The insufficient development of new antibiotics and the growing incidence of infections with multi-resistant Gram-negative bacteria have led to older antibiotics returning into use. One of them is colistin, used in human medicine since 1958, although its use has been limited due to its nephrotoxicity and neurotoxicity. Until recently, the only known resistance mechanisms against colistin were the consequence of chromosomal mutations, which, however, did not present a great risk for rapid spread among bacteria species. A plasmid-mediated resistance to colistin was first reported in November 2015 in China. An mcr-1 gene that encodes the phosphoethanolamine transferase enzyme was discovered in an Escherichia coli pig isolate. Since its discovery the presence of the gene in E. coli, Salmonella spp. and K pneumoniae species has been reported from all over the world. The research attempted to discover the susceptibility of the Slovene isolates of Enterobacteriaceae for colistin, and whether the plasmid-coded resistance against colistin is present in Slovenia. A micro-dilution method was used to test 409 Enterobacteriaceae. Between the tested isolates of Enterobacteriaceae bacteria species, 392 (95.8 %) isolates were susceptible and 17 (4.2 %) isolates were resistant against colistin. In vitro, colistin performed well against Enterobacteriaceae isolates (MIC90 = 0.5-1 g/ml). Six (10.7 %) isolates of the genus Enterobacter, seven (6.8 %) isolates of the species K. pneumoniae, one (3.3 %) isolate of the genus Citrobacter and three (1.4 %) isolates of the species E. coli, were colistin-resistant. In resistant isolates, the polymerase chain reaction was used to check the presence of mcr-1 gene. In one E. coli isolate from a urine sample, the plasmid-coded resistance against colistin was detected.
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