The Role of Outer-Membrane Vesicles in Intercellular Communication in Pseudomonas Aeruginosa
Gram-negative bacteria produce outer-membrane vesicles (OMVs) that have biological roles ranging from biofilm formation, modulation of host-cell interactions & delivery of virulence factors. Several studies have shown a role for OMVs to act as intracellular signals to co-ordinate the behaviour of bacteria. This study showed OMVs generated at sub-lethal ciprofloxacin concentrations were capable of programming naïve P. aeruginosa cultures resulting in premature entry into stationary-phase and a significantly lower final culture density reached after 14 hrs. Pyoverdine production was also initiated after 6 hrs in cultures treated with OMVs. Heat-inactivation of OMVs failed to impede OMV-mediated growth inhibition & pyoverdine production. Chloroform-disruption of OMVs prevented OMV-mediated growth inhibition but did not inhibit OMV-induced pyoverdine production. It is likely that these effects are mediated by multiple signals as opposed to a single mechanism. This suggests that a protein is not responsible for OMV-mediated growth inhibition and an intact OMV lipid bilayer is required. Induction of pyoverdine production is likely due to a lipid (such as a homo-serine lactone) or small molecule present within OMVs. Preincubation with OMVs for 2-4 hrs resulted in a substantial decrease in the final culture density from cultures that were exposed to OMVs during the course of growth. This suggests that OMV fusion is capable of programming naïve bacteria to set a predetermined division limit on subsequent daughter cells. We coin this as the ‘Dayflick’ limit due to the similarities of the Hayflick limit in eukaryotic cells. This shows that OMVs act as intercellular messaging vehicles between bacteria that communicate and program naïve bacteria to adapt to the environment under which they were generated in, aiding survival in harsh environments. Further study is needed to determine what OMV components are responsible for initiating these responses and to determine how long the programming is stable.