It is very important, before starting the manufacture of any vaccine from any microorganism estimation of LD₅₀ of that microorganism to determine their pathogenicity and virulence. Estimated LD₅₀ was very important to be used in challenge tests later to estimate the protection level of the manufactured vaccine in experimental animals. So, this study was aimed to estimate LD₅₀ of local methicillin-resistant Staphylococcus aureus (MRSA) bacterial isolate. A pilot study has been done to determine approximately LD₅₀ of used MRSA in the study by using different bacterial concentrations of MRSA to determine approximate LD₅₀ that can be able to kill half numbers of animals used in the study to be used later in the estimation of exact LD₅₀ by using of Up-and-Down method. Ninety Wistar albino rats have been used for this purpose, eighty-four animals which divided into fourteen groups by six animals for each group (for pilot study) and remained six animals for (Up-and-Down method). The results showed that 9 X 10¹⁰ CFU/ml was led to killing half number of animals used in the study, this dose has been used as starting dose in the Up-and-Down method to the estimation of the exact LD₅₀ dose. The results showed that 5.526 X 10¹⁰ CFU/ml was the exact LD₅₀ of local MRSA isolate, which will be used later in the challenge test to estimate the protection level of a locally prepared vaccine against MRSA isolate.
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