Complex interbacterial interactions in mixed biofilms as a key determinant of their antimicrobial treatment efficacy

Biofilms are formed by closely adjacent microorganisms embedded into an extracellular matrix this way providing them with strong protection from antimicrobials, which is often further reinforced in polymicrobial biofilms. Despite of the well-known antagonistic interactions between S. aureus and P. aeruginosa, the most common pathogens causing various nosocomial infections, they often form mixed consortia characterized by increased pathogenicity and delayed recovery in comparison with single species infections. Here we show that, while S. aureus could successfully avoid a number of antimicrobials by embedding into the biofilm matrix of P. aeruginosa despite of their antagonism, the very same consortium was characterized by 10–fold higher susceptibility to broad-spectrum antimicrobials compared to monocultures. Moreover, quantitatively similar increase in antimicrobials susceptibility could be achieved when P. aeruginosa was introduced into S. aureus biofilm, compared to S. aureus monoculture. In a reverse experiment, intervention of S. aureus into the mature P. aeruginosa biofilm significantly increased the efficacy of ciprofloxacin against P. aeruginosa. A broader perspective is provided by antagonistic bacteria intervention into already preformed monoculture biofilms leading to the considerable enhancement of their antibiotic susceptibility. We believe that this approach has a strong potential of further development towards innovative treatment of biofilm-associated infections such as transplantation of the skin residential microflora to the wounds and ulcers infected with nosocomial pathogens to speed up their microbial decontamination. Author summary Biofilms formation is one of the key mechanisms providing pathogenic bacteria with extreme resistance to antimicrobials. On the S. aureus and P. aeruginosa mixed culture model we show explicitly that antimicrobials efficacy against bacteria in mixed biofilms differs considerably from monoculture biofilms. From the one hand, S. aureus avoids vancomycin and ampicillin by the rearrangements to the lower layers of the P. aeruginosa biofilm matrix. On the other hand, in the same consortium susceptibility to ciprofloxacin and aminoglycosides increases nearly 10– fold compared to monocultures. This finding allowed suggesting that intervention of antagonistic bacteria into already preformed monoculture biofilms could be used as an innovative approach to their treatment by increasing their antibiotic susceptibility. Thus, by introducing P. aeruginosa into preformed S. aureus biofilm, susceptibility of S. aureus to aminoglycosides was increased 4-fold, compared to monoculture. The intervention of S. aureus into the mature P. aeruginosa biofilm significantly increased the efficacy of ciprofloxacin against P. aeruginosa. We believe that this approach has a strong potential of further development towards innovative treatment of biofilm-associated infections such as introduction of the skin residential microflora to the wounds and ulcers infected with nosocomial pathogens to speed up their microbial decontamination.