Molecular Characterization of Extended-Spectrum Cephalosporinase-Producing Salmonella enterica Serovar Choleraesuis Isolates from Patients in Thailand and Denmark

Salmonella enterica is a common cause of human gastroenteritis and bacteremia worldwide (18, 31), and a wide variety of animals, particularly food animals, have been identified as reservoirs for non-Typhi Salmonella (11, 22, 23). Although approximately 2,600 serovars of Salmonella enterica have been identified, most human infections are caused by a limited number of serovars, and in general these infections are self-limiting. Some Salmonella serovars, including Salmonella Choleraesuis (swine) and Salmonella Dublin (cattle), which are adapted to a specific animal host, have a propensity to cause extraintestinal infections in humans. Compared to those with other serovars of non-Typhi Salmonella, infections with these serovars are associated with higher rates of bacteremia, meningitis, and mortality (4, 5, 24). For patients with severe salmonellosis, antimicrobial chemotherapy may be life-saving. Due to the increasing prevalence of fluoroquinolone resistance, extended-spectrum cephalosporins are increasingly used for the treatment of Salmonella infections in humans (17, 21, 25) and especially for children, for whom treatment of highly resistant Salmonella infections with fluoroquinolones is not advised, since such treatment has been associated with treatment failures (12, 13, 21). Therefore, these compounds have been designated critically important for human health by the World Health Organization (10). We recently reported that the prevalence of human infections with Salmonella serovar Choleraesuis in Thailand increased from 1.5% (n = 87) in 1994 to 9.2% (n = 190) in 2006 (19). The group of people at highest risk for these infections was those between 6 and 40 years of age in the central region of Thailand (19). A 2007 study of Salmonella serovar Choleraesuis isolates from Thailand observed an increasing resistance to both extended-spectrum cephalosporins (ceftriaxone) and fluoroquinolones. Fifty-four isolates obtained between 2003 and 2005 were tested, of which 30% were found to be resistant to an extended-spectrum cephalosporin (ceftriaxone) (25). To date, only two reports, both from Taiwan, have described mechanisms for extended-spectrum cephalosporin resistance in Salmonella serovar Choleraesuis. The first report was published in 2004 with the discovery of the blaCMY-2 AmpC β-lactamase gene located on a 140-kb F-like plasmid (6). The following year, the same authors detected blaCTX-M-3 in a Salmonella serovar Choleraesuis isolate from a patient admitted to a university hospital (30). In 2007, a massive increase of fluoroquinolone- and ceftriaxone-resistant Salmonella serovar Choleraesuis isolates was described in Thailand (25). In Taiwan, the usage of antimicrobials in veterinary medicine and as growth promoters in animal feed may have promoted the emergence of resistance (5). Likewise, in Thailand, the extended-spectrum cephalosporin ceftiofur is used as a growth promoter in swine production (25). However, data on antimicrobial usage in disease prevention and as growth promoters are not accessible in both countries. The objective of the present study was to characterize the mechanisms responsible for the emergence of resistance to extended-spectrum cephalosporins in isolates of Salmonella serovar Choleraesuis recovered from patients in Thailand and Denmark. Additional objectives were to determine the clonality of the isolates resistant and susceptible to cephalosporins (ceftriaxone and cefoxitin) using pulsed-field gel electrophoresis (PFGE) and antimicrobial susceptibility testing and to find biological evidence of transmission through international travel.