Investigating the Interactions Between Bartonella quintana and Human Neutrophils

Bartonella quintana is a fastidious Gram-negative bacterium that occupies the intra-erythrocytic niche. Spread by the human body louse, B. quintana is the etiological agent of urban trench fever, a disease of poverty, poor hygiene, and alcoholism. Unfortunately, B. quintana is a pathogen that thrives amongst those most vulnerable, including the immunocompromised and homeless, with countries in Europe and Africa recognising it as a re-emerging pathogen. B. quintana infections can result in many clinical manifestations, ranging from asymptomatic to potentially life-threatening chronic bacteraemia and endocarditis. Despite the increasing prevalence of B. quintana and its ability to cause serious disease, very little is known about the pathogen and many aspects of its infection strategy are yet to be characterised. Neutrophils are the most abundant white blood cell and are key for controlling microbial infections. Given their abundance and function, it is likely neutrophils interact with B. quintana during infection, but little research exists on the B. quintana - immune system interplay. Due to this knowledge gap, the goal of this project was to establish the basic interactions between B. quintana and neutrophils, using HL-60 cells differentiated to resemble a neutrophil model (nHL-60).

Some bacterial pathogens can combat the antimicrobial properties of neutrophils and survive intracellularly. Research demonstrates that some Bartonella species can survive in host cells such as endothelial cells and some are able to manipulate the antimicrobial functions of neutrophils. B. quintana intracellular survival in nHL-60s was assessed using a gentamicin protection assay. Infected nHL-60s were lysed and bacterial colonies were enumerated to indicate bacterial intracellular survival. B. quintana was found to survive up to 72 hours within nHL-60 cells despite their antimicrobial properties. Induction of premature nHL-60 death was also assessed using a cytotoxicity assay, which revealed no difference in the viability of infected or uninfected nHL-60 cells. A major virulence factor in the B. quintana arsenal is the VirB/D4 Type IV secretion system and the effectors (Beps) it injects into host cells to divert cell functions. To investigate targeting of nHL-60 cells by two well-studied Beps, BepC and BepE, translational fusion proteins containing a Bep fused to beta-lactamase were created. Expression of some of these proteins was confirmed in S17-1 E. coli, however expression could not be detected in B. quintana. Further studies are required to understand why.