Differential effects of Pseudomonas aeruginosa on biofilm formation by different strains of Staphylococcus epidermidis.

Pseudomonas aeruginosa and Staphylococcus epidermidis are common opportunistic pathogens associated with medical device-related biofilm infections. 16S rRNA fluorescence in situ hybridization and confocal scanning laser microscopy were used to study these two bacteria in dual-species biofilms. We have shown that fresh isolates of S. epidermidis form biofilms more avidly on polymer surfaces than laboratory strains. In dual-species biofilms the presence of P. aeruginosa reduced biofilm formation by S. epidermidis, although the different clinical isolates differed in their susceptibility to this effect. The most resistant fresh isolate coexisted with P. aeruginosa for up to 18 hours and was also resistant to the effects of culture supernatant from P. aeruginosa biofilms which caused dispersal of other S. epidermidis strains from established biofilms. Thus, different strains of S. epidermidis differed in their capacity to withstand the action of P. aeruginosa, with some fresh isolates being better equipped than laboratory strains to coexist in biofilms with P. aeruginosa. Our data suggest that where S. epidermidis and P. aeruginosa are present on abiotic surfaces such as medical devices, S. epidermidis biofilm-formation can be inhibited by P. aeruginosa through two mechanisms: disruption by extracellular products, possibly polysaccharides, and, in the later stages, by cell lysis