Engulfment, persistence and fate of Bdellovibrio bacteriovorus predators inside human phagocytic cells informs their future therapeutic potential

Abstract

In assessing the potential of predatory bacteria, such as Bdellovibrio bacteriovorus, to become live therapeutic agents against bacterial infections, it is crucial to understand and quantify Bdellovibrio host cell interactions at a molecular level. Here, we quantify the interactions of live B. bacteriovorus with human phagocytic cells, determining the uptake mechanisms, persistence, associated cytokine responses and intracellular trafficking of the non-growing B. bacteriovorus in PMA-differentiated U937 cells. B. bacteriovorus are engulfed by U937 cells and persist for 24 h without affecting host cell viability and can be observed microscopically and recovered and cultured post-uptake. The uptake of predators is passive and depends on the dynamics of the host cell cytoskeleton; the engulfed predators are eventually trafficked through the phagolysosomal pathway of degradation. We have also studied the prevalence of B. bacteriovorus specific antibodies in the general human population. Together, these results quantify a period of viable persistence and the ultimate fate of B. bacteriovorus inside phagocytic cells. They provide new knowledge on predator availability inside hosts, plus potential longevity and therefore potential efficacy as a treatment in humans and open up future fields of work testing if predators can prey on host-engulfed pathogenic bacteria.

DivIVA Controls Progeny Morphology and Diverse ParA Proteins Regulate Cell Division or Gliding Motility in Bdellovibrio bacteriovorus

“…This was then subcloned into the vector pK18mobsacB and then introduced into the B. bacteriovorus via a single crossover event at the 5' end of the gene, such that the ORF-tag is transcribed from the endogenous promoter in a merodiploid strain. To illuminate the cytoplasm of the B. bacteriovorus cells within prey, we used a previously published strain with full gene replacement of cytoplasmic marker protein Bd0064 with Bd0064mCerulean (Raghunathan et al, 2019). Genomic deletions of specific B. bacteriovorus genes were made using methods previously described (Capeness et al, 2013;Sockett, 2013, Lambert et al, 2016).…”

Insight into the Possible Use of the Predator Bdellovibrio bacteriovorus as a Probiotic

“…In response to the emergent increase of antimicrobial-resistant bacterial infections as a global health issue, numerous options are being examined to treat drug-resistant bacterial infections. Among these, recently, the use of Bdellovibrio bacteriovorus has been taken into consideration [110]. Sun and collaborators examined the ability of B. bacteriovorus to prey on multidrug-resistant (MDR) and extensively drug-resistant (XDR) Gram-negative clinical bacteria like Klebsiella pneumoniae, Escherichia coli, Acinetobacter baumanni, and Pseudomonas aeruginosa in sessile and planktonic culture.…”

“…B. bacteriovorus did not cause any significant alterations in the cytokine levels for the four cell lines tested, but it elicited cytokine production in activated blood macrophages, although the induction of IL-1β and TNF-α was lower than with E. coli exposed cells [117]. Raghunathan and collaborators studied the interactions of live B. bacteriovorus HD100 with human phagocytic cells (PMA-differentiated U937 cells) to determine the uptake mechanisms, its persistence inside phagocytes, and associated cytokine responses [110]. U937 cells were found to passively engulf B. bacteriovorus, which persisted within for 24 h without affecting cell viability.…”

Shedding Light on the Cell Biology of the Predatory Bacterium Bdellovibrio bacteriovorus

“…First, due to the exquisite proliferation mode of this species, biological questions that were not accessible with canonical model organisms can be tackled, which may illuminate novel mechanistic principles, for instance regarding the determinants of non-binary division. Second, whereas evidence accumulates supporting the use of B. bacteriovorus as a potential ally against pathogenic bacteria [e.g., (Atterbury et al, 2011;Shanks et al, 2013;Tyson and Sockett, 2016;Willis et al, 2016;Im et al, 2017;McNeely et al, 2017;Findlay et al, 2019;Raghunathan et al, 2019)], there is no doubt that a fundamental understanding of its biology and the underlying molecular dynamics are absolutely needed before concretely envisioning therapeutic applications. However, only a few studies aimed at visualizing proteins or other intracellular components of the predator cell inside or outside its periplasmic niche so far (Fenton et al, 2010a,b,c;Milner et al, 2014;Lambert et al, 2015;Kuru et al, 2017;Makowski et al, 2019).…”