Both clinical and environmental Caulobacter species are virulent in the Galleria mellonella infection model

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.

Genes required for the formation of virulence-provoking bacterial sphingolipids

“…In previous work, Moore and Gitai (Moore and Gitai, 2020) suggested that the virulence-provoking compound was contained in the LPS fraction. However, this present study shows that genes required for SphL, PSphL, and specifically CPG2 formation are needed for C. crescentus virulence on larvae of the greater wax moth G. mellonella .…”

“…Notable humoral immune responses include the formation of antimicrobial peptides and the activation of phenol oxidase leading to melanin formation and melanization of the larvae when counteracting an immune challenge due to invasive microbes (Pereira et al ., 2018). Recent studies have shown that distinct Caulobacter species are virulent in the G. mellonella infection model (Moore and Gitai, 2020). In order to analyze whether bacterial SphLs might be responsible for this virulence, we tested the effect of various C. crescentus strains when injected into G. mellonella larvae.…”

“…However, an increase in clinical Caulobacter isolates (Justesen et al ., 2007; Bridger et al ., 2011) put this perception into question. A recent report (Moore and Gitai, 2020) shows that both clinical and non-clinical isolates of Caulobacter show virulence on greater wax moth ( Galleria mellonella ) larvae due to a heat-resistant, cell-associated factor.…”

CauloKO: an ordered transposon mutant library inCaulobacter crescentus

“…C. crescentus has emerged as a powerful model system for a wide range of important behaviors including its canonical "crescent" cell shape, cell cycle, and polarity [17][18][19]. Recent studies have also suggested that C. crescentus can function as an animal pathogen [20]. While many of these processes have been studied using classical genetic techniques, the continued discovery of novel determinants in C. crescentus is aided by a number of tools generated in the organism, including a cosmid library [21], a fluorescently-tagged localisome library of open reading frames (ORFs) [22], and a web-based visualizer of all published high-throughput genomic data [23].…”

Influence of Citrobacter freundii Infection on Ion Levels of Model Organism Galleria mellonella Larvae

“…In recent years, invertebrate infection models have been used to determine the effects of pathogenic bacteria on animals. Especially, G. mellonella larvae are the most preferred invertebrate infection models (Asai et al, 2020;Moore & Gitai, 2020;Ochoa et al, 2021). G. mellonella larvae are an important infection model that is suitable for injection, can be quickly mass production and can survive at 30-37 °C after injection.…”