Submitted to: Applied and Environmental Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/12/2009
Publication Date: 4/1/2009
Citation: Lindsey, R.L., Cray, P.J., Frye, J.G., Meinersmann, R.J. 2009. Inc A/C Plasmids are Prevalent in Multidrug-Resistant Salmonella enterica Isolates. Applied and Environmental Microbiology. 75(7):1908-1915. Interpretive Summary: Salmonella are food borne bacterial pathogens that can result in serious illness in humans and animals. Salmonella may also be acquired by the oral ingestion of contaminated water. Approximately 600 people die each year in the US after being infected. Salmonella carries assorted plasmids which are pieces of deoxyribonucleic acid (DNA) that can be easily transferred from one bacterial strain to another. There is a lot of interest in characterizing and tracking plasmids because they may carry genes or sequences which benefit survival of the bacterium. Some of these genes may confer resistance to antimicrobials. While treatment of salmonellosis is not typically recommended, in cases where it is necessary, resistance to antimicrobials, and in particular multiple antimicrobials, may compromise outcome. Replicon typing is a method to characterize 18 of the 26 known types of plasmids in Salmonella. Plasmid replicon type Inc A/C is of interest because it is often associated with multidrug resistance. This study used antimicrobial resistance profiles, plasmid replicon typing and pulse-field gel electrophoresis to characterize 437 Salmonella enterica isolates collected from ill animals in 2005 in which Salmonella infection was associated with the illness. Data suggests that Salmonella harbored the Inc A/C plasmids and multiple drug resistant genes for a long period of time, probably starting before antimicrobials were discovered. Conversely, plasmids like Inc I1 are much more frequently shared between different types of bacteria. Therefore, most “new” antimicrobial resistance genes are likely only new in the sense that they were not previously found, probably because of their exceedingly low occurrence before pressure was placed by use of antimicrobials. These data are useful for physicians, veterinarians, scientists and epidemiologists as they develop models to better understand the evolution of antimicrobial resistance.
Technical Abstract: Bacteria plasmids are fragments of extra-chromosomal double stranded deoxyribonucleic acid (DNA) that can contain a variety of genes beneficial to the host organism like antibiotic resistance. The objective of this study was to characterize a collection of 437 Salmonella enterica isolates from different animal sources for their antimicrobial resistance phenotype, replicon type and in some cases pulsed-field gel electrophoresis (PFGE) in an effort to learn more about the distribution of MDR in relation to replicon types. A PCR-based replicon typing assay consisting of three multiplex PCR reactions was used to detect 18 of the 26 known plasmid types in Enterobacteriaceae based on their Incompatibility (Inc) groups. Linkage analysis was completed with antibiograms, replicon types, serovars and Inc A/C groups. Inc A/C plasmids were prevalent in multidrug resistant (MDR) isolates with the notable exception of Salmonella enterica serovar Typhimurium. Cluster analysis based on PFGE of a subset of 216 isolates show 155 unique types suggesting a variable population, but distinct clusters of isolates with Inc A/C plasmids were apparent. Significant linkage of serovar was also seen with Inc groups B/O, I1, Frep and HI1. The present study showed the combination of Salmonella, the Inc A/C plasmids and multiple drug resistant genes is very old. Our results suggest that some strains notably serovar Typhimurium and closely related types, used to carry the plasmid and had a transfer of resistance genes to the chromosome with subsequent loss of the plasmid.