Can commensals alter pathogen’s antibiotic resistance during co-culture?

Introduction. Bacterial infections of skin wounds can increase hospitalization duration and lead to worse patient prognoses, especially for burn wounds and diabetic foot ulcers. The two main pathogens which infect these wounds are Pseudomonas aeruginosa and Staphylococcus aureus. However, many other species can be present in wound infections, including skin commensal bacteria such as Staphylococcus epidermidis and Micrococcus luteus. 

Hypothesis. It was hypothesized that co-infection alters the antibiotic resistance of each species present. 

Aim. To investigate dual-species commensal-pathogen co-culture and assess the potential influence on the antibiotic resistance of each species. 

Methodology. The commensal and pathogenic species were grown either separately or in dual-species co-culture, potentially allowing biofilm formation for 24 h and were subsequently treated with antibiotics (ciprofloxacin or tobramycin). The impact of the co-culture growth was compared with single species cultures and the effect of the antimicrobial treatment on both conditions were assessed through Minimum Biofilm Eradication Concentrations (MBECs) and bacterial viable counts. 

Results. The viability of each bacterial species was reduced in the presence of other species, and this translated to reduced antibiotic resistance (lower MBECs) of P. aeruginosa in particular. The resistance of the other species appeared more dependent on the specific inter-species effects. 

Conclusion. The inclusion of a commensal species with pathogens in co-culture reduced the antibiotic resistance, and interspecies effects influenced the viability of the pathogens. More realistic antimicrobial resistance assessment protocols accounting for microbial communities could therefore lead to more effective treatments.