pediatric patients, warranting further multicenter studies to define it properly.
The present study successfully gathered data from a single-center surveillance study, addressing critical gaps in epidemiological, clinical, and microbiological knowledge about Pantoea species.
However, some limitations should be acknowledged, including its retrospective nature and the fact that it was conducted in a single center, albeit one with a considerable capacity and encompassing all major medical and surgical specialties.Although our findings probably apply to similar settings, the narrow nature of our results, due to the limited sample size, does not allow generalized conclusions to be drawn.Therefore, the results of antimicrobial susceptibility tests should be interpreted with caution, especially for species represented by a single isolate.

Study Design and Data Collection
In this five-year study (October 2018-October 2023), we included all Gram-negative isolates recovered from positive blood cultures of patients admitted at the "Città della scienza e della salute di Torino", a 1900-bed tertiary referral hospital in Turin, Northwestern Italy, a region with a high prevalence of multidrug-resistant organisms with complex resistant phenotypes [80].Duplicate isolates obtained within a 20-day interval from the same patient were considered as part of a single positive blood culture episode and thus excluded from the analysis.The prevalence of Pantoea species positive blood culture episodes was investigated.Electronic medical charts of patients who suffered from Pantoea species bloodstream infection were retrospectively reviewed and demographic, clinical and microbiological features investigated.

Definitions
A Pantoea species bloodstream infection was defined as a bloodstream infection event documented by blood culture positivity for a Pantoea strain and concomitant Systemic Inflammatory Response Syndrome signs.Bloodstream infection onset was defined as the collection date of the index BC.Healthcare-associated Pantoea species bloodstream infection was defined as infection occurring while the patient was receiving healthcare, infection developed in a hospital or other healthcare facility that first appeared 48 h or more after hospital admission, or infection within 30 days after having received healthcare.The source of Pantoea species bloodstream infection was assessed according to the National Healthcare Safety Network, and primary bloodstream infection was defined as not secondary to infection at another body site.Catheter-related bloodstream infection was defined according to criteria of the Infectious Diseases Society of America clinical practice guideline [81].

Microbiological Diagnostics
The BACT/ALERT FA and FN Plus blood culture bottles (bioMérieux, Marcy l'Ètoile, France) were incubated in the BACT/ALERT Virtuo (bioMérieux, Marcy l'Ètoile, France).Flagged positive blood cultures were subjected to Gram staining and subculture on appropriate solid medium (Blood Agar and MacConkey Agar).Microbial species identification was performed on overnight subcultures by using matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS, Bruker DALTONIK GmbH, Bremen, Germany).Pantoea species identification was validated with a spectral score >2.00 and subsequently confirmed by 16S rRNA gene sequencing.Antimicrobial susceptibility testing was performed through a microdilution method (Panel NMDR on automated Microscan WalkAway 96 Plus System, Beckman Coulter, Nyon, Switzerland), and results were interpreted according to the current EUCAST clinical breakpoints for Enterobacterales (v.13.1) [82].

Statistical Analysis
Descriptive data are shown as absolute (n) and relative (%) frequencies for categorical data and median and interquartile range (IQR) for continuous variables.Summary statistics for MIC values included the MIC range.Data analysis was performed using Microsoft®Excel®for Microsoft 365 MSO (Version 2311 Build 16.0.17029.20028).

Conclusions
This study showed both the low prevalence and 28-day mortality rate of Pantoea species bloodstream infections.Malignancy in adults and malignancy and anoxic brain injury in pediatric patients as comorbidities and shivering and/or fever following parenteral infusion as a symptom/sign of Pantoea species bloodstream infection onset were the most commonly observed clinical features.Despite being predominantly detected in healthcare-associated bloodstream infections, Pantoea species showed a profile of antimicrobial multi-susceptibility.The prescription of targeted therapy including broad-spectrum antibiotics could indicate an underestimation of the specific involvement of the Pantoea species in the onset of the disease, warranting further multicenter studies for defining their pathogenic potential.

Table 1 .
Clinical features of patients with Pantoea species bloodstream infection.

Table 2 .
Antimicrobial susceptibility of Pantoea species strains included in the study.