Diversity and Antibiotic Susceptibility of Acinetobacter Strains From Milk Powder Produced in Germany

Abstract

Forty-seven <i>Acinetobacter</i> 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 <i>rpo</i>B gene sequencing, as well as by whole genome sequencing of selected isolates and their <i>in silico</i> 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 <i>Acinetobacter baumannii</i>. Of the 47 strains, 42 were identified as <i>A. baumannii</i>, 4 as <i>Acinetobacter Pittii</i>, and 1 as <i>Acinetobacter calcoaceticus</i> based on 16S rRNA gene sequencing. <i>In silico</i> DDH with the genome sequence data of selected strains and <i>rpo</i>B gene sequencing data suggested that the five non-<i>A. baumannii</i> strains all belonged to <i>A. pittii</i>, suggesting that the <i>rpo</i>B gene is more reliable than the 16S rRNA gene for species level identification in this genus. Rep-PCR genotyping of the <i>A. baumannii</i> 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 <i>Acinetobacter</i> strains or resistance genes.