Introduction: Polymicrobial infections in chronic lung disease
Microbial communities shape energy flow and disease outcomes across ecosystems, including human chronic infections. In the lungs of COPD and cystic fibrosis patients, Pseudomonas aeruginosa often coexists with oral streptococci such as Streptococcus salivarius. These interactions influence virulence, antibiotic response, and disease progression. This study investigates how the antibiotic aztreonam reshapes the competitive and cooperative dynamics between P. aeruginosa strains (lasR-intact and lasR-mutant) and S. salivarius isolated from a COPD patient, integrating phenotypic assays, transcriptomics, and a Caenorhabditis elegans virulence model.
Experimental design: pairing and perturbing two species
Two P. aeruginosa–S. salivarius pairs were studied: a lasR-intact P. aeruginosa (PAcx1) with its S. salivarius counterpart (SScx1), and a lasR-mutant P. aeruginosa (PAcx2) with its S. salivarius partner (SScx2). The team conducted MIC testing, agar-based interaction assays, and dynamic co-culture experiments at sub-inhibitory aztreonam concentrations (8 µg/mL) to mimic selective pressures during therapy. They also performed mixed-ratio competitions, pyocyanin assays, transcriptomics, qPCR validation, and C. elegans lethality assays to connect gene expression with functional outcomes.
Key finding: aztreonam shifts competitive balance in co-culture
Initial static assays showed that P. aeruginosa generally suppresses S. salivarius growth on lawns, while the latter can influence P. aeruginosa under certain conditions. When aztreonam was present at sub-MIC levels for P. aeruginosa, the usual suppression by P. aeruginosa on S. salivarius was reversed. Subtle changes in growth density did not erase this effect; rather, S. salivarius gained a competitive edge and could invade P. aeruginosa backgrounds. In dynamic mix experiments, initial ratios that favored P. aeruginosa under antibiotic-free conditions shifted toward S. salivarius takeover in the aztreonam environment. The result: a recalibration of interspecific interactions, with antibiotic exposure enabling a shift toward coexistence or even S. salivarius-driven suppression of P. aeruginosa dominance.
Transcriptomic landscape: S. salivarius activation and P. aeruginosa silencing
Across cross-feeding setups, aztreonam activated S. salivarius transcription, markedly upregulating hundreds of genes tied to growth and biosynthesis, while P. aeruginosa showed widespread downregulation of quorum-sensing (QS) circuits and key metabolic pathways. In monocultures, aztreonam did not drastically alter S. salivarius gene expression, but in co-culture it triggered significant changes, suggesting interspecies cues in the presence of the drug. RNA-seq revealed that S. salivarius extracellular products substantially dampened QS-regulated genes in P. aeruginosa, including lasR, rhlR, mvfR, and genes involved in pyocyanin biosynthesis (phzA1, phzS). The downregulation extended to biofilm, transport, and redox-related pathways, reducing P. aeruginosa’s virulence potential even when growth was not fully suppressed. Conversely, S. salivarius showed enhanced expression of growth-related pathways under aztreonam in cross-feeding conditions, indicating a metabolic shift that supports its competitive advantage.
Virulence and host-model validation
Pyocyanin production, a virulence proxy controlled by QS networks, was significantly reduced in the presence of S. salivarius extracellular products, especially when aztreonam was present. qPCR validation corroborated strong repression of QS genes in P. aeruginosa when exposed to S. salivarius supernatants, with further suppression under aztreonam. The Caenorhabditis elegans infection model echoed these findings: aztreonam and S. salivarius extracellular products individually reduced P. aeruginosa lethality toward nematodes, and their combination yielded additive protective effects, extending worm survival beyond either factor alone.
Implications for therapy and microbiome management
This work highlights that antibiotic therapy does more than kill or inhibit a pathogen; it reshapes interspecies relationships within polymicrobial communities. Sub-inhibitory aztreonam reconfigures the P. aeruginosa–S. salivarius interaction, enabling S. salivarius to counteract P. aeruginosa virulence through QS disruption and metabolic reprogramming. In COPD and other chronic lung diseases where Streptococcus species are core lung microbiome residents, such interactions may influence treatment outcomes, pathogen persistence, and host inflammation. The results point to potential strategies that exploit interspecies interactions—using targeted antibiotics at sub-MICs or adjunctive probiotic-like species to recalibrate pathogenic communities and reduce virulence, rather than relying solely on bactericidal approaches.
Limitations and future directions
The study focuses on two strain pairs derived from COPD patients, and disentangling the relative contributions of aztreonam toxicity versus extracellular bacteriocin-like products remains complex. Further work should assess broader patient-derived communities, in vivo relevance, and the clinical utility of exploiting S. salivarius–mediated QS suppression as an adjunct to conventional antibiotics.
Conclusion: a new angle on antibiotic stewardship
Aztreonam can recalibrate microbial interspecies dynamics, turning a dominant P. aeruginosa into a more manageable partner through S. salivarius activation and QS attenuation. This work adds to the growing recognition that polymicrobial ecology matters for infection dynamics and treatment success.
