**Alexander Hladicz 1,2 ID , Clemens Kittinger <sup>2</sup> and Gernot Zarfel 2,\***


Received: 4 September 2017; Accepted: 30 September 2017; Published: 3 October 2017

**Abstract:** Antibiotic-resistant bacteria are spreading worldwide in medical settings but also in the environment. These resistant bacteria illustrate a major health problem in our times, and last-line antibiotics such as tigecycline represent an ultimate therapy option. Reports on tigecycline non-susceptible *Enterobacteriaceae* are presented with regard to medical settings but are rare with that for the environment. The aim of this study was to characterize two tigecycline non-susceptible *Klebsiella pneumoniae* isolates from the river Mur, and to question the resistance mechanism. The screening for chromosomal mutations revealed a deletion and a silent point mutation in one isolate and a point mutation in the other isolate all within the *ramR* allele. RamR acts as repressor and prevents overexpression of *ramA*. These mutations are likely to cause a resistant phenotype due to the overexpression of AcrAB-TolC. MLST revealed that the isolates belonged to two unrelated MLST types (ST2392 and ST2394). Both isolates only revealed resistance to tigecycline and tetracycline. This is one of the rare reports of tigecycline-resistant *Klebsiella pneumoniae* from surface water. The presence of two genetically different isolates suggests that the river water may bear substances that favor mutations that can lead to this efflux pump-driven resistance.

**Keywords:** ramA; efflux pump; multilocus sequence typing; surface water
