Abstract
BackgroundColistin is regarded as a last-resort antimicrobial against multi-drug resistant Gram-negative bacteria (GNB), therefore the dissemination of colistin resistance in the environment is of great concern. Horizontal transfer of mobile colistin resistance (mcr) genes to potential pathogens poses a serious problem. This study aimed to describe the presence of colistin resistant GNB and mcr genes in river and storm water in regions of the Western Cape.MethodsWater samples were collected from three rivers during May 2019 and January 2020 and two storm water samples were collected in November 2019. Colistin resistant GNB were cultured on MacConkey agar containing colistin and identified by MALDI-TOF. Colistin resistance was confirmed using broth microdilution (BMD). mcr-1-5 genes were detected by PCR performed directly on the water samples and on the colistin resistant isolates. mcr functionality was assessed by BMD after cloning the mcr genes into pET-48b(+) and expression in SHuffle T7 E. coli.Resultsmcr-5.1 and various mcr-3 gene variants were detected in the Plankenburg-, Eerste- and Berg rivers and in storm water from Muizenberg, and only mcr-5.1 was detected in storm water from Fish Hoek. Colistin resistant GNB were isolated from all of the water sources. Aeromonas spp. were the most common colistin resistant organisms detected in the water sources; 25% (6/24) of colistin resistant Aeromonas spp. isolated from the Berg river contained novel mcr-3 variants; mcr-3.33 (n = 1), mcr-3.34 (n = 1) mcr-3.35 (n = 1) mcr-3.36 (n = 2) and mcr-3.37 (n = 1), which were confirmed to confer colistin resistance.ConclusionsThe mcr-5.1 and mcr-3 colistin resistance gene variants were present in widely dispersed water sources in regions of the Western Cape. The mcr genes were only detected in water sampled downstream of and alongside communities, suggesting that their presence is driven by human influence/contamination. This is the first documentation of mcr-3 and mcr-5 gene variants in any setting in South Africa. Spill-over of these genes to communities could result in horizontal gene transfer to pathogenic bacteria, exacerbating the challenge of controlling multidrug resistant GNB infections.
Highlights
The emergence and spread of mobile colistin resistance genes threaten the efficacy of colistin, a last resort antibiotic for treating infections caused by Snyman et al Antimicrob Resist Infect Control (2021) 10:96 multidrug-resistant Gram-negative bacteria (GNB)
This study investigated the presence of colistin resistant GNB and mcr genes in samples from three surface water sources, the Plankenburg, Eerste- and Berg rivers, as well as storm water from Muizenberg and Fish Hoek in the Western Cape province, South Africa, and characterised the role of the mcr variants in colistin resistance
The river and storm water samples were heavily contaminated with bacteria, as they contained an abundance of E. coli and coliforms (Additional file 1: Table 2)
Summary
The emergence and spread of mobile colistin resistance (mcr) genes threaten the efficacy of colistin, a last resort antibiotic for treating infections caused by Snyman et al Antimicrob Resist Infect Control (2021) 10:96 multidrug-resistant Gram-negative bacteria (GNB). Mcr-1 (30 variants), mcr-2 (7 variants), mcr-3 (40 variants), mcr-4 (6 variants), mcr-5 (4 variants), mcr-6, mcr-7, mcr-8 (3 variants), mcr-9 (3 variants) and mcr-10 [1,2,3,4,5,6,7,8,9,10], have been described in various GNB, including Acinetobacter spp., Aeromonas spp., Citrobacter spp., Enterobacter spp., Escherichia coli, Klebsiella spp., Kluyvera spp., Proteus spp., Providencia spp., Pseudomonas spp., Raoultella spp., and Salmonella spp., isolates [11] Some of these mcr variants do not cause colistin resistance [12, 13].
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