Ultrastructural polymicrobial Staphylococcus aureus–Pseudomonas aeruginosa interactions and antimicrobial resistance in ex vivo cornea model

Aim: To investigate antagonistic interactions among pathogens, in ex vivo donor corneas infected with monomicrobial or polymicrobial combinations of antibiotic susceptible and resistant clinical isolates of Staphylococcus aureus (MSSA, MRSA) and Pseudomonas aeruginosa (S-PA, MDR-PA). Materials & methods: Scanning electron microscopy and antimicrobial susceptibility testing (AST, broth microdilution for minimum inhibitory and bactericidal concentrations [MIC/MBC]) pre-and post-polymicrobial interactions, in infected donor corneas. Results: MSSA lost viability with S-PA/MDR-PA, while MRSA formed larger cells, biofilm and lower MIC (teicoplanin) with S-PA, but lost viability with MDR-PA. S-PA had lower MIC (ceftazidime, meropenem, chloramphenicol) with MSSA, and lower MBC (cefoperazone, ciprofloxacin) and fewer cells with MRSA. MDR-PA had abundant cells and no change in AST with MSSA or MRSA. Conclusion: Significant antagonistic interactions occur in ocular polymicrobial infections, affecting antibiotic susceptible isolates more than resistant ones. Bacteria that cause human disease are becoming difficult to treat using common antibiotics due to resistance. One alternative way of treating infections being researched is by looking at how different bacteria interact. Just like humans, bacteria exist in communities and have ‘friends’ and ‘enemies’, against whom they compete for food and space. We wanted to see if this was also true at an infection site. We looked at two harmful bacteria, Staphylococcus aureus and Pseudomonas aeruginosa. We found that these bacteria did interact and were made weaker in some respects when they were together at an infection site. How they become weaker and how this could be used to improve treatment requires further research. This study investigates polymicrobial interactions between ocular isolates of Staphylococcus aureus and Pseudomonas aeruginosa in an ex vivo cornea model. Scanning electron microscopy was used to observe morphological changes, and broth microdilution for antimicrobial susceptibility testing. Antagonistic/competitive polymicrobial interactions reduce antimicrobial resistance and virulence among co-infectants. Drug-susceptible isolates were more affected by these antagonistic interactions compared to drug-resistant counterparts. Our results potentially offer a new approach to combating antimicrobial resistance in ocular infections.