In vitro interspecies interactions between methicillin-resistant Staphylococcus aureus and Pseudomonas aeruginosa:...

Polymicrobial bone infections (PBIs) are associated with greater clinical severity and present more significant therapeutic challenges than monomicrobial infections. Methicillin-resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa (PA) are the most frequently isolated pathogens in PBIs. However, despite previous studies have suggested both competitive and cooperative interactions between these two pathogens, their underlying molecular mechanisms and the impact on antibiotic susceptibility remain incompletely understood.

Under in vitro co-culture conditions, bacterial growth and biofilm formation of MRSA and PA were evaluated by colony-forming unit (CFU) enumeration, crystal violet staining, and live/dead fluorescence staining. Minimum inhibitory concentrations (MICs) of clinically relevant antibiotics were determined for each species following co-culture to evaluate reciprocal alterations in antibiotic susceptibility. Additionally, RNA sequencing (RNA-seq) was performed on both species separately after co-culture to profile transcriptomic changes underlying their polymicrobial interaction.

At an MRSA: PA ratio of 1:1, MRSA CFU remained close to the initial inoculum throughout the experiment, with maximum CFU reaching only 1% of that in monoculture. MRSA biofilm biomass in the presence of PA cell-free supernatant was reduced by 33% at 36 hours. Notably, antibiotic susceptibility testing showed that MRSA exhibited increased MIC values for levofloxacin and daptomycin, with MICs 1.53 ± 0.00-fold and 2.00 ± 0.00-fold higher, respectively, than those in the monoculture group. Meanwhile, PA showed increased MIC values to colistin, with a 2.00 ± 0.00-fold higher MIC, but reduced MIC values for levofloxacin and ceftazidime, both showing a 0.50 ± 0.00-fold lower MIC compared with the monoculture group. Transcriptomic analysis of MRSA revealed significant increases in gene expression related to ribosome biogenesis, oxidative phosphorylation, and stress response pathways. PA co-cultured with MRSA downregulated multidrug-resistant efflux pump genes and upregulated pore-forming protein genes.

PA inhibited MRSA growth in vitro co-culture at a 1:1 ratio, accompanied by altered antibiotic susceptibility profiles and transcriptional reprogramming. This study provides mechanistic insights into MRSA-PA interactions and establishes an experimental framework for developing targeted therapies against PBIs.