Comparative characterization of nontyphoidal Salmonella isolated from humans and food animals in China, 2003-2011.

Food animals are major reservoirs from which specific pathogenic Salmonella strains emerge periodically. Probing the identity and origin of such organisms is essential for formulation of highly-focused infection control measures and analysis of factors underlying dissemination of such strains. In this work, the genetic and phenotypic features of animal and human clinical isolates collected at different geographical localities in China during the period 2003-2011 were characterized and compared. Animal-specific serotypes were identified, with S. Enteritidis, S. Cremieu and S. Fyris being recovered almost exclusively from chicken, ducks and pigs respectively. Nevertheless, only four serotypes were commonly found to be transmitted among both animal and human clinical isolates: S. Enteritidis, S. Typhimurium, S. Derby and S. Indiana. Strains of the serotypes S. Enteritidis and S. Typhimurium not only accounted for up to 50% of all human clinical isolates tested, but often shared identical genetic profiles with the animal isolates. Using a recently identified mobile efflux gene, oqxAB, as genetic marker for assessing the efficiency of transmission between animal and human isolates, we demonstrated that a newly emerged genetic trait could be simultaneously detectable among both animal and human clinical isolates. Findings in this work show that transmission of Salmonellae between animal and human is highly efficient and serotype dependent.

Prevalence and Molecular Characterization of mcr-1-Positive Salmonella Strains Recovered from Clinical Specimens in China.

The recently discovered colistin resistance element, mcr-1, adds to the list of antimicrobial resistance genes that rapidly erode the antimicrobial efficacy of not only the commonly used antibiotics but also the last-line agents of carbapenems and colistin. This study investigated the prevalence of the mobile colistin resistance determinant mcr-1 in Salmonella strains recovered from clinical settings in China and the transmission potential of mcr-1-bearing mobile elements harbored by such isolates. The mcr-1 gene was recoverable in 1.4% of clinical isolates tested, with the majority of them belonging to Salmonella enterica serotype Typhimurium. These isolates exhibited diverse pulsed-field gel electrophoresis (PFGE) profiles and high resistance to antibiotics other than colistin and particularly to cephalosporins. Plasmid analysis showed that mcr-1 was carried on a variety of plasmids with sizes ranging from ∼30 to ∼250 kb, among which there were conjugative plasmids of ∼30 kb, ∼60 kb, and ∼250 kb and nonconjugative plasmids of ∼140 kb, ∼180 kb, and ∼240 kb. Sequencing of representative mcr-1-carrying plasmids revealed that all conjugative plasmids belonged to the IncX4, IncI2, and IncHI2 types and were highly similar to the corresponding types of plasmids reported previously. Nonconjugative plasmids all belonged to the IncHI2 type, and the nontransferability of these plasmids was attributed to the loss of a region carrying partial or complete tra genes. Our data revealed that, similar to the situation in Escherichia coli, mcr-1 transmission in Salmonella was accelerated by various plasmids, suggesting that transmission of mcr-1-carrying plasmids between different species of Enterobacteriaceae may be a common event.