SOS Response Regulating Mechanism To Resistance And Virulence Of Escherichia Coli

SOS response (SOSr) system, which is widespread in bacteria and regulated by a repressor, LexA, and an activator protein, RecA, is an important regulating response to DNA damage in which DNA repair and mutagenesis are induced. Nornally, LexA, as a dimer, binds to the promoter region of SOS genes and inhibits the expression of these genes. When bacteria are in stress state, the RecA is activated by ssDNA and the genes, inhibited by LexA dimer, are depressed and expressed. So the DNA damage is repaired. Simultaneously the mutation rate is increasing and the bacteria may adapt to environmental stress. The new habitat is developing because of antibiotic frequent used. Pathogenic bacteria are confronted with antibiotic and the inhibition of the host and the resistance and virulence must be regulated. So the model of SOS response inhibited was constructed and the correlation was explored between resistance and virulence.1. The MIC and resistant rate of recA-deleted bacteria to enrofloxacin was remarkably decreasedThe recA of E. coli ATCC 25922 and O157:H7, was knocked out by Red recombination system and the ATCC 25922/△recA and 0157:H7/△recA were gained. Then the change of the SOS genes expression, recA and umuC, was detected by real-time PCR. It was indicated that the expression of umuC was inhibited because of deleting recA. The result of broth dilution method was that the MIC of the recA-deleted isolates to enrofloxacin was 8-fold lower than those of their respective parents. In enrofloxacin environment, ATCC 25922/△recA was more difficult to mutagenesis than ATCC 25922 by the gradient plates.2. The effect of deleting recA on virulence of O157:H7The change of expression quantity of main virulent genes, stx2 and eaeA was detected by real-time PCR in different situation. It was suggested that the expression of stx2 and eaeA were improved in enrofloxacin condition. The expression of stx2 was decreased and the expression of eaeA was increased when recA was deleted. The results were consistent with cell test.3. The regulation of SOS response system to resistance and virulence of O157:H7 was explored by Seq-RNA20 genes, probably regulated by SOS response, were found out and the expression was signally up-regulated or down-regulated in enrofloxacin condition. It was speculated that 6 genes were associated with resistance,2 genes were associated with virulence and 3 genes were associated with DNA and nucleotide. Especially, the result of eaeA was coincident among Seq-RNA, real-time PCR and cell adhesion test. It was confirmed that eaeA was regulated by SOS response.4. The inhibitor of SOS response system Based on these, the inhibitor of RecA was synthesized. And the inhibiting effect of RecA inhibitor with enrofloxacin on growth and resistance of E. coli ATCC 25922 was evaluated. Through real-time PCR, the expression quantity of umuC, eaeA and stx2 was inhibited by RecA inhibitor in enrofloxacin condition.