Journal of Bacteriology Research Vol. 2(4), pp. 30-40, September 2010 Available online at http://www.academicjournals.org/JBR ISSN 2006- 9871 ©2010 Academic Journals Full Length Research Paper Factors influencing multi-drug resistant and plasmid DNA harbouring Aeromonas hydrophila isolated from Lake Manzala, Egypt Mahmoud M. M. Zaky 1 *, Fathy A. Mansour 2 and Kenneth M.Persson 3 1 Botany Department, Faculty of Science, Port-Said University, Egypt. 2 Botany Department, Faculty of Science, Mansoura University, Egypt. 3 Water Resources Engineering, Lund University, Sweden. Accepted 13 July, 2010 Lake Manzala is considered one of the most important coastal Lakes in Egypt. This study revealed the highly polluted condition of water and fish samples taken from three of the more important sites (Kapoty, Bashtier and Mataryia areas), representative to human activity and different ecosystems in the Lake water environment. Total suspended solids (TSS), ammonia and nitrates, of the samples taken showed figures exceeding the national and international standards. The samples from the study yielded high levels of bacterial content, which were found through plate counts and by using different growth media such as agar and endo agar. Total viable bacteria (TVB) reached more than 10 4 cfu/ml in water samples and 10 5 cfu/g in fish samples, particularly in Kapoty and Mataryia areas. Faecal coliform counts reached 10 2 cfu/ml in water samples and 10 3 cfu/g in fish samples. Statistical analysis revealed a positive correlation between Aeromonas spp. counts and some minerals, such as chlorides, calcium and magnesium (r = 0.9, 0.8, and 0.6 respectively; p  0.001), which revealed the interactions and adaptability of such organisms to environmental factors. A total of 88 isolates from Aeromonas differential agar were identified using the API 20E system; 17% of these isolates were Aeromnas hydrophila, and most of these strains were resistant to three or more antibiotics including pencillinG, ampicillin and chloramphenicol. Isolation of plasmid DNA from these strains of A. hydrophila, showed that most harboured plasmids ranging from less than 2.2 - 4 Kb P . Such findings could explain the adaptability and multi-drug resistance of such bacterial pathogens, which may have an impact on the course of infection and active causal agents of diarrhoeal diseases in this important coastal area of Egypt on the Mediterranean Sea. Key words: Aeromonas hydrophila, multi-drug resistance, plasmid DNA, Lake Manzala. INTRODUCTION Lake Manzala is considered one of the most important coastal Lakes in Egypt. It is situated in the northeastern part of the country, on the Delta of the Nile River bordering of the Mediterranean Sea. It receives a high load of different polluted waste water effluents including sewage water, industrial and agricultural waste from *Corresponding author. E-mail: zakymahmoud@yahoo.co.uk. many drains, such as Bahr El-Bakar, Ramsis, El-Sirw and Hadous. The effluents from these drainages negatively affect the Lake environment to the eutrophic level as well as human health among communities around the Lake. Aeromonas spp. particularly Aeromonas hydrophila and Aeromonas sobria are widely distributed in fresh, brac- kish and saline water environments, as well as in lakes and sewage water; they have also been isolated from drinking water and food, such as meat, fish, shellfish, raw milk and vegetables (Araujo et al., 1990; Gonzalez et al., 2002; Hanninen et al., 1997; Kirov et al., 1990; Montesw