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Both clinical and environmental Caulobacter species act as opportunistic pathogens

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 and human health implications 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. 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 is mediated by a dose-dependent, cell-associated toxic factor that does not require active bacterial cells or host cellular innate immunity to elicit its toxic effects. Finally, we show that C. crescentus does not grow well in standard clinical culture conditions, suggesting that Caulobacter infections may be more common than generally appreciated but rarely cultured. Taken together, our findings redefine Caulobacters as opportunistic pathogens and highlight the importance of broadening our methods for identifying and characterizing pathogens. AUTHOR SUMMARY Bacterial species have historically been classified as either capable of causing disease in an animal (pathogenic) or not. Caulobacter species represent a class of bacteria that were thought to be non-pathogenic. Caulobacters have been widely studied and proposed to be used for various industrial and medical applications due to their presumed safety. However, recent reports of human Caulobacter infections raised the question of whether disease-causing Caulobacters have acquired special factors that help them cause disease or whether the ability to infect is a more general feature of most Caulobacters. By combining genomic sequencing and animal infection studies we show that a clinical Caulobacter strain is similar to lab Caulobacters and that all Caulobacters studied can infect a model host. We explore the mechanism of this infectivity and show that it is due to a toxic factor that associates with Caulobacter cells. We also provide a possible explanation for why Caulobacters have not traditionally been isolated from human patients, owing to their inability to tolerate the salt levels used in most medical culturing systems.