Persistent enhancement of bacterial motility increases tumor penetration

Abstract

Motile bacteria can overcome the transport limitations that hinder many cancer therapies. Active bacteria can penetrate through tissue to deliver treatment to resistant tumor regions. Bacterial therapy has had limited success, however, because this motility is heterogeneous and within a population many individuals are non-motile. In human trials, heterogeneity led to poor dispersion and incomplete tumor colonization. To solve these problems, a swarm-plate selection method was developed to increase swimming velocity. Video microscopy was used to measure the velocity distribution of selected bacteria and a microfluidic tumor-on-a-chip device was used to measure penetration through tumor cell masses. Selection on swarm plates increased average velocity four fold, from 4.9 to 18.7 μm/sec (P<0.05) and decreased the number of non-motile individuals from 51 to 3% (P<0.05). The selected phenotype was both robust and stable. Repeating the selection process consistently increased velocity and eliminated non-motile individuals. When selected strains were cryopreserved and subcultured for 30.1 doublings, the high-motility phenotype was preserved. In the microfluidic device, selected Salmonella penetrated deeper into cell masses than unselected controls. By ten hours after inoculation, control bacteria accumulated in the front 30% of cell masses, closest to the flow channel. In contrast, selected Salmonella accumulated in the back 30% of cell masses, farthest from the channel. Selection increased the average penetration distance from 150 to 400 μm (P<0.05). This technique provides a simple and rapid method to generate high-motility Salmonella that have increased penetration and potential for greater tumor dispersion and clinical efficacy.

Factors Affecting Bacterial Propagation towards Tumor Microenvironments

“…Since the directional efficiency, p, increases, the bacteria find their way to the tumor better than when p = even when the distance between the region of injection and the tumor increases. The numerical values we obtain match well with existing experimental results in [28] that show that control bacteria reach the tumor growing edge in about 8 hours, in vitro, where the distance between the injection site of bacteria and tumor edge is about 150 m µ . …”

“…While the distance between the tumor microenvironment and the region of injec tion also play a significant role, the population is the least effective parameter, particularly beyond a certain threshold value (called the saturation threshold of population). The results obtained from out model matches well with existing experimental results [28] that measure the time taken by control bacteria to reach the tumor microenvironment. To the best of our knowledge, this is the first research that characterizes the first passage times of injected bacteria reaching the tumor microenvironment, to clearly bring out the dependencies on the population, directional efficiency of the bacteria and the distance between the region of injection and the tumor.…”

“…Thus, recent research has focused on improving bacterial targeting strategies to tumor sites. Thornlow et al [28] show that increasing the motility of anerobic bacteria, Salmonella and generating homogeneous population of highly motile Salmonella, enhanced tumor penetration in vitro. Volume 4 Issue 1 -2016 The chemo taxis capability and their effectiveness in DDS motivated the study of diffusion models for bacteria.…”

Utilizing Salmonella to treat solid malignancies

“…Furthermore, prolonged colonization of melanoma tumors was associated with significant reduction in tumor burden (volume) and increased survival. The mobile capabilities of ST (ie, flagella) allow them to follow nutrient gradients generated by tumors and allows them to actively penetrate through stromal layers much more effectively than simple diffusion of non‐biological therapeutics . One study showed that ST can preferentially chemotax toward and proliferate in quiescent tumor cells, which would be beneficial for deep tumor penetration .…”

“…The mobile capabilities of ST (ie, flagella) allow them to follow nutrient gradients generated by tumors and allows them to actively penetrate through stromal layers much more effectively than simple diffusion of non-biological therapeutics. 34 One study showed that ST can preferentially chemotax toward and proliferate in quiescent tumor cells, which would be beneficial for deep tumor penetration. 35 Various attenuated strains have been tested extensively for efficacy in preclinical tumor models and include VNP20009, 36 SL7207, 37,38 and BRD509, with VNP20009 being the only strain used and studied extensively in clinical trials.…”