Bacterial infections seriously threaten human health and safety [1], [2], [3], making the precise and rapid diagnosis of bacterial infections urgent. The precise localization of the lesion site is the crucial step for in-time diagnosis and treatment of bacterial infections in clinical. The current clinical diagnosis of infectious diseases is mainly based on the combination of physical examination, an invasive biopsy of infected tissues, and noninvasive imaging techniques, which is time-consuming and labor-intensive [4], [5], [6]. In situ imaging of bacterial infections offers prospects for rapid and accurate diagnosis of early diseases. Current clinical imaging modalities, such as computed tomography (CT) [7,8], magnetic resonance imaging (MRI) [9,10], and ultrasound (US) [11], cannot accurately distinguish bacterial infections from other sterile inflammations [12,13]. It is, therefore, vital to develop noninvasive imaging techniques to detect bacterial infections with high specificity so that patients can receive timely and effective treatment.

In recent years, various imaging modalities based on molecular imaging probes designed with bacterial targeting molecules (e.g., antibodies [14], antibiotics [15], [16], [17], [18], antimicrobial peptides [19], and glucose polymer [20], [21], [22]) have been developed to improve sensitivity, specificity, and earlier detection of bacterial infections in vivo [23]. Due to its high spatial resolution, nonradioactive, high soft tissue contrast, and unrestricted penetration depth, MRI is a noninvasive imaging model for detecting bacterial infections [10]. Gadolinium (Gd) and magnetic nanoparticles (MNPs)-based contrasts have been used to enhance the contrast of MRI in clinical. Unfortunately, these conventional contrasts would bring nonspecific enhancement of pathologies due to the lack of specificity [24,25]. Thus, various bacterial-targeted MRI probes [26], [27], [28] have been developed to accurately and precisely image bacteria in vivo. Although these MRI probes have improved their specificity for bacterial infections, the MRI signal of these MRI probes is always “on”, which inevitably generates nonspecific background MRI signals, affecting the accuracy of MRI to a certain extent and limiting their clinical application. Interestingly, activated MRI agents with modulation of either T1 or T2-weighted signal upon target binding, enzymatic activity, or disease-related biological process could bring a strong signal, resulting in accurate detection of target sites due to the high signal-to-noise ratio. Recently, distance-dependent magnetic resonance tuning (MRET) MRI probes, activated by enzymes, acidic, reactive oxygen, and electric-field related to physiological or pathological change, have been developing for noninvasive in vivo exploration [29]. The “turn-on” or “turn-off” of the MRET probe's T1-weighted signal depends on the distance between a paramagnetic T1 enhancer and a superparamagnetic quencher [30]. In addition, by manipulating the aggregation or dispersion state of contrast agents, T2/T1 contrast agent switching could also be achieved to obtain stimuli-responsive MRI in vivo with high accuracy [31,32].

Inspired by the MRET-based activation of the T1-weighted signal and the structure-switching-based transformation of the T2 contrast agent to the T1 contrast agent, we designed a kind of MRET MRI probes (MDVG-1) assembled with the DNA-modified MNPs (MDV) and DOTA-Gd labeled complementary DNA(Gd-DNA3-Gd) with initial low T1-weighted signal. Compared with no disassembling and low T1-weighted signal (MRET “on”) of MDVG-1 in the normal tissue (pH∼7.4), upon encountering the acidic bacterial microenvironment [33], [34], [35], [36], the MRET probes disassembled as several individual units, resulting the T2-weighted contrast switching to T1-weighted contrast. The MRET enhancer of Gd-DNA3-Gd detaching from the MRET quencher of MDV assembly, enhancing the T1-weighted signal, which could specifically and sensitive image Staphylococcus aureus (S. aureus) infection in site (as shown in Scheme 1). Specifically, the paramagnetic enhancer of Gd-DNA3-Gd was the conjugation of DOTA-Gd with pH-responsive i-motif nucleic acid linkers (DNA3) [37], and the precursor of superparamagnetic quencher (MDV) was obtained by conjugating nucleic acid molecule (DNA1 or DNA2, partially complementary strands to DNA3) and bacterial targeting ligands (vancomycin, Van) with MNP. Once administrated MDVG-1 in vivo, the probes targeted the site of S. aureus infection due to the specific binding of Van to the D-Ala-D-Ala moiety in the Gram-positive bacterial cell walls [38]. Moreover, under the acidic bacterial microenvironment, Gd-DNA3-Gd transformed from single-stranded to intercalated quadruple-helical structure [37], which triggered the T2 contrast agent of MDVG-1 to disassemble and turn to the T1 contrast agent. At the same time, Gd-DNA3-Gd detached from the surface of MDV, resulting in the recovery of the T1 signal of Gd-DNA3-Gd recovered, further enhancing the T1-weighted signal (MRET OFF) in the site of S. aureus infection. Therefore, the solid inverse contrast of darkness in normal tissue to brightness in the site of S. aureus infection under T1 imaging mode showed great potential for precise diagnosis of bacterial infections in clinical.