Infectious diseases, the second leading cause of death worldwide, exert a grave threat to the public health. This situation is aggravating due to the progressively emerging microbial resistance and the lack of new drugs into the clinic [1,2]. Mycobacterial infection has had its notorious name engraved on the Georgia Guidestones. For instance, human tuberculosis, which is mainly caused by Mycobacterium tuberculosis, causes approximately more than 1.5 million deaths each year [3]. The stinky devil shows no regret in escalating its influence and evolves multidrug-resistant tuberculosis levering up the costs of corresponding treatment [4,5]. It brooks no delay to search for effective drugs against [6].

Natural products remain a major source for antibiotic discovery and drug development [7-9]; however, the accessibility and efficiency for chemical synthesis of these compounds, as well as the associated investigation into their mechanisms of action, often get choked by challenges arising from the structural complexity. Synthetic chemists and pharmaceutical chemists have their own opinions to develop antiinfective agents, while extracting untapped functions from existing drugs is outcropping as an accepted and efficient way. During the in vitro screen of new anti-tuberculosis agents amongst known drugs, an archetypal thiopeptide antibiotic, thiostrepton, showed to behave a remarkable bioactivity against either wild type or multidrug-resistant M. tuberculosis with a very low minimum inhibitory concentration [10]. Thiostrepton has long been wildly used as an animal feed additive, and no obvious side effects caused by thiostrepton have been reported. Unfortunately, the drawbacks of thiopeptides (e.g. poor aqueous solubility and pharmacokinetics) limited their potential clinical applications [11]. As synthetic biologists, we sought to make a stunning turnaround via a biosynthetic strategy to obtain more potent thiopeptide antibiotics with improved pharmaceutical properties [12,13].

In the previous work, we developed a robust and efficient chemoenzymatic protocol to synthesize quinaldic acid and its analogs, which serve as key building blocks in the biosynthetic pathway of Thiostrepton [14]. As a “chemical module” in synthetic biology, the produced quinaldic acid analogs could be incorporated into Thiostrepton skeleton via. a mutational biosynthesis strategy and used for replacing multiple gene functions [15]. The obtained thiosrtepton derivatives that varied with respect to the quinaldic acid moiety of the side ring (Figure 1) possessed improved antibacterial activity and water-solubility [16].

Figure 1: Chemical structures of thiostrepton (TSR) and its side ring-modified derivatives (6’-fluoro-TSR, 5’-fluoro-TSR, and 12’-methyl-TSR) that vary in quinaldic acid (QA) substitution; the QAmoiety in the structure is indicated in blue.

Meanwhile, utilizing these obtained thiopeptide molecules as chemical probes and drug leads to treat the zebra fish infected by intracellular pathogen M. marinum (an important model strain of M. tuberculosis used in lab), we uncovered a unique mode of action of TSR-type antibiotics against parasitic mycobacteria [17]. In addition to directly targeting the ribosome of bacterial parasites, TSRs can induce autophagy to enhance host cell defense by activating Endoplasmic Reticulum (ER) stress and Unfolded Protein Response (UPR) pathways in eukaryotes (Figure 2). This unusual property of TSR is most likely attributed to its role as a dual functional inhibitor for both prokaryotic ribosomes and eukaryotic proteasomes.

Figure 2: TSR antibiotics exhibite a dual mode of action against intracellular pathogens that involves effects on both the host and the microbe.

Although a number of endoplasmic reticulum stress inducers and ribosome inhibitors are reported and some of them have been developed into clinically utilized first-line drugs [18,19], to our best knowledge, thiostreptonss are the only type of antibiotics that intuitively act on both the bacterial pathogens and infected cells. Distinct from the current antibacterials used in clinic that only affect bacterial cells, thiostreptons activate autophagy that plays a key role in host antimicrobial immunity [20,21], to eliminate intracellular pathogens parasitizing host macrophages. Several molecular mechanisms have been gained by M. tuberculosis or other intracellular pathogens to prevent the host cell from activating autophagy during the process of evolution [22,23].

However, it will be hard for bacteria to generate resistance against Thiostrepton-type antibiotics that can deliver a double punch by acting on two totally different targets. This newly elucidated dual mode of action may inspire the future changes in the treatment of intracellular pathogens by taking host response into account, and facilitate developing new drugs for clinical applications in dealing with mycobacterial diseases. Meanwhile, recent developments in drug delivery systems will also accelerate the upcoming clinical use of thiopeptide antibiotics with large molecular weights and poor water solubilities [24,25].