Monitoring Colistin Resistance in Food Animals, An Urgent Threat

Polymyxins, which were discovered in the 1950s, are antimicrobial agents with significant activity against Gram-negative bacteria. They act as a bactericidal agent by targeting the polyanionic lipid A of lipopolysaccharides in the outer membrane of Gram-negative bacteria [1]. Polymyxins were gradually replaced by relatively less toxic antibiotics in the 1970s [2]. However, over the last decade, the rapid rise of multidrug-resistant (MDR) and extensively drug-resistant Gram-negative bacteria, especially carbapenem-resistant Gram-negative bacteria, has resulted in the reintroduction of polymyxins. In particular, polymyxin E (colistin) is being used as a lastresort antibiotic for the treatment of severe infections caused by MDRGram-negative pathogens such as Klebsiella pneumoniae and Acinetobacter baumannii in humans [1,3]. Therefore, colistin is currently playing a key role in human health, and its resistance has gained extensive attention worldwide. Many studies determined the susceptibility of Gram-negative bacteria to colistin. However, data on colistin susceptibility should be interpreted with caution because of potential selective testing aimed atMDRGram-negative bacteria andmethodological issues in determination of minimal inhibitory concentrations, like the diffusional limitations of polymyxins in agar, the naturally cationic polymyxins, the heteroresistance to polymyxins, and the method lacking a reliable reference [1]. Because of their relatively recent reintroduction to clinical practices, the overall prevalence of colistin resistance in human clinical isolates remains fairly low (Table 1) [1,4]. However, there has been a remarkable increase in colistin-resistant strains in recent years because of the rapidly increasing use of colistin in hospitals, which has led to increased selection pressure for resistance [5]. It should be noted that different Gram-negative bacteria vary greatly in their sensitivity to colistin. The annual report of the European Antimicrobial Resistance Surveillance Network reported that colistin resistance in K. pneumoniae attained a high level (> 20%) among MDR K. pneumoniae isolates in Greece and Italy [6]. Unlike its usage in human medicine, colistin has been used for decades in veterinary medicine as a drug for treatment and prevention of infectious diseases or as a feed additive to promote growth in food-producing animals [1]. The consumption of colistin in animals has been increasing since the mid-twentieth century and has been particularly high in lowand middle-income countries. It increased by 13% in Brazil, Russia, India, China, and South Africa from 2000 to 2010 [7]. Annual use of colistin in food-producing animals in China ranged from 2470 to 2875 metric tons from 2011 to 2015, which made China the largest user of polymyxins in the world [8]. In the European Union, although colistin is available for the treatment and prevention of infectious diseases, it was banned as a growth promoter in food animals in 2006 [9]. The European Surveillance of Veterinary Antimicrobial Consumption network reported that polymyxins were the fifth most widely consumed antimicrobials during 2013–2015, whereas no polymyxin was sold in Finland, Iceland, and Norway [10,11]. In several developed countries, such as Canada and the United States, colistin has never been approved for use in animals [10]. Despite the extensive use of colistin in food animals for over the last 50 years, data on colistin resistance in veterinary medicine are sparse compared with its data in human medicine. Based on the limited available resistance data, the occurrence of colistin resistance among isolates from food animals (0.9–76.9%) is more frequent than isolates from humans (0.1–8.8%) (Table 1) [12–16]. In North America, data on colistin resistance from animals are scarce because colistin has not been widely available in livestock in these regions [10]. The generally higher rates of colistin resistance among isolates from food animals is in accordance with the higher selection pressure posed by the heavy use of colistin in veterinary medicine. Data from 26 European countries indicated statistically significant positive associations between polymyxin consumption in food-producing animals and colistin resistance in indicator E. coli from food-producing animals [16]. Before 2015, all reported polymyxin resistance mechanisms were chromosomally mediated and involved with modulation of a two-component regulatory system (e.g. pmrAB, phoPQ, and the negative regulator mgrB) leading to modification of lipid A with moieties, such as phosphoethanolamine, 4-amino-4-arabinose, or, in rare instances, the total loss of lipopolysaccharides [2,13]. However, in 2015, we reported the first case of plasmid-mediated colistin resistance mechanism [13]. In November 2013, during an annual resistance surveillance for E. coli in farm animals, we found that the prevalence of colistin resistance among E. coli isolates from a pig farm located in Shanghai was surprisingly high (over 55%) [17]. We suspected that such high prevalence of colistin resistance might indicate the existence of transferable colistin resistance mechanism. To test this hypothesis, we selected one of the colistin resistant E. coli strain SHP45 for conjugation experiment, and then the colistin resistance was successfully transferred to the