CarO, the outer membrane protein of Acinetobacter baumannii involved in carbapenem resistance, exists as a family of proteins encoded by multiple alleles

MUSSI, MARÍA ALEJANDRA; LIMANSKY, ADRIANA S; ARAKAKI, ADRIÁN; VIALE, ALEJANDRO M

Córdoba, Argentina

Congreso; XI Congreso Argentino de Microbiología, AAM,; 2007

Asociación Argetina de Microbiología

The protein CarO is a major constituent of the outer membrane of Acinetobacter baumannii, an emergent nosocomial pathogen. The loss of this protein has been associated to carbapenem resistance acquisition in clinical strains of this organism, and one of the mechanisms leading to its dissapearence from the OM was shown to be mediated by the natural insertional inactivation of its coding gene. Data concerning CarO immunogenicity and gene evolution, both of which are important in understanding the biologic role of the protein in the organism, are still incomplete. The aim of the present work was to assess the genetic variation of carO within the A. baumannii population. For this purpose, we analyzed a collection of 53 A. baumannii clinical strains obtained from three distint status of Argentina, recovered during the period 1990-2006. Direct nucleotide sequence determination of PCR products clearly identified 5 different carO gene groups (I-V) encoding unique amino acid sequence variants among the whole population.  Identities between these groups ranged from 70.5 to 82.3 % and 72.2 to 88.4 % at the nucleotide and amino acid level, respectively, revealing that most changes at the nucleotide level resulted in amino acidic changes. Of the 5 carO alleles, alleles I and II were only minoritary, appearing in 1 and 2 of the 53 strains, respectively. carO alleles III and IV represented 26 and 18%, respectively, while carO allele V was the most common, appearing in 49% of the strains. It is also interesting to note that sequence diversity within each group of carO variants was minimal, suggesting that a clonal dissemination of the variants has taken place. Sequence comparisons between the CarO variants revealed that genetic variation in carO is not randomly distributed but strikingly concentrated in three regions designated variable region (VR) 1, 2 and 3, comprising 36% of the whole carO sequence. These variable regions recruit 82% of the total amino acidic changes and interestingly superimpose with the predicted external loops 2, 3 and 5, respectively. Besides, the CarO variants exhibited differential inmunogenicity patterns as determined by using specific antibodies, indicating the existance of specific epitopes between the variants. Programs dedicated to search for positive selected mutations (http://selecton.bioinfo.tau.ac.il) did not show statistically significant result, whereas programs designed to detect recombination indicated the presence of breakpoints boundering the variable sequences. These results pointed out the differential phylogenetic origins of the variable regions relative to the conserved ones, and suggested that recombination may account for the polymorphism observed in carO.