The Phosphate-Specific Transport System Gene pstA1 Contributes to Rifampin Tolerance in Mycobacterium tuberculosis
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Tuberculosis (TB) caused an estimated 10.8 million new cases and 1.25 million deaths in 2023. Antibiotic tolerance, the ability of bacteria to survive exposure to bactericidal concentrations of antibiotics without genetic resistance mutations, contributes to the need for prolonged treatment. Adjunctive targeting of antibiotic tolerance mechanisms could promote accelerated clearance of Mycobacterium tuberculosis ( Mtb ), thereby improving medical adherence. Our previous forward genetic screen for rifampin tolerance genes identified pstA1 , which is involved in phosphate-specific import. The ABC-type transporter permease PstA1 has previously been implicated in Mtb virulence, as mutants lacking this gene exhibit defective survival during phosphate limitation in vitro , in infected macrophages, and in immunocompetent mice. The minimum inhibitory concentration (MIC) of rifampin was not altered in a pstA1 deletion mutant (Δ pstA1 ), suggesting this difference in susceptibility is not due to antibiotic resistance. Consistent with a role in rifampin tolerance, time-kill assays revealed a shift in the mean duration of killing (2-log reduction) from 1.6 days in wild-type to 1.0 day in Δ pstA1 , and complementation partially restored this phenotype. We found that pstA1 is specifically required for Mtb survival in the absence of exogenous inorganic phosphate and is important for adaptation to growth in culture without detergent, and within macrophages in an interferon-γ-dependent manner. Differential expression analysis revealed that Δ pstA1 exhibited substantial transcriptional reprogramming with 58 differentially expressed genes (49 upregulated and 9 downregulated, adj. P < 0.05, |log₂ fold change| > 1), including altered expression of metabolic, DNA damage repair, and secretory pathways. PstA1 represents a novel drug target, and inhibitors could be used as adjunctive therapies to shorten curative treatment times, reducing opportunities for the emergence of drug resistance.
Importance
Tuberculosis remains one of the world’s deadliest infectious diseases, causing over a million deaths annually. Current treatment requires months of antibiotic therapy, and poor adherence to these lengthy regimens contributes to the emergence of drug-resistant strains that are increasingly difficult to treat. A major barrier to shorter treatment is antibiotic tolerance, which allows bacteria to survive drug exposure without genetic resistance mutations. This study identifies the phosphate transport component PstA1 as a critical factor enabling Mycobacterium tuberculosis to tolerate the frontline antibiotic rifampin. Bacteria lacking pstA1 are eliminated more rapidly by rifampin, demonstrating that this transporter actively promotes bacterial survival during treatment. These findings suggest that drugs targeting PstA1 could be combined with standard antibiotics to accelerate bacterial clearance, potentially shortening treatment duration and improving patient adherence. Such adjunctive therapies targeting tolerance mechanisms represent a promising strategy to combat tuberculosis and reduce the global burden of drug-resistant disease.
