(228aw) Motility Is Necessary to Enable Efficient Intratumoral Dispersion of Salmonella

Small molecule cancer therapies are limited by diffusion. This prevents the therapies from reaching the entire tumor in high concentrations. Moreover, these diffusional limitations are one reason for drug resistant cancer relapse.

Facultative anaerobic bacteria, like Salmonella, on the other hand, are not restricted by diffusion since the bacteria can swim and colonize tumors 10,000 fold more than any other organ. But, the bacteria predominantly accumulate in quiescent and necrotic but not actively dividing tissue. This prevents Salmonella from delivering cancer therapies to a tumor uniformly. We hypothesize that highly motile Salmonella can colonize tumors uniformly, migrate far away from tumor vasculature and colonize actively dividing tissue.

To test this hypothesis, we transformed an arabinose inducible flhDC gene circuit into non-pathogenic Salmonella. We overexpressed flhDC to investigate the effect on aqueous motility. Mice were used to determine how highly motile Salmonella dispersed in in vivo tumors.

Overexpression of flhDC enabled a significant increase in aqueous motility. Upon arabinose induction, the Salmonella demonstrated a 50% overall increase in aqueous motility. Highly motile Salmonella populations had fewer non-motile bacterium when compared to control (P<.01). While 60% of control bacteria exhibited motilities between 0-15 micrometers/second, only 20% of flhDC induced Salmonella exhibited the same range of motility. Moreover, 85% of flhDC induced bacteria exhibited aqueous motilities between 16-30 microns/second compared to only 20% in the control.

Salmonella with overexpressed flhDC were also smaller than a control strain of Salmonella (P<.01). While the control Salmonella was approximately 10 µm in length, flhDC overexpressing Salmonella were 4 µm in length. This finding was consistent with other results reporting that aqueous motility is inversely proportional to bacterial size. This finding further confirms that flhDC overexpression strongly influences bacterial lifestyle and motility.  

The same flhDC overexpressing Salmonella was administered to mice along with a less motile control. Histological staining detected extravasated, highly motile Salmonella within tumors. This finding demonstrated that highly motile Salmonella could circumvent dispersion limitations and be an effective cancer drug delivery vehicle.

Engineering Salmonella to disperse uniformly after extravasation will improve drug distribution in a tumor. By developing a bacterial drug delivery vehicle that disperses, cancer therapies can be administered to the entire tumor. This can potentially limit drug resistance and relapse of the disease.