| Acinetobacter baumannii is a common opportunistic pathogen in clinical practice,and it often leads to severe community-related and nosocomial infections with its strong survival and pathogenic abilities.When A.baumannii overproduces surface polysaccharides,its colony will exhibit a characteristic mucinous phenotype.As an important adaptive defense response for A.baumannii,the mucoid phenotype increases its environmental stress resistance,reduces its antibiotic susceptibility,and enhances its virulence and the ability to resist the immune system clearance,which poses a serious challenge to the prevention and treatment of related infections.At the same time,A.baumannii can adhere to and colonize various abiotic surfaces,i.e.,equipment and instruments in hospitals,to form biofilms,and enhance its durability and transmission ability,which makes it one of the most challenging pathogens in hospitals.Therefore,the use of surfactants to effectively inhibit and remove the biofilms on solid surfaces is particularly important in preventing and controlling A.baumannii-related infection.In this thesis,the sodium dodecyl sulfate(SDS),cetyltrimethylammonium bromide,betaine,rhamnolipids,and Triton X-100 were selected as representative surfactants and comprehensively evaluated their effects on both the planktonic cells and biofilms of A.baumannii ATCC19606.The results showed that specific concentrations of SDS and Triton X-100 were able to effectively inhibit the initial cell attachment,biofilm formation,and dispersal of planktonic cells on hydrophobic surfaces by the 19606 strain,as well as remove its mature biofilms.Except for betaine,all the other surfactants significantly increased the permeability of the inner and outer membranes of 19606 cells,thereby achieving inhibition and killing of its planktonic cells.Meanwhile,it was shown that 1 g/L SDS could induce an increase in surface polysaccharides of 19606 cells,leading to mutual adhesion of their colonies on solid plates and the appearance of a typical mucoid phenotype.Further analysis of multiple clinical isolates of A.baumannii showed that the induction of mucoid phenotype by a specific concentration of SDS has a certain universality.Next,through the construction of insertion inactivation mutants of two genes,bfmS and bfmR,and transcriptional level of related genes and phenotype analysis compared with the wild-type strain 19606,it was confirmed that A.baumannii might perceive the stimulation of SDS on cells through the two-component system BfmR/S,upregulate the transcription of the structural gene cluster K locus,synthesize a large amount of capsule polysaccharides(CPS),and thus present a mucoid phenotype.By comparing the deletion mutant of outer membrane protein Omp38 and the induction of wild-type cells with SDS,similarities and potential correlations in outer membrane morphology,polysaccharide synthesis,and colony phenotype were revealed.Finally,a possible molecular mechanism for the induction of mucoid phenotype in A.baumannii by a specific concentration of SDS was proposed.When the outer membrane of cells suffers specific damage,such as the absence of the critical protein like Omp38 or treatment with 1 g/L SDS,this stimulus signal is perceived by BfmS and no longer phosphorylates the regulatory factor BfmR,leading to a significant upregulation of the transcription level of the K locus gene cluster and a large amount of capsule polysaccharide synthesis,thereby producing a mucoid phenotype.As an important adaptive defense response of A.baumannii,the mechanism underlining SDS-induced mucoid phenotype can provide some reference information and strategic guidance for the prevention and treatment of A.baumannii infections in clinical settings. |
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