The World Health Organization has cautioned that antimicrobial resistance (AMR) will be responsible for an estimated 10 million deaths annually by 2050. To facilitate prompt and accurate diagnosis and treatment of infectious disease, we investigated the potential of amino acids for use as indicators of bacterial growth activity by clarifying which amino acids are taken up by bacteria during the various growth phases. In addition, we examined the amino acid transport mechanisms that are employed by bacteria based on the accumulation of labeled amino acids, Na⁺ dependence, and inhibitory effects using a specific inhibitor of system A. We found that ³H-L-Ala accurately reflects the proliferative activity of Escherichia coli K-12 and pathogenic EC-14 in vitro. This accumulation in E. coli could be attributed to the amino acid transport systems being different from those found in human tumor cells. Moreover, biological distribution assessed in infection model mice with EC-14 using ³H-L-Ala showed that the ratio of ³H-L-Ala accumulated in infected muscle to that in control muscle was 1.20. By detecting the growth activity of bacteria in the body that occurs during the early stages of infection by nuclear imaging, such detection methods may result in expeditious diagnostic treatments for infectious diseases.

Pseudomonas aeruginosa infection is an infectious disease that must be controlled because it becomes chronic and difficult to treat, owing to its unique system of toxin production/injection and elimination of other bacteria. Here, we noninvasively monitored P. aeruginosa using single-photon emission computed tomography (SPECT) imaging. Determining the amount and localization of the P. aeruginosa will enable making faster clinical diagnoses and selecting the most appropriate therapeutic agents and methods. Nonclinically, this information can be used for imaging in combination with biofilms and toxin probes and will be useful for discovering drugs targeting P. aeruginosa. To study P. aeruginosa accumulation, we conducted in vitro and in vivo studies using iodine-123 β-methyl-p-iodophenyl-pentadecanoic acid (¹²³I-BMIPP), which we previously reported using for Escherichia coli. In vitro, ¹²³I-BMIPP accumulated in P. aeruginosa by being taken up into the bacteria and adsorbing to the bacterial surface. In vivo, ¹²³I-BMIPP accumulated significantly more in infected sites than in noninfected sites and could be quantified by SPECT. These results suggest that ¹²³I-BMIPP can be used as a probe for P. aeruginosa for SPECT. Establishing a noninvasive monitoring method using SPECT will allow further progress in studying P. aeruginosa.

Background: The prevalence of nontuberculous mycobacteria (NTM) infection is on the rise, surpassing that of pulmonary tuberculosis in Japan. Current standard therapy for NTM infection involves long-term treatment of at least 1.5 years, with low success rates and a high relapse rate. ^99mTc-diethylenetriaminepentaacetic acid-galactosyl-human serum albumin (^99mTc-GSA) is used for human liver imaging. In this study, we utilized ^99mTc-GSA as a probe to detect Mycobacterium avium (M. avium), a major pathogen in NTM pulmonary diseases (NTM-PDs). Our aim was to investigate the non-invasive detection of M. avium using ^99mTc-GSA on Single-Photon Emission Computed Tomography (SPECT). Methods: The accumulation of ^99mTc-GSA in M. avium was investigated in vitro. In vivo, SPECT images were obtained after the administration of ^99mTc-GSA to an M. avium thigh infection model. Subsequently, the contrast difference in accumulated ^99mTc-GSA between infected and non-infected thighs was calculated using SPECT imaging. Furthermore, SPECT images were obtained for thighs infected with varying bacterial loads, and the accumulation was compared between them. Results: In vitro, we observed that ^99mTc-GSA accumulates in M. avium. In vivo, SPECT images demonstrated the specific accumulation of ^99mTc-GSA at the infection site, with this accumulation being correlated with the bacterial load. Conclusions: ^99mTc-GSA specifically accumulates in M. avium, and SPECT can be used to monitor the distribution and quantity of M. avium in animals. By utilizing these measures, ^99mTc-GSA can be targeted to the site of infection and used as a bacterial probe.

Background: The number of bacterial infections that—for various reasons—are challenging to cure continues to increase. One such reason is persister cell infection. To investigate persister formation and persister infections, viable bacteria must be evaluated in the same animal over time. In this study, the feasibility of monitoring viable bacteria by SPECT using two labeled peptides was evaluated. Results: Two types of ubiquicidin (UBI) 29-41 labeled with technetium-99m, ^99mTc-HYNIC(GH)₂-UBI 29-41 and ^99mTc-HYNIC(Tricine)₂-UBI 29-41, were synthesized. The in vitro binding of these labeled peptides to Staphylococcus aureus was measured. For the in vivo study, each labeled peptide was injected into S. aureus infected mouse thigh after treatment with various doses of ciprofloxacin (CPFX). Two hours after injection, the accumulation of each labeled peptide at the infection site was assessed by SPECT, and then the number of viable bacteria was determined from the accumulation detected. The peptide labeling was successful, and the radiochemical purity was 91±9% (GH, n=8) and 100% (Tricine, n=8). The in vitro binding of the labeled peptides to S. aureus (5×10⁸ cfu) without serum was 78.9% (GH) and 85.5% (Tricine) of the total ^99mTc activity. With serum, the binding rate was 67.5% (GH) and 13.3% (Tricine). The accumulation of labeled peptide was calculated from the SPECT images, and that in the bacterial infection site (left thigh) was higher than that in the non-infection site (right thigh) for both peptides. Good correlation was found between the target-to-non-target (T/NT) ratios of each labeled peptide and the viable bacterial count at the infection site, and ^99mTc-HYNIC(Tricine)₂-UBI 29-41 had a wider range than ^99mTc-HYNIC(GH)₂-UBI 29-41.Conclusion: Using the SPECT/labeled peptide method, it was possible to monitor viable bacterial count in the range 10³–10⁸ cfu, which is appropriate for tracking viable bacterial counts in the same animal over time.