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Onion (Allium cepa), is a herbaceous biennial plant and has edible bulb. The onion is probably native to southwestern Asia but is now grown throughout the world, chiefly in the temperate zones. The plant belongs to the lily family, Liliaceae, most members of which have an underground storage system, such as a bulb or tuber.   The deterioration of raw onions may result from physical factors; actions of their own enzymes, microbial or combination of these factors. In this Four samples of onions with signs of spoilage were bought from Owode-Ede market and Okefia market, Osogbo, Osun State, Nigeria.  A total of four bacteria and one fungus were isolated from four samples of onions with signs of spoilage purchased at Owode-Ede market and Oke fia market in Osogbo,  Osun state.

 The isolates were subjected to biochemical tests such as Gram staining, oxidase test, starch hydrolysis, methyl red growth at different pH and so on. They were identified to be Bacillus megaterium (3), Bacillus licheniformis (1), Bacillus polymyxa (1), and Staphylococcus aureus (1).  The only fungus isolated was Aspergillus niger from both the samples.  The four bacteria and one fungus isolated were subjected to further tests and these include physiochemical test and antimicrobial susceptibility test.  The effect of dfferent temperature 300C, 700C and 1000C on the isolates showed that the organisms thrive well at 300C but declined at higher temperature i.e 700C and 1000C.  Evaluation of the effect of different pH ranges (pH3, pH5, pH7 and pH9) on the growth of organisms was carried out and it was observed that the rate of growth of the organisms increased as the rate of the pH of the medium changed from acidity to alkalinity.

 The high occurrence of Bacteria and fungi in the spoilt onion bulbs is a public health risk. Appropriate control measures must be employed during the harvesting, processing, transportation, and handling of the fruits. Good storage facilities should also be put in place to protect the onion bulbs from attacks by these fungi, thereby minimizing wastes due to deterioration and unacceptability. The consumption of raw onions without prior heat treatment should be discouraged.