_____________________________________________________________________________________________________ *Corresponding author: E-mail: austinraney.unimke@gmail.com; Journal of Advances in Microbiology 4(4): 1-9, 2017; Article no.JAMB.34847 ISSN: 2456-7116 Assessment of the Microbial Diversity of Spent-Oil Contaminated Soil in Calabar, Nigeria A. A. Unimke 1* , A. O. Mmuoegbulam 1 , I. U. Bassey 1 and S. E. Obot 1 1 Department of Microbiology, Faculty of Biological Sciences, University of Calabar, P.M.B. 1115, Calabar, Cross River State, Nigeria. Authors’ contributions This work was carried out in collaboration between all authors. Author AAU designed the study, performed the statistical analysis, wrote the protocol and wrote the first draft of the manuscript. Authors IUB and AOM managed the analyses of the study. Authors AAU and SEO managed the literature searches. All authors read and approved the final manuscript. Article Information DOI: 10.9734/JAMB/2017/34847 Editor(s): (1) Satarupa Dey, Microbiology Laboratory, Department of Botany, University of Calcutta, Kolkata, India. (2) Graciela Castro Escarpulli, Laboratorio de Bacteriologia Medica, Instituto Politécnico Nacional, Mexico. Reviewers: (1) Omoregbee Osazuwa, University of Benin, Nigeria. (2) Kohji Ishihara, Okayama University of Science, Japan. Complete Peer review History: http://www.sciencedomain.org/review-history/20114 Received 15 th June 2017 Accepted 2 nd July 2017 Published 19 th July 2017 ABSTRACT This study was designed to assess the total heterotrophic and hydrocarbon oxidizing microbial diversity, determine the physicochemical parameters of oil contaminated soil and evaluate the potentials of microorganisms isolated to utilize spent oil. Three mechanic workshops within Calabar South axis of Cross River State Nigeria were selected and three soil samples were collected from each site. The total heterotrophic bacteria were enumerated on nutrient agar and the hydrocarbon utilizing bacteria on minimal salt medium using the vapor phase method after employing the ten-fold dilutions from 1:10 to 1:100000. The total heterotrophic mean count ranged from 1.28×10 4 to 3.58×10 4 cfu/g while mean count of hydrocarbon utilizing bacteria varied from 1.94×10 4 to 7.34×10 4 cfu/g. Physicochemical parameters of samples were determined. Such parameters included pH, electrical conductivity and compounds such as nitrate, nitrite, phosphate, calcium, magnesium and potassium. The bacterial isolates evaluated belonged to the genera of Bacillus, Klebsiella, Proteus, Pseudomonas, Enterococcus, Flavobacterium and Micrococcus spp. Klebsiella spp. emerged as the most prevalent heterotrophic bacteria with 29.41% while Bacillus spp, and Enterococcus spp. were Original Research Article