Impact of Gamma Radiation and Some Natural Compounds on Quorum Sensing System of Some Pathogenic Gram Negative Bacteria

Abstract

Inhibition of Quorum Sensing (QS), a cell-density- dependent regulation of virulent bacterial gene expression represents a challenging strategy for the discovery of novel antimicrobials to combat the resistant pathogenic bacterial strains, based on inhibition of virulence factors expression regulated in many pathogens by this QS system Among 207 collected clinical specimens (collected from cancer and non cancer patients), 168 Gram negative rods and 39 Gram positive cocci isolates were recovered. According to preliminary identification of the Gram negative rods, they turned out to be 87 Pseudomonas species(P. spp.), 24 Escherichia coli (E coli), 31 Klebsiella (K.spp.), 21 Acinetobacter (A. spp.) and 5 Providencia (Pv spp.). The isolates were screened for the production of some virulence factors, namely, production of protease, lipase, elastase, ability to produce slime, pyocyanine pigmentation, twitching motility and cytotoxicity. Twenty three isolates that showed highest production of virulence factors were screened for their ability to produce quorum sensing signals by both cross feeding assay and disc diffusion assay using the bioreporter Chromobacterium violaceum (CV026). Four Pseudomonas spp. (P17, P38, P40 and P77) and three Providencia spp. (Pv1, Pv3 and Pv5) were able to stimulate the production of purple pigment of the bioreporter i.e. produce Acyl Homoserine Lactone(AHL) signal which was confirmed by GC/MS. Antibiotic senstivity test was carried out for those isolates to determine the extent of their resistance to different antibiotics. Some plant extracts (at sub-MICs level) as well as a therapeutic gamma radiation dose (24.4 Gray level, employed in protocol of cancer treatment) were evaluated for their ability to affect several virulence factors produced by the selected strains quantitatvely through interfering with their quorum sensing system. Extracts of clove oil, garlic, rosemary and gamma radiation therapeutic dose demonstrated a potential reduction in protease that ranged from 23.3 to 61.7% among the tested isolates, lipase (7 to 61.5%), elastase (19 to 56.9%), cytotoxicity (7.7 to 50.7%) and biofilm capability (weakly or non-adherent). This coincides with the results of the GC/MS carried for the selected isolates which showed reduction of quorum sensing signal molecules, Butyryl homoserine lactone (BHL) with a reduction range from 50.7 to 80.66% and Hexanoyl homoserine lactone (HHL) from 15 to 80.72%. Identification by 16S rRNA gene sequencing revealed that the selected isolates were four Pseudomonas aeruginosa, two Providencia vermicola and one Providencia stuartii.