Abstract
Salmonellosis is one of the most important zoonoses in Europe and the world. Human infection may evolve in severe clinical diseases, with the need for hospitalization and antimicrobial treatment. Colistin is now considered an important antimicrobial to treat infections from multidrug- resistant Gram-negative bacteria, but the spreading of mobile colistin-resistance (mcr) genes has limited this option. We aimed to evaluate colistin minimum inhibitory concentration and the presence of mcr (mcr-1 to mcr-9) genes in 236 Salmonella isolates previously collected from different animals and the environment between 2000 and 2020. Overall, 17.79% of isolates were resistant to colistin; no differences were observed in relation to years of isolation (2000–2005, 2009–2014, and 2015–2020), Salmonella enterica subspecies (enterica, salamae, diarizonae, and houtenae), origin of samples (domestic animals, wildlife, and environment), or animal category (birds, mammals, and reptiles); only recently isolated strains from houseflies showed the most resistance. Few isolates (5.93%) scored positive for mcr genes, in particular for mcr-1, mcr-2, mcr-4, mcr-6, and mcr-8; furthermore, only 2.54% of isolates were mcr-positive and colistin-resistant. Detected resistance to colistin was equally distributed among all examined Salmonella isolates and not always related to the presence of mcr genes.
Highlights
Academic Editors: Ilias KaraiskosColistin is an old antibiotic, discovered in 1947, and belonging to the class of polymyxins
Considering the years of isolation of the investigated strains and the first report of mcr genes in 2015, to evaluate the distribution over time of colistin resistance, isolates were arranged in three groups: salmonellae collected between 2000 and 2005, between 2009 and 2014, and between 2015 and 2020
The discovery of mobile genes conferring colistin resistance caused many concerns. These genes were more commonly observed among E. coli strains, but they were detected in other Gram-negative bacteria, such as Salmonella [28]
Summary
Colistin is an old antibiotic, discovered in 1947, and belonging to the class of polymyxins. The target of this antimicrobial is the outer membrane lipopolysaccharides (LPS) of. Colistin use has been largely abandoned since the mid-1970s as a consequence of some adverse events, primarily nephrotoxicity and neurotoxicity, combined with the discovery of new antibiotics [2]. Colistin was largely used in veterinary medicine worldwide to treat infections, mainly from Enterobacteriaceae, in terrestrial and aquatic animals. It was used for decades as a growth promoter in farm animals in many countries; this practice is still allowed in some territories [3]. The increase in antimicrobial resistance led, by the mid-1990s, to the rediscovery of colistin for human therapy as a last-resort antibiotic in multidrug-resistant
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