Background and Aims:
Colorectal cancer (CRC) is the second-leading cause of cancer-related deaths in Australia. As such, there is incentive to improve treatment options, with a promising avenue being tumour-homing bacteria. Advancements in synthetic biology have enhanced bacterial genetic engineering capabilities, allowing for the development of programmable ‘micro factories’ for bacterial drug delivery with improved specific anti-tumour activity and safety. Bacteria, however, have notoriously rapid evolution, and it is unknown how bacteria adapt to the tumour microenvironment.
This project aims to characterise the tumour-specific evolution of Escherichia coli Nissle 1917 (EcN) in mouse and human CRC in order to identify genetic targets that can be engineered to enhance tumour-specific colonisation for future therapeutic strains.
Methods:
Mouse Study
Tumour-bearing mice were orally administered with EcN, then two weeks later EcN were re-isolated from colonised tumours, then readministered into the next cohort of tumour-bearing mice, for a total of five passages. EcN and parental isolates will undergo whole genome sequencing (WGS) to identify genomic changes.
Clinical Study
EcN isolated from human CRC tissue via plating on nonselective agar were identified using EcN-specific qPCR for WGS analysis.
Results:
Mouse Study
Across all five passages, the EcN retained their tumour-specific colonisation capabilities. Unexpectedly, the tumour colonisation rate displayed an oscillating pattern, from 44% for cohort 1 (7/16 tumours across four mice), with succeeding cohorts yielding 87% (13/15 tumours, eight mice), 50% (6/12 tumours, four mice), 78% (7/9, five mice), and 44% (4/9, three mice).
Clinical Study
From three patient tumours, approximately 100 EcN colonies each have been isolated and will undergo WGS alongside the parental strain.
Conclusions and Significance/Impact:
Despite the variation in tumour colonisation rates seen in the mouse model, EcN has remained tumour-localised across all five passages, which highlights the safety of this strain as a potential therapeutic option for CRC. Future WGS analysis of bacterial isolates is likely to reveal a genomic mechanism that can be exploited to engineer stronger and more robust tumour-colonising capabilities.