Pathogenicity Of Prevalent Cloning Strains Of Carbapenem Resistant Acinetobacter Baumannii

Background： Acinetobacter baumannii is a nonfermentingGram-negative opportunistic Pathogen. It was thought to be little virulentand sensitive to almost all the antibiotics available in1970s. However, since1990s，it has turned to become one of the most emerging nosocomialbacteria due to its great ability to tolerate harsh environment especiallydesiccation, development of a serial of drug-resistant mutations andexcellent capacity of acquiring resistant genes from other species, whichlead to an epidemic of multidrug-resistant or pan-drug-resistantAcinetobacter baumannii worldwide, especially in ICU, and difficult to treat.Till now, most of the researches on Ab have been focused on theepidemiology、resistance mechanism and therapy methods, which havedemonstrated the association between clone epidemicity and drug resistance.But limited pathogenic mechanism or virulent attributes about this bacteriahave been illuminated, mainly including biofilm formation、siderophore-mediated iron acquisition systems、outer membrane protein. Also factors ofadherence such as twitching motility、pili、hemagglutination activity and some kinds of enzymes including gelatinase have been identified to affectvirulence.However, whether the virulence factors attribute to cloneepidemicity is not clear. Meanwhile, whether acquired resistance has effecton the virulence and adaptation of Ab is unknown.For some reports,it hasbeen shown decreased fitness and virulence in vitro and in vivo of Ab whichis resistant to Ciprofloxacin、colistin or carbapenem. Given the researchabove, we consider the the influence of drug resistance on the virulence andfitness of clinical strains, and also the association between virulence andepidemicity, which could be contribute to explain the cause of cloneepidemicity, and also provide guidance on distinguishing colonization andinfection.Object：Limited data on the virulence of nosocomial carbapenemresistant Acinetobacter baumannii are known. We aimed to study thevirulence of prevalent carbapenem resistant Acinetobacter baumanniicompared with the susceptible strains in a teaching hospital in2009.Methods：A total of87strains including three Carbapenem-resistantprevalent clones of A/B/C and Carbapenem sensitive types isolated fromclinical species were studied for twitching motility by motorial plate,gelatinase activity by tube test, D-mannose related O/AB hemagglutinationactivity by serially diluted micro-hemagglutination test, biofilm formationby crystal violet staining.Results：All of the three carbapenem-resistant clones showed no twitching motility, whereas33.3%（7/21）of the sensitive ones positive.Gelatinase activity was detected only in one strain of clone B. The incidenceof human group O and AB erythrocytes agglutinated by clone A、B、C andcarbapenem-sensitive strains in the presence of D-mannose were2.7%(1/37)、10.8%(4/37),45%(9/20)、75%(15/20)，0、0,19%(4/21)、19%(4/21),respectively, while their ability of biofilm formation varied that the proportionsof negative、weak positive、medium positive、strong positive of eachgroup were43%、49%、8%、0，5%、15%、60%、20%，56%、33%、11%、0，5%、33%、29%、33%，respectively.Conclusion：The weaker ability of twitching motility of cloneA、B、C and biofilm formation of clone A、C compared with carbapenem-sensitive strains indicate lower virulence of carbapenem-resistant isolates;clone B presents stronger hemagglutination activity and biofilm ability thanclone A and C,which shows different virulence among carbapenem-resistantgroups; furthermore,this study shows that it is antimicrobial resistancepressure that leads to the prevalence of carbapenem-resistant Acinetobacterbaumannii in the hospital. The ability of carbapenem resistance may lead todecreased virulence of Ab,but may not cause increase of infection rate.