Effects of plasmid acquisition by Pseudomonas aeruginosa clinical isolates on methicillin-resistant Staphylococcus aureus biofilm formation

This work aimed to study the influence of plasmid acquisition on the mutual interaction of certain Pseudomonas aeruginosa clinical isolates and methicillin resistant Staphylococcus aureus (MRSA) clinical isolates. In this study, two mixed cultures were used: i) P. aeruginosa clinical isolate P14 which co-existed with MRSA isolate S14; and ii) P. aeruginosa clinical isolate P12 which co-existed with MRSAisolate S12. The respective mixed cultures were selected to study the existence of the plasmids on the inhibitory effect of the culture supernatant of P. aeruginosa on the biofilm formation of co-existing MRSA clinical isolates. The culture supernatant of P. aeruginosa harboring no plasmids exerted a significant reduction effect on the biofilm formation (about 57% reduction; P < 0.005) of the co-existing MRSA isolate and such effect was also confirmed using plasmid transformation experiments. To ensure that such effect was habitually due to the presence of plasmids and not to any mutational or conformational changes in the gene/protein sequences of the key genetic elements involved in interspecies interaction, the genes, araC, pqsS and ahlScoded for AraC family transcriptional regulator (1026 bp, 341 aa), and 2-heptyl-3-hydroxy-4(1H)-quinolone synthase (1149 bp, 382 aa) and acyl-homoserine-lactone synthase (540 bp; 179 aa) of P. aeruginosa clinical isolate P14 were amplified using PCR, analyzed and submitted into the GenBank database under the accession codes, KT693035, KT693034, KT693033, respectively. AraC was a model of AraC transcription regulator with a conserved N-terminal arabinose-binding domain and C-terminal H-T-H motive. PqsS was a model of putative 2-heptyl-3-hydroxy-4(1H)-quinolone synthase with a conserved domain of a NAD(P)-binding Rossmann-like domain. AhlS showed a conserved domain with the acyl-homoserine-lactone synthase (LasI) of the protein family COG3916. The open reading frames (ORFs) of the respective genes showed no mutation or deviation in the predicted tertiary structures.Inconclusion, plasmid acquisition significantly decreased the inhibitoryeffect of P. aeruginosa on the biofilm formation of the co-existing MRSA isolates.