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Introduction: Bacterial colonization of medical devices is promoting hospital-acquired infections leading to worsening patient outcomes and high costs for society. Sequential bacterial colonization of surfaces may provide altered conditions that benefit pathogens.

Methods: In this study we have investigated the interactions between two pairs of clinical isolates collected from patients that were on mechanical ventilation. Two patients were first colonized by Staphylococcus aureus and thereafter Pseudomonas aeruginosa settled. The two P. aeruginosa isolates were weak colonizers in monoculture. We investigated two hypotheses: (1) S. aureus preconditions material surfaces, facilitating adhesion of later colonizers. (2) S. aureus provides an altered nutrient environment promoting the growth and settlement of other bacteria.

Results: Surface preconditioning did not seem to enhance colonization of P. aeruginosa. However, bacterial growth, biofilm formation, ratio of colony forming units, and metabolic profiles were influenced by co-cultivation. The effects varied depending on nutrient content in the medium.

Discussion: In general, co-cultures appeared to benefit clinical isolates to a higher degree, compared to reference strains. The results indicate that differences in airway microenvironment between patients may have a large effect on the infection process and which pathogens that persist.