professor Julie MorrisseySAAD ABDULLAH SAAD ALKHOWAITER2022-06-042022-01-252022-06-04109875https://drepo.sdl.edu.sa/handle/20.500.14154/65924In conclusion, in order to determine the effect of lexA variants and BC on S.aureus biofilm formation, we constructed a lexA repressor mutant in S.aureus USA300. We examined how BC and lexA variants affected S. aureus mutagenesis and small colony variants SCV formation. We did not find significant differences in the formation of biofilms in the wild strain with different environments in the presence of carbon and in the environment of the pre-treated media. Also when studying the effect of BC exposure on WT strain S.aureus USA 300 LAC and mutant strain ΔagrB, Δsae in biofilm formation, there were no significant differences in the results of biofilm formation in all tested environments (with carbon and with pre-treated media). The results of small colonies assay after testing the wild strain S.aureus USA 300 LAC when grown in an environment containing ciprofloxacin (to increase DNA damage) then cultured in an environment resistant to rifampicin showed that there was a noticeable effect on increasing SCV formation both with ciprofloxacin alone and with ciprofloxacin in the presence of carbon black .The effect of black carbon on mutation formation in culture conditions in media containing black carbon and ciprofloxacin alone was also tested with ciprofloxacin in the presence of black carbon in an environment resistant to gentamycin. Ciprofloxacin increased the percentage of Rifampicin resistance in CFU, but interestingly this is decreased in the presence of BC, which indicates that BC decreased the rate of mutagenesis in response to Ciprofloxacin. As a result, both the WT and lexA strains were tested for mutagenesis.52enThesis