Manganese (Mn) is an essential trace element involved in numerous biological processes [[1], [2], [3], [4], [5], [6], [7], [8]]. It acts as a cofactor for various enzymes, supporting cellular responses to oxidative stress and metabolic functions critical for cell survival [9]. In bacterial cells, Mn is especially important for maintaining homeostasis and counteracting environmental stress. One key role of Mn is to protect bacterial cells from oxidative damage by serving as a cofactor for superoxide dismutase (SOD), which neutralizes harmful reactive oxygen species (ROS) [10]. Additionally, Mn is involved in the activation of enzymes responsible for DNA synthesis and repair, highlighting its multifaceted role in bacterial physiology, particularly under stressful conditions [11]. Mn also contributes to other metabolic processes, such as carbohydrate metabolism, where it supports enzymes involved in glycolysis and the tricarboxylic acid (TCA) cycle, further emphasizing its importance in bacterial growth and survival [12,13].

To meet their need for manganese, bacteria have developed multiple strategies to acquire this essential metal from their environment. Two major types of Mn importers have been identified in bacteria: the NRAMP (natural resistance-associated macrophage protein) family transporters and the ABC (ATP-binding cassette) transporters [14]. The NRAMP family transporters, such as MntH (Manganese transport protein H), were first identified in Salmonella enterica serovar Typhimurium and Escherichia coli based on their similarity to eukaryotic transporters. MntH is a highly specific Mn transporter that co-transports Mn along with protons [15]. In Brucella abortus, MntH functions as the sole high-affinity Mn transporter, enhancing bacterial resistance to oxidative stress [16]. In Pseudomonas aeruginosa, two NRAMP transporters, MntH1 and MntH2, collaborate to ensure efficient Mn uptake [17]. The ABC transporter family, which also plays a key role in Mn acquisition, has different names across bacterial species, including SitABC (Iron/manganese ABC transporter system), SloABC, and MtsCBA [14]. These transporters are ATP-dependent and are critical for ensuring sufficient Mn levels for bacterial growth and stress resistance. The ability of these transporters to adapt to fluctuating Mn levels is a key factor that allows pathogenic bacteria to thrive in hostile environments, including host cells when Mn availability is limited.

In E. coli, Mn uptake and export are managed by MntH and MntP, respectively [18]. MntH is responsible for Mn import and is negatively regulated by the regulator MntR, while MntP exports excess Mn to prevent toxicity [19]. Deletion of mntP results in elevated intracellular Mn levels and increased Mn sensitivity [20]. The coordination of MntH, MntR, and MntP is crucial for maintaining Mn homeostasis in E. coli. [14]. In addition to MntH, some bacteria also possess an ABC transporter for Mn uptake, such as SitABCD in S. typhimurium [21]. SitABCD is the Mn2+ transporter in S. typhimurium that specializes in Mn uptake in different physiological environments [22]. Specifically, SitA functions as the periplasmic Mn2+-binding protein critical for high-affinity manganese acquisition under metal-limiting conditions, while SitBCD form the transmembrane channel and ATPase module [21]. The combination of MntH- and SitABCD-type systems promotes S. typhimurium survival and virulence by keeping adequate Mn levels. The ability to manage Mn uptake and export effectively allows these bacteria to maintain metabolic functions and resist the damaging effects of oxidative stress, which is particularly important during infection when the host's immune system generates reactive oxygen species to kill invading pathogens [23].

The regulation of Mn uptake is tightly controlled to balance the need for this essential element while preventing toxicity. Under conditions where Mn and iron are abundant, the transcription of mntH is repressed by regulators such as Fur and MntR [15,18,22,[24], [25], [26]]. Moreover, transcription factors such as OxyR, which respond to oxidative stress, add an additional layer of regulation by modulating Mn transporter expression based on the oxidative state of the cell. Importantly, hydrogen peroxide influences mntH expression through OxyR [25]. Our previous studies have also shown that bacterial secretion systems, like the type VI secretion system (T6SS), can secrete manganese-binding proteins, such as TesM to enhance Mn uptake, with the secretion of TesM regulated by OxyR [3]. This regulation highlights the complex interplay between Mn acquisition and oxidative stress response, allowing bacteria to fine-tune Mn levels in response to changing environmental conditions. This intricate regulation is vital for bacteria to avoid Mn toxicity while ensuring sufficient availability for critical cellular processes, particularly during periods of stress when Mn demand may be elevated.

Mn also plays a crucial role in host-pathogen interactions. Recent studies have shown that Mn is involved in regulating host innate immune activation, particularly through the cGAS-STING pathway, which detects cytosolic double-stranded DNA and initiates an antiviral and antitumor response [5,6]. Mn ions can enhance bacterial infection elicited innate immune response through the cGAS-STING pathway, aiding the host in responding to bacterial infections [27,28]. This could help the host to detect and respond to viral and bacterial infections more effectively. The host's innate immune system also limits the availability of essential nutrients, including manganese, during bacterial infection by producing metal-binding proteins like calprotectin [9,23]. This mechanism, known as nutritional immunity, is a strategy used by the host to starve invading pathogens of essential metals [29]. To counteract host-induced Mn limitation, bacterial Mn transport systems are vital for maintaining virulence. The ability to overcome nutritional immunity through specialized Mn transporters is a key factor that contributes to the success of many bacterial pathogens in establishing infections, particularly in environments where competition for essential nutrients is intense [29].

The function and regulation of Mn2+ transports were first identified by using E. coli and S. typhimurium as model organisms. Enterohaemorrhagic E. coli (EHEC) is a zoonotic pathogen that causes diarrhea, hemorrhagic colitis, and hemolytic-uremic syndrome, and S. typhimurium can infect a wide range of animals and can result in several manifestations of disease [30,31]. During enteropathogenic bacterial infections, the inflamed host gut also creates an environment that controls microbial infections [32]. These pathogens use their Mn transport systems to survive within the host gut. In this study, we demonstrate that Mn transport proteins MntH in EHEC and MntH and SitABCD in S. typhimurium are essential for Mn uptake, stress resistance, and virulence. MntH and SitABCD facilitate the growth of bacteria and could increase the resistance of EHEC and S. typhimurium to different stresses. In turn, such advantage conferred by Mn transport systems enhance the competition ability of EHEC and S. typhimurium. Importantly, the Mn transport systems help the bacteria evade the Mn2+-mediated innate immune response and suppress innate immune response via the STING pathway. In summary, our findings reveal that the Mn transport systems in two important enteric bacteria enhance their stress resistance and enable them to evade the Mn-mediated innate immune response. This insight broadens our understanding of how bacterial Mn transport systems modulate host cell innate immunity, contributing to pathogen survival and immune evasion.