Bacterial imaging in tumour diagnosis

Abstract

Some bacteria, such as Escherichia coli (E. coli) and Salmonella typhimurium (S. typhimurium), have an inherent ability to locate solid tumours, making them a versatile platform that can be combined with other tools to improve the tumour diagnosis and treatment. In anti-cancer therapy, bacteria function by carrying drugs directly or expressing exogenous therapeutic genes. The application of bacterial imaging in tumour diagnosis, a novel and promising research area, can indeed provide dynamic and real-time monitoring in both pre-treatment assessment and post-treatment detection. Different imaging techniques, including optical technology, acoustic imaging, magnetic resonance imaging (MRI) and nuclear medicine imaging, allow us to observe and track tumour-associated bacteria. Optical imaging, including bioluminescence and fluorescence, provides high-sensitivity and high-resolution imaging. Acoustic imaging is a real-time and non-invasive imaging technique with good penetration depth and spatial resolution. MRI provides high spatial resolution and radiation-free imaging. Nuclear medicine imaging, including positron emission tomography (PET) and single photon emission computed tomography (SPECT) can provide information on the distribution and dynamics of bacterial population. Moreover, strategies of synthetic biology modification and nanomaterial engineering modification can improve the viability and localization ability of bacteria while maintaining their autonomy and vitality, thus aiding the visualization of gut bacteria. However, there are some challenges, such as the relatively low bacterial abundance and heterogeneously distribution within the tumour, the high dimensionality of spatial datasets and the limitations of imaging labeling tools. In summary, with the continuous development of imaging technology and nanotechnology, it is expected to further make in-depth study on tumour-associated bacteria and develop new bacterial imaging methods for tumour diagnosis.

FIGURE 1

Bacteria‐mediated imaging of live animals. Bacteria‐mediated imaging in mouse models of different tumours.

FIGURE 2

Optical imaging promotes bacteria‐mediated tumour imaging. (A). The diagram of tumour‐targeting engineered EcN bioluminescence imaging in vivo. (B) In vivo bioluminescence imaging of EcN‐FluC‐LRE and EcN‐Fluc through intravenous injection over 7 days. (C) Bioluminescence quantification at time points that correspond to panel b. Panels B–C are adapted with permission of the reference (Jiang et al., 2021), based on the knowledge sharing signed permission CC‐BY‐NC‐ND 4.0 (https://creativecommons.org/licenses/by‐nc‐nd/4.0/).

FIGURE 3

In vivo imaging and CFU of tumour‐targeting EcN in mouse tumours. (A) In vivo bioluminescence imaging of EcN‐Fluc and EcN‐FLUC‐LRE triggered by the addition of d‐fluorescein and d‐cysteine. (B) Bioluminescence quantification at time points that correspond to panel a. (C) In vivo bioluminescence imaging of EcN‐Fluc and EcN‐FLUC‐LRE at specific time points. (D) Bioluminescence quantification at time points that correspond to panel c. (E) Quantification of bacteria are isolated from tumour tissue at time points that correspond to panel c. Panels a–e are adapted with permission of the reference (Jiang et al., 2021), based on the knowledge sharing signed permission CC‐BY‐NC‐ND 4.0 (https://creativecommons.org/licenses/by‐nc‐nd/4.0/).

FIGURE 4

Ultrasonic imaging promotes bacteria‐mediated tumour imaging. (A). The diagram of engineered pBAD‐bARGSer‐AxeTxe EcN ultrasonic imaging in vivo. MC26 tumours were implanted subcutaneously in the mice and pBAD‐bARGSer‐AxeTxe EcN was injected through the tail vein on day 14. On day 17, the expression of bARGSer or FP was induced by intraperitoneal injection of L‐arabinose. (B) Representative B‐mode, pAM and BURST ultrasonic images of pBAD‐bARGSer‐AxeTxe EcN‐colonized tumours induced by l‐arabinose on day 18. (C) On day 19, the bargser or FP region within the tumour was disintegrated and reinduced to express and ultrasonic imaging was performed again. (D) Ultrasonic imaging of pBAD‐bARGSer‐AxeTxe EcN‐colonized tumours that were not induced by l‐arabinose on day 18. (E) Ultrasonic imaging of the pBAD‐bARGSer‐AxeTxe EcN‐colonized tumour in the control group on day 18. (F) Optical images of H&E stained tumour tissue sections. (G) Optical images of tumour tissue sections stained with anti‐E. coli antibodies. (h) Panel g indicates BURST images of the same tumour. (I) xAM images of panel g of the same tumour. (L) Quantification of bacteria isolated from tumour tissue on day 20. Panels B–L are adapted with permission of the reference (Hurt et al., 2023), based on a Creative Commons Attribution 4.0 International Licence CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/).

FIGURE 5

MR imaging promotes bacteria‐mediated tumour imaging. (A). The diagram of C. novyi‐NT spore germination MR imaging in vivo. (B) The left is the spore form of the bacteria, the middle is the nutrient form after germination and the right is the exchange of protons carried by the bacteria with water particles. (C) The top image marks T2‐weighted (T2w) images of tumours 12 h after spore injection, the middle image indicates MTRasym signals at 2.6 ppm and the bottom image refers to co‐localization of CEST contrast combined with T2w images. (D) The left panel indicates H&E staining of the tumour area. The right panel is gram staining of C. novyi‐NT. (E) Comparison of CEST signals of tumour ROI before and after spore injection. The Y‐axis scale on the right represents z‐spectra and the Y‐axis scale on the left represents the MTRasym curve. (F) Histograms of CEST contrast of tumour ROI before (blue) and after (red) spore injection. (G) Comparison of average MTRasym before and after C. novyi‐NT injection. Panels B–G are adapted with the permission of the reference (Liu et al., 2013), based on the licence under John Wiley and Sons Publisher (Licence number: 5778270078825).

FIGURE 6

Nuclear medicine imaging promotes bacteria‐mediated tumour imaging. (A). The diagram of E. coli MG1655 nuclear medicine imaging in vivo. (B) 18F‐FDS PET imaging of E. coli MG1655 in CT26 tumour‐bearing mice before and 1, 3 and 5 days after intravenous administration. (C) PET signals (SUV_max) from the transplanted tumour and normal organs (the gut, muscles, the heart, lungs, the liver and the brain) at each time point from panel b. (D) SUV ratios obtained by dividing SUV_max after bacterial injection by SUV_max before bacterial injection in normal organs and transplanted tumours after 1, 3 and 5 days. (E) Bioluminescence and autoradiography after 3 days of intravenous administration of LUX‐expressing E. coli. (F) In vitro images of 2 mm thick tumour sections, bioluminescent images, 18F‐FDS radiological images and fusion images of bioluminescent and radiological images. (G) H&E staining of tumour tissue (×400). (H) Immunofluorescence staining of tumour tissue. Sections are stained with E. coli antibodies (red). The nucleus is stained with DAPI (blue). Panels b‐h are adapted with permission of the reference (Kang et al., 2020), based on the terms of the Creative Commons Attribution Licence CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).