Multimodal Tracking of Controlled Staphylococcus aureus Infections in Mice

Abstract

There is a need to develop diagnostic and analytical tools that allow noninvasive monitoring of bacterial growth and dissemination in vivo. For such cell-tracking studies to hold translational value to controlled human infections, in which volunteers are experimentally colonized, they should not require genetic modification, and they should allow tracking over a number of replication cycles. To gauge if an antimicrobial peptide tracer, 99mTc-UBI29–41-Cy5, which contains both a fluorescent and a radioactive moiety, could be used for such in vivo bacterial tracking, we performed longitudinal imaging of a thigh-muscle infection with 99mTc-UBI29–41-Cy5-labeled Staphylococcus aureus. Mice were imaged using SPECT and fluorescence-imaging modalities at various intervals during a 28 h period. Biodistribution analyses were performed to quantitate radioactivity in the abscess and other tissues. SPECT and fluorescence imaging in mice showed clear retention of the 99mTc-UBI29–41-Cy5-labeled bacteria following inoculation in the thigh muscle. Despite bacterial replication, the signal intensity in the abscess only modestly decreased within a 28 h period: 52% of the total injected radioactivity per gram of tissue (%ID/g) at 4 h postinfection (pi) versus 44%ID/g at 28 h pi (15% decrease). After inoculation, a portion of the bacteria disseminated from the abscess, and S. aureus cultures were obtained from radioactive urine samples. Bacterial staining with 99mTc-UBI29–41-Cy5 allowed noninvasive bacterial-cell tracking during a 28 h period. Given the versatility of the presented bacterial-tracking method, we believe that this concept could pave the way for precise imaging capabilities during controlled-human-infection studies.

Cyclodextrin/Adamantane-Mediated Targeting of Inoculated Bacteria in Mice

“…This observation is in line with previous reports using 99m Tc-Ad-MAA. 11 , 15 Unfortunately, the short half-life of 99m Tc meant that the signal intensity in the 140 keV detection window after about 4 physical half-lives was weak (see Figure 3 A). Imaging in the 240 keV window of 111 In revealed accumulation in the spleen and the liver, which was not observed in the control mice (no functionalization; Figure 3 A), suggesting bacterial dissemination.…”

“…The efficiency of the bacterial functionalization was expressed as the % of the total added amount of radioactivity: (([MBq] added /[MBq] associated with bacteria ) × 100). 15 …”

“…To demonstrate the proof-of-principle, we made use of S. aureus inoculation models where functionalized bacteria would serve as a target for the delivery of cargo. For this purpose, S. aureus bacteria functionalized with Ad (using the bacteria-specific UBI 29–41 vector as previously described 15 ) were inoculated into thigh muscle or liver to establish infection ( Scheme 1 ). Subsequently, this infection site, characterized by the presence of the Ad-functionalizations (the guest), then drove the local accumulation of an intravenously administered cargo, 111 In-Cy5 0.5 CD 9 PIBMA 39 (the host).…”

Radioanalytical Techniques to Quantitatively Assess the Biological Uptake and In Vivo Behavior of Hazardous Substances

“…SPECT/CT imaging and biodistribution study make it possible to analyze the initial uptake values and changes in accumulation of both orally administered and intravenously injected E. coli . In a subsequent study, Welling et al achieved multimodal tracking of Staphylococcus aureus ( S. aureus ) in a mouse model [ 48 ]. They labeled an antimicrobial peptide with 99m Tc and Cy5 ( 99m Tc-UBI29-41-Cy5) for use as a tracer.…”

“…Organs are marked as (1) inoculation site and (2) bladder. Reprinted with permission from Welling et al [ 48 ].…”