en Microbiology and Anatomy Microbiology and Anatomy Original Research Original Research <p>&nbsp;&nbsp;<strong>&nbsp; Background: </strong>Nanoscale poly (amidoamine) dendrimers have been investigated for their biological demands, but their antibacterial activity has not been widely discovered. Thus, the sixth generation of poly (amidoamine) dendrimer (PAMAM-G6) was synthesized and its antibacterial activities were evaluated on Gram-negative bacteria; P. aeruginosa, E. coli, A. baumannii, S. typhimurium, S. dysenteriae, K. pneumoniae, P. mirabilis, and Gram-positive bacteria, and S.aureus and B. subtilis, which were isolated from different clinical specimens and standard strains of these bacteria.<br> <strong>&nbsp;&nbsp; Methods:</strong> In this study, 980 specimens including urine (47%), blood (27%), sputum (13%), wounds (8%), and burns (5%) were collected from clinical specimens of 16 hospitals and clinics in city of Sabzevar, Iran. Then, the target bacteria were isolated and identified using standard methods. Minimum inhibitory concentration and minimum bactericidal concentrations against Gram-positive and Gram-negative bacteria were determined according to guidelines described by clinical and laboratory standards institute (CLSI). Standard discs were prepared using 0.025, 0.25, 2.5, and 25 &mu;g/mL concentrations of PAMAM-G6 on Mueller-Hinton agar plates to determinate the zone of inhibition. The cytotoxicity of PAMAM-G6 dendrimer was evaluated in HCT116 cells by MTT assay.<br> <strong>&nbsp;&nbsp; Results: </strong>The most important isolated bacteria were E. coli (23.65%), S. aureus (24.7%), P. aeruginosa (10.49%), B. subtilis (7.7%), S. typhimurium (8.87%), A. baumannii (7.02%), K. pneumoniae (7.1%), P. mirabilis (6.46%), and S. dysenteriae (3.6%). Moreover, it was found that poly (amidoamine)&ndash;G6 exhibited more antibacterial efficacy on standard strains than isolated bacteria from clinical samples (p<0.05). The cytotoxicity of PAMAM-G6 to the cells showed that cytotoxicity depended on the concentration level and exposure time.<br> <strong>&nbsp;&nbsp; Conclusion:</strong> The PAMAM-G6 dendrimer showed a positive impact on the removal of dominant bacterial isolated from clinical specimens and standard strains.&nbsp;<br> &nbsp;</p> Antimicrobial resistance, Cytotoxicity, Poly (amidoamine)-G6, Health care-associated infections, Nosocomial pathogens, Novel antibacterial, Synthesis 368 376 http://mjiri.iums.ac.ir/browse.php?a_code=A-10-2906-1&slc_lang=en&sid=1 Ayoob Rastegar rastegar.89@gmail.com 200319475328460034031 200319475328460034031 No Department of Environmental Health Engineering, School of Public Health, Sabzevar University of Medical Sciences, Sabzevar, Iran & Student Research committee, Faculty of Public Health Branch, Iran University of Medical Sciences, Tehran, Iran. Shahram Nazari shahramnazari73@yahoo.com 200319475328460034032 200319475328460034032 Yes Student Research committee, Faculty of Public Health Branch, Iran University of Medical Sciences, Tehran, Iran. Ahmad Allahabadi Allahabadi@gmail.com 200319475328460034033 200319475328460034033 No Department of Environmental Health Engineering, School of Public Health, Sabzevar University of Medical Sciences, Sabzevar, Iran. Farahnaz Falanji rad6791@yahoo.com 200319475328460034034 200319475328460034034 No Cellular and Molecular Research Center, Sabzevar University of Medical Sciences, Sabzevar, Iran. Fakhreddin Akbari Dourbash fakhraddin.akbari91@gmail.com 200319475328460034035 200319475328460034035 No Department of Materials science and Engineering, Tarbiat Modares University, Tehran, Iran. Zahra Rezai Reazi12@gmail.com 200319475328460034036 200319475328460034036 No Department of Environmental Health Engineering, School of Public Health, Sabzevar University of Medical Sciences, Sabzevar, Iran. Soudabeh Alizadeh Matboo soudabehalizadeh@ymail.com 200319475328460034037 200319475328460034037 No School of Public Health, Ardabil University of Medical Sciences, Ardabil, Iran. Reza Hekmat-Shoar R.hekmatshoar@gmail.com 200319475328460034038 200319475328460034038 No Department of Occupational Health Engineering, School of Public Health, Sabzevar University of Medical Sciences, Sabzevar, Iran. Seyed Mohsen Mohseni mohsen_vincent@yahoo.com 200319475328460034039 200319475328460034039 No Department of Environmental Health Engineering, School of Public Health, Shahid Beheshti University of Medical Sciences, Tehran, Iran. Gharib Majidi 200319475328460034040 200319475328460034040 No Research Center for Environmental Pollutants, Qom University of Medical Sciences, Qom, Iran.