According to World Health Organization (WHO), lower respiratory tract infections are the third most common cause of death worldwide. These infections are mainly caused by multidrug-resistant (MDR) bacteria. Between 8% - 28% of patients receiving mechanical ventilation are affected by ventilator associated pneumonia (VAP).The causative microorganisms are extremely resistant rendering the proper choice of antibiotics for treatment difficult. The drugs which disrupt virulence factors of bacteria are more promising as the bacteria cannot develop resistance against them. Quorum sensing inhibitors are used to disrupt intra and inter bacterial communications giving better chance for host defense mechanishms. The aim of current study is to characterize bacteria isolated from VAP patients, to evaluate the effectiveness of some antimicrobial agents, characteraize MDR strains, screen isolates for quorum sensing systems and to evaluate efficacy of different essential oils as quorum sensing inhibitors. Clinical bacterial isolates were recovered from patients having pneumonia associated with mechanical ventilation from intensive care units of Zagazig University Hospital and identified using standard microbiological methods. The specimens were cultured on nutrient agar, blood agar, Mac-Conkey agar and mannitol salt agar and incubated for 24 hours at 37°C. Isolated bacteria were characterized using cultural and biochemical characteristics. A total of 233 isolates were recovered from 153 sputum samples, compromising 203(87.1%) Gram negative and 30 (12.9%) Gram positive isolates. The major isolates were Klebsiella pneumoniae (36.9%), Escherichia coli (21.03%), Acinetobacter baumannii (14.59%), Pseudomonas aeruginosa (14.16%) and Staphylococcous aureus (12.02%). Bacterial isolates were tested for their in vitro susceptibility to antimicrobial agents by disk diffusion method. The isolates were highly resistant to antimicrobial agents. Two hundreds and twelve isolates (90.99%) were MDR and one hundred seventy two isolates (73.8%) were extensively drug resistant (XDR). K. pneumoniae isolates demonstrated high resistance rate to cefepime (94.19%), ceftraixone (97.68%) and aztreonam (93.02%). All K. pneumoniae isolates (100%) were multi drug resistant (MDR) and seventy eight isolates (90.7%) were extensively drug resistant (XDR). E. coli isolates showed high resistance to ceftraixone (97.96%), aztreonam (95.92%) and cefepime (95.92%). Fourty eight isolates (97.96%) were MDR and thirty one isolates (63.27%) were XDR. All Acinetobacter baumannii isolates were resistant to ceftazidime, piperacillin, ofloxacin , meropenem ,imipenem, ceftraixone , cefepime and levofloxacin, while they were sensitivite to colistin. All A. baumannii isolates (100%) were MDR and thirty one isolates (91.2%) were XDR. Pseudomonas aeruginosa isolates demonstrated resistance to ceftarixone and carbenicillin, twenty eight isolates (84.84%) were MDR and twenty six isolates (78.78%) were XDR. All Staphylococcus spp demonstrated resistance to ceftraixone, cefotaxime and methicillin while were sensitivie to linezolid. Sixteen isolates (53.3%) were MDR and six isolates (21.4%) were XDR. The MDR isolates of Klebsiella pneumoniae, E. coli and S. aureus were screened for the presence of quorum sensing genes by PCR. Also, MDR isolates of Klebsiella pneumoniae were screened for both LuxR and LuxS genes using real time PCR. The results showed that quorum sensing genes were widely spread among different bacterial species. Essential oils of clove, peppermint, eucalyptus, chenopodium, ginger, thyme, rosemary, jasmine and oegano oils were screened for their quorum sensing inhibition activity using the biosensor Chromobacterium violaceium CV026. These EOs were also tested for their effect on some virulence of PAO1 known to be controlled by QS system. The virulence factors tested were pyocyanin production, twitching and swimming motility, protease activity and biofilm formation. The results showed violacein production inhibition in CV026 for clove, thyme, chenopodium and eucalyptus oils and pyocyanin inhibition by clove, chenopodium and rosemary oils. Eucalyptus and jasmine oil showed high biofilm formation inhibition. Clove and thyme oil completely inhibited protease enzyme activity. The effect of subinhibitory concentrations of essential oils on susceptibility of MDR isolates of K. pneumoiae to different antimicrobial agents (ciprofloxacin, cefepime, imipenem and amikacin) were tested. Seven MDR isolates were used in this combination. The MIC values of antimicrobials were determined in presence of subinhibitory concentrations of EOs. MIC values of cefepime were reduced by 2-64 and 4-32 fold in presence of sub inhibitory concentrations of thyme and clove oils respectively. MIC values of amikacin were reduced by 4-16 fold by thyme oil. MIC values of ciprofloxacin were reduced in presence of sub inhibitory concentrations of thyme oil by 8-32 fold and jasmine oil by 2-32 fold. In conculsion, the data of this study showed high incidence of VAP and high resistance rates to various antimicrobial agents among MDR isolates. K. pneumoniae and E. coli showed high drug resistance to all tested antimicrobial agents. Colistin were highly effective against Acinetobacter baumannii. Vancomycin and linezolid retained excellent activity against Staphylococcus spp. Clove and thyme oil showed high violacien inhibition. MDR isolates of K. pneumoniae showed high expression of both LuxS and LuxR genes. For better management of VAP, the study recommends that periodic epidemiological investigation of the most encountered pathogens causing VAP and antimicrobial susceptibility test should be performed before antimicrobial therapy to help choosing the appropriate antibiotics and avoid emergence of MDR strains . The high rate of resistance render antivirulent agents as quorum sensing inhibitors more promising.