Abstract
Forty-seven Acinetobacter spp. isolates from milk powder obtained from a powdered milk producer in Germany were investigated for their antibiotic resistance susceptibilities, in order to assess whether strains from food harbor multiple antibiotic resistances and whether the food route is important for dissemination of resistance genes. The strains were identified by 16S rRNA and rpoB gene sequencing, as well as by whole genome sequencing of selected isolates and their in silico DNA-DNA hybridization (DDH). Furthermore, they were genotyped by rep-PCR together with reference strains of pan-European groups I, II, and III strains of Acinetobacter baumannii. Of the 47 strains, 42 were identified as A. baumannii, 4 as Acinetobacter Pittii, and 1 as Acinetobacter calcoaceticus based on 16S rRNA gene sequencing. In silico DDH with the genome sequence data of selected strains and rpoB gene sequencing data suggested that the five non-A. baumannii strains all belonged to A. pittii, suggesting that the rpoB gene is more reliable than the 16S rRNA gene for species level identification in this genus. Rep-PCR genotyping of the A. baumannii strains showed that these could be grouped into four groups, and that some strains clustered together with reference strains of pan-European clinical group II and III strains. All strains in this study were intrinsically resistant toward chloramphenicol and oxacillin, but susceptible toward tetracycline, tobramycin, erythromycin, and ciprofloxacin. For cefotaxime, 43 strains (91.5%) were intermediate and 3 strains (6.4%) resistant, while 3 (6.4%) and 21 (44.7%) strains exhibited resistance to cefepime and streptomycin, respectively. Forty-six (97.9%) strains were susceptible to amikacin and ampicillin-sulbactam. Therefore, the strains in this study were generally not resistant to the clinically relevant antibiotics, especially tobramycin, ciprofloxacin, cefepime, and meropenem, suggesting that the food route probably poses only a low risk for multidrug resistant Acinetobacter strains or resistance genes.
Highlights
Species of the genus Acinetobacter (A.) belong to the γProteobacteria, and group in the order Pseudomonadales and family Moraxellaceae (Bouvet and Jeanjean, 1989)
This study aimed to determine whether foodborne Acinetobacter isolates in dried milk possessed similar antibiotic resistances as the clinical strains, and to assess whether Acinetobacter strains can function as potential reservoirs of antibiotic resistance along the food transmission route and my later become problematic in a hospital setting when hosts are immunocompromised
Acinetobacter spp. including A. baumannii commonly occur in raw bulk tank milk (Straley et al, 2006; Gurung et al, 2013; Tamang et al, 2014), with one investigation reporting up to 7.7% of raw, bulk tank milk samples being positive for species of this genus (Gurung et al, 2013)
Summary
Species of the genus Acinetobacter (A.) belong to the γProteobacteria, and group in the order Pseudomonadales and family Moraxellaceae (Bouvet and Jeanjean, 1989). There are currently 42 validly published species names, which include at least one pair of synonyms (Touchon et al, 2014; Krizova et al, 2015). It is difficult to differentiate accurately between A. baumannii, A. pittii, A. nosocomialis, and Acinetobacter calcoaceticus strains, as these are highly similar with respect to their phenotypic and biochemical properties. These strains show close relationship in DNA-DNA hybridization (DDH) studies. These have commonly been grouped together in the so-called A. calcoaceticus–A. baumannii complex (Sheng et al, 2011)
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