EFFECTS OF PROTAMINE ON PSEUDOMONAS AERUGINOSA CELL ENVELOPE COMPONENTS: SURFACE REMODELLING
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The main objective of the study was to understand the mode of interaction of protamine (Ptm), a cationic antibacterial peptide from fish milt on the Gram negative bacterial envelope. The present study was designed to resolve the question of Ptm translocation across the seemingly impermeable Gram negative cell envelope. The Gram negative pathogen Pseudomonas aeruginosa was studied as an example of a microorganism that is Ptm-sensitive but doesn’t lyse even at bactericidal concentrations. Acquired resistance to Ptm was induced in P. aeruginosa by continuous sub-culturing in nutrient rich media containing increasing concentrations of Ptm. Alterations in bacterial surface charge, LPS composition, cell morphology and Ptm localisation on acquiring resistance were also examined. Expression of outer membrane proteins significantly decreased as P. aeruginosa acquired resistance to Ptm. OprF, the major porin in P. aeruginosa was found to be stably expressed in control, revertant (Ptm-Rev) and resistant (Ptm-Res) groups. No change in expression of efflux proteins was observed as a result of induced Ptm resistance, indicating that efflux is not among the Ptm resistance mechanisms at least in P. aeruginosa. OprM, which is part of the major efflux system (MexAB-OprM) in P. aeruginosa, was found to be down-regulated in Ptm-resistant P. aeruginosa. Another outer membrane protein down-regulated in Ptm-resistant P. aeruginosa was found to be petidyl-prolyl cis trans isomerase (PPIase) which plays a major role in proper folding and maturation of channel proteins in the outer membrane. Among the sarcosinate soluble proteins, DNA dependent RNA polymerase ? and ?’ subunits were found to be down-regulated in Ptm-resistant group indicating lower transcription levels in them. Lipopolysaccharide (LPS) from the three groups of P. aeruginosa under study was isolated and separated by SDS-PAGE. LPS composition of Ptm-Res P. aeruginosa was found to be significantly different from that of the control and Ptm-Rev but was found to be similar with that of LPS from O-antigenic mutant (A+B-, which possessed only A band structures). Comparison of the zetapotential of control, Ptm-Rev and Ptm-Res P. aeruginosa, proved that electrostatic shielding was coincidental in acquired resistance to Ptm in P. aeruginosa. The MIC of the parent strain of P. aeruginosa (A+B+) and the O-antigenic mutants (A+B-, A-B+ and A-B-) were found to be the same which may be indicating that alterations in O-antigenic components alone cannot contribute to Ptm resistance. Effects of Ptm treatment on morphologies of E. coli, S. typhimurium and P. aeruginosa whole cells and spheroplasts were also studied using transmission immuno-electron microscopy. Condensation of cytoplasmic contents was observed when whole cells and spheroplasts were treated with Ptm. Also, Ptm-treated cells and spheroplasts were stained with colloidal gold-labelled antibodies against Ptm to determine distribution within the target cells. It was quite evident that Ptm internalised in whole cells and spheroplasts without lysis and was found to be concentrated in the cytoplasm. Morphological changes observed in Ptm-Rev P. aeruginosa when exposed to Ptm were comparable with that of the control. Condensation of cytoplasmic contents was not observed in Ptm-Res P. aeruginosa when challenged with Ptm. Most of the Ptm was localized at or near the outer membrane of Ptm-treated Ptm-Res P. aeruginosa, indicating decreased outer membrane permeability. Results obtained from these experiments confirm that the resistance to Ptm observed in P. aeruginosa is at the very least, coincidental with the pleiotropic mutations involving change in outer surface including change in LPS composition, loss of porins and or alterations of porin size in OprF.