Beschreibung
Although A. baumannii (Ab) is considered as one of the most important pathogens of nosocomial infections in human medicine, little is known about the potential reservoir and the role of animals as carriers and vectors of this species. Based on this work, information on the occurrence of multidrug-resistant, in particular carbapenem-resistant (CR) Acinetobacter spp., should be observed from clinical animal samples. The aim was the genotypically and phenotypically characterization of animal Acinetobacter spp. isolates. For this purpose, the influence of CR on bacterial fitness, biofilm formation and virulence was investigated in addition to the phylogenetic background, antimicrobial resistance profile and acquired resistance determinants. A total of 1,540 Acinetobacter spp., including 821 Ab, were collected between November 2000 and September 2018. The collection was based on isolates from the Institute of Hygiene and Infectious Diseases of Animals as part of routine diagnostics as well as CR Acinetobacter spp. of a commercial veterinary laboratory and human CR Acinetobacter. Species identification was performed using MALDI-TOF MS analysis and PCR. Antimicrobial susceptibility was determined by VITEK2 and for carbapenem antibiotics by E-test. Acquired carbapenemase genes were detected using PCR. Among 1,483 Acinetobacter spp. from animals, 74 (5 %) isolates, including 48 (3.2 %) Ab, eight (0.5 %) A. pittii, eight (0.5 %) A. lwoffi, five (0.3 %) A. johnsonii, three (0.2 %) A. indicus, and two (0.1 %) A. species, were identified as carriers of carbapenemase genes. OXA-58 occurred as the most frequent carbapenemase (n=47), followed by OXA-23 (n=24), and OXA-72, OXA-143, and NDM-1 (1 isolate each). CR isolates were obtained from dogs (n=39; 52.7 %) and cats (n=18; 24.3 %), livestock (n=10; 13.5 %), pets (n=4; 5.4 %), horses (n=2; 2.7 %), and from an air conditioner of a veterinary clinic (1.4 %). The Acinetobacter strains were isolated from urine (n=19; 25.7 %), the respiratory tract (n=17; 23 %), wounds (n=10; 1.4 %) and other clinical sites (n=28; 37.8 %). Most carbapenemase-positive Acinetobacter spp. were carriers of other acquired resistance genes, such as str, aac or aad, tet, sul1/2 and their variants.
In CR Ab isolates, these acquired resistance genes were partially detected within resistance islands. Thus, ten of the ST1Past-OXA-58-positive Ab isolated between 2016 and 2018 carried AbaR3-like and one isolate carried an AbaR10-like island. In contrast, Ab isolated before 2016 did not carry resistance islands. Acquisition of resistance genes was mostly reflected in a multidrug-resistant phenotype. However, not all isolates, which carried a carbapenemase gene showed a phenotypic resistance to the tested carbapenem antibiotics. This was due to (i) the absence of expression-enhancing insertion elements, (ii) partial deletions in relevant genetic structures, or (iii) the absence of synergistic resistance mechanisms, such as efflux pumps.
Using MLST analysis (Pasteur scheme), the animal Ab isolates could be assigned to six different sequence types (STs), including ST1Past and ST25Past, which are distributed worldwide in human medicine. Furthermore, the distribution of acquired resistance genes and virulence-associated genes correlated with the respectively assigned STs. Using core genome MLST (cgMLST), the CR Ab could be assigned to twelve different clusters (CTs). The majority of ST1Past-OXA-58-positive Ab (93.1%) were assigned to CT-1808. In Germany OXA-23-Ab isolates CT-2177 dominated, whereas isolates from France and Italy belonged to eight different CTs. This suggests a clonal spread of CT-1808 respectively CT-2177 in companion animals in Germany, whereas the spread of OXA-positive Ab in France and Italy seems to be independent.
The genetic context of carbapenemase genes, both Ab and non-baumannii Acinetobacter overlapped with the structure of CR isolates occurring in human nosocomial infections. In particular, the embedding within mobile genetic elements such as blaOXA-23 in the transposon structure Tn2008 as well as blaNDM-1 in Tn125 provides evidence for a possible transmission of resistance genes between Acinetobacter spp. of animal and human origin.
The examination of the influence of CR on bacterial fitness, carrying the resistance plasmid showed a negative influence on the fitness of Ab. The same can be observed in biofilm formation: CR Ab showed significantly lower biofilm formation than the isogenic plasmid-free variants. In contrast, the CR Ab isolates showed a significantly more virulent phenotype than the corresponding carbapenem-sensitive variant in the Galleria mellonella infection model. According to the available data, the acquisition of carbapenemase genes has a negative impact on bacterial fitness and biofilm formation, but not on the virulence of the isolates.
Based on phylogenetic analyses of this study, it could be shown that the animal isolates from this work potentially represent a separate group, which clusters with human isolates from Australia, South Korea, Thailand, the Czech Republic and the USA. While human Ab from Europe were less similar to the veterinary isolates. Related to human medicine, successful clones have become established in veterinary medicine. These clones spread nosocomially in the veterinary clinical setting. It must be assumed that Ab isolates and/or their resistance determinants are transferred between animals and humans. Whereas animals intitially acted as recipients, mutual transmission cannot be ruled out. Accordingly, targeted monitoring of multidrug-resistant Ab, in particular CR isolates, in veterinary medicine should be considered. This is mandatory to implement effective intervention measures in veterinary hospitals to reduce infections with this pathogen. After all, a common consideration of human and animal isolates is required to assess the spread and transmission of CR Ab isolates within a One Health context.
