Antimicrobials were once believed to reduce bacterial fitness and virulence even at sub-inhibitory (SI) concentrations. This notion is being challenged in an increasing number of publications. In our study, we demonstrate that Thymol, a major monoterpene phenol obtained from thyme or oregano essential oils, and Tetracycline, a protein synthesis inhibiting antibiotic, at (SI) concentrations trigger the expression of virulence factors in the major opportunistic bacterial pathogen Pseudomonas aeruginosa. This organism is considered one of the top ten nosocomial pathogens, and is prevalent in cystic fibrosis (CF) lung infections, urinary tract infections and bacteremia. We show that exposing P. aeruginosa PAO1 to SI levels of Thymol or Tetracycline resulted in a dose dependant increase in secretion of virulence factors such as pyoverdine, proteases, elastase and exopolysaccharide. We also found that biofilm formation and cytotoxicity against macrophages were significantly elevated following SI exposure to both antimicrobials. These results indicate that natural antimicrobial agents, such as essential oils, as well as antibiotics, at low concentrations up-regulate virulence factors and therefore influence bacterial pathogenesis. We suggest that the increased virulence is not related to the specific toxic agent, but is rather due to the general biological response to SI exposure.
