Abstract
The Caulobacter genus, including the widely-studied model organism Caulobacter crescentus, has been thought to be non-pathogenic and thus proposed as a bioengineering vector for various environmental remediation and medical purposes. However, Caulobacter species have been implicated as the causative agents of several hospital-acquired infections, raising the question of whether these clinical isolates represent an emerging pathogenic species or whether Caulobacters on whole possess previously-unappreciated virulence capability. Given the proposed environmental and medical applications for C. crescentus, understanding the potential pathogenicity of this bacterium is crucial. Consequently, we sequenced a clinical Caulobacter isolate to determine if it has acquired novel virulence determinants. We found that the clinical isolate represents a new species, Caulobacter mirare that, unlike C. crescentus, grows well in standard clinical culture conditions. C. mirare phylogenetically resembles both C. crescentus and the related C. segnis, which was also thought to be non-pathogenic. The similarity to other Caulobacters and lack of obvious pathogenesis markers suggested that C. mirare is not unique amongst Caulobacters and that consequently other Caulobacters may also have the potential to be virulent. We tested this hypothesis by characterizing the ability of Caulobacters to infect the model animal host Galleria mellonella. In this context, two different lab strains of C. crescentus proved to be as pathogenic as C. mirare, while lab strains of E. coli were non-pathogenic. Further characterization showed that Caulobacter pathogenesis in the Galleria model is mediated by lipopolysaccharide (LPS), and that differences in LPS chemical composition across species could explain their differential toxicity. Taken together, our findings suggest that many Caulobacter species can be virulent in specific contexts and highlight the importance of broadening our methods for identifying and characterizing potential pathogens.
Highlights
The free-living, gram-negative genus Caulobacter was first described and classified as a group of rod-shaped, stalk possessing bacteria in 1935 [1, 2]
Caulobacter species represent a class of bacteria that were thought to be non-pathogenic
There was only one bacterial species that could be cultured from the peritoneal fluid using Danish blood agar medium, and the infection responded to gentamycin treatment suggesting that this species was the likely cause of the infection [7]
Summary
The free-living, gram-negative genus Caulobacter was first described and classified as a group of rod-shaped, stalk possessing bacteria in 1935 [1, 2]. Since their identification, Caulobacter have been observed in rhizosphere, soil, and aqueous environments, including drinking water reservoirs [3, 4]. Caulobacter have been observed in rhizosphere, soil, and aqueous environments, including drinking water reservoirs [3, 4] This genus has been considered non-pathogenic due to lack of presence in infection cases, no obvious pathogenicity islands, and increased bacterial mortality at human body temperatures [5]. It remains unclear whether clinical isolates have acquired virulence mechanisms absent from other Caulobacters, or if Caulobacter species generally have the capacity for human disease in the right context
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