A spotlight on Raoultella ornithinolytica: A newly emerging life-threatening zoonotic pathogen

The genus Raoultella contains Raoultella ornithinolytica (R. ornithinolytica), which was previously named as Klebsiella ornithinolytica. R. ornithinolytica is a Gram-negative bacillus that belongs to the family Enterobacteriaceae. This pathogen is normally present in aquaculture, and it has been isolated from fish, insects, and wild and domestic birds. Seafood and poultry products are incriminated as sources of R. ornithinolytica infection. Community-acquired human infection with R. ornithinolytica is common. This infection is commonly associated with sepsis, bacteremia, food poisoning, purities, and urinary and respiratory tracts’ infections. Rapid acquired resistance of R. ornithinolytica strains is common, and it is related to the development of resistant genes. R. ornithinolytica is considered as a newly emerging life-threatening pathogen worldwide. Hence, it is very important to determine the role of animals and birds in the epidemiological situation of this pathogen as well as the detection of antibiotic resistance genes before treatment.


Introduction
Raoultella ornithinolytica (R. ornithinolytica) belongs to the family Enterobacteriaceae which is Gram-negative, non-motile, aerobic or facultative anaerobic, and encapsulated bacillus [1,2]. This pathogen is a member of Raoultella genus which was previously designated Klebsiella till differentiation through the phylogenetic analysis of the organism in 2001 [3][4][5]. The underreported incidence of R. ornithinolytica may be due to the inaccuracy of conventional identification methods causing uncertainties [6,7] as well as the similarities with species of Klebsiella, namely, Klebsiella pneumoniae and Klebsiella oxytoca [8].
R. ornithinolytica is commonly found in aquatic environments [9], and it could be isolated from fish and insect species such as ticks and termites [2,10,11]. The role of R. ornithinolytica in animal infection has not yet been well studied. Some reports showed isolation of this pathogen from wild and domestic birds [12][13][14][15][16][17]. Moreover, R. ornithinolytica is a potent histamine producer [18,19], so it became a pathogen of great importance in food products. The pathogen has also been isolated from poultry products [20] as well as seafood products [21,22]. It has been found that Raoultella species produced more than 1.000 mg histamine/kg in fresh tuna [19].
It is well known that R. ornithinolytica is an emerging, lethal and intermittent nosocomial pathogen and associated with community-acquired and/or hospital-acquired human infections [8,23,24]. The reported cases of R. ornithinolytica infections in humans included urinary, respiratory, and gastrointestinal tracts infections as well as wound, skin, and catheter-related blood-stream infections [25][26][27].
Although R. ornithinolytica can be treated easily, the development of resistant strains from natural environments and clinical material is common [28][29][30]. For example, a carbapenem-resistant R. ornithinolytica strain has been isolated from the urban river sediment in China [9].
Accordingly, this review article has been designed to spotlight on R. ornithinolytica pathogen, host susceptibility, and human infection as well as its treatment.

The Pathogen
The order Enterobacterales include genus Raoultella which is an aerobic, non-motile, Gramnegative, and encapsulated rods [3,31]. It is difficult to probably distinguish Raoultella from Klebsiella genus using conventional biochemical and phenotypic methods [32,33]. Raoultella was first classified in cluster II of Klebsiella genus and termed Klebsiella ornithinolytica [3,31,34]. Hence, ornithine decarboxylase has been used to differentiate between Raoultella and Klebsiella [28,35]. Based on sequence analysis of 16S rRNA, rpoB, gyrA, and gyrB genes, the cluster II of the genus Klebsiella was renamed as a new genus Raoultella [3,36]. The genus Raoultella contains four species; R. ornithinolytica, Raoultella planticola, Raoultella terrigena, and Raoultella electrica [26,37]. The only way to achieve reliable identification of R. ornithinolytica and other species of Raoultella is by applying supplemental biochemical tests and/or using Analytical Profile Index E20 and Matrix-Assisted Laser-Desorption Ionization Time-of-Flight Mass Spectrometry [13,16,20,33,38,39]. It is oxidase-negative and catalase-positive and grows at low temperatures using sorbose as a source of carbon [3,6,31].
R. ornithinolytica is considered as an emerging pathogen that has some virulence factors such as capsule, and CFA/I and CFA/II colonization factors, in addition to the production of siderophores, histamine, and bacteriocins [8,40,41].
The isolation of R. ornithinolytica from animals or birds is infrequent. For instance, it has been detected in the cloacal samples of apparent healthy vulture [14] and in a case of hepatitis in ring-neck [12]. In addition, R. ornithinolytica has been described during a routine evaluation of aerobic enterobacteria in cloacal microbiota of birds in Argentina [16]. Mallard ducks also have been found to harbor R. ornithinolytica in their intestines and could act as a reservoir for this bacterium [13]. Moreover, R. ornithinolytica isolate was found in the droppings of the migratory common crane in Slovakia [15]. Recently, in Spain, R. ornithinolytica has been isolated from wild Canarian Egyptian vultures after feeding on fish [17].
In Thailand, previous studies demonstrated that R. ornithinolytica and R. terrigena could form a biofilm during seafood processing [21,22,46]. For the first time in Egypt, strains of R. ornithinolytica have been detected in chicken products including nuggets, strips, burgers, luncheon meats, pane, frankfurters, and minced chicken meat. Thirty-three bacterial isolates out of forty samples (82.50%) were Enterobacteriaceae and three isolates out of 33 (9.09%) were regarded as R. ornithinolytica [20].
To obtain good results after the treatment of R. ornithinolytica infection, the antimicrobial resistance of the isolates should be detected. The multidrug resistance of R. ornithinolytica against some antimicrobials as quinolones, cephalosporins, and carbapenems has been recorded [68,69]. This type of resistance is primarily associated with the development of the resistance genes [70]. The first reported case of R. ornithinolytica resistance was in India, where metallo-β-lactamase-1 producing R. ornithinolytica was isolated from the wound in a patient with urethral injury [30]. Later, R. ornithinolytica antibiotic resistance has been reported in many countries like China [67], India [71] and Brazil [72].
The resistance of Raoultella strains to ampicillin [10,56] that is related to the expression of

Conclusion
R. ornithinolytica is considered a newly emerging life-threatening pathogen world-wide. Hence, it is very important to determine the role of animals and birds in the epidemiological situation of this pathogen as well as the detection of antibiotic resistance genes before treatment.

Author's Contributions
WAA has collected and drafted the manuscript, formatted it, and approved the final manuscript.

Acknowledgments
The author did not receive any funds for this study.

Competing Interests
The author declares that she has no competing interests.