Title: Analysis of Antimicrobial Resistance Genes Detected in MDR Salmonella enterica Serovar Typhimurium animal isolates from the National Antimicrobial Resistance Monitoring System Authors
Submitted to: American Society for Microbiology Conference
Publication Type: Abstract Only
Publication Acceptance Date: September 21, 2009
Publication Date: October 2, 2009
Citation: Frye, J.G., Glenn, L.M., Jackson, C.R., Cray, P.J. 2009. Analysis of Antimicrobial Resistance Genes Detected in MDR Salmonella enterica Serovar Typhimurium animal isolates from the National Antimicrobial Resistance Monitoring System. American Society for Microbiology Conference. 155A:84-85. Technical Abstract: Background: The presence of Multi-Drug Resistant (MDR) Salmonella in food animals is concerning. To understand how antimicrobial resistance (AR) develops, the genetic elements responsible for MDR phenotypes in Salmonella animal isolates were investigated. National Antimicrobial Resistance Monitoring System (NARMS) animal isolates collected from 1997-2007 were assayed for antimicrobial resistant phenotypes, and a subset of MDR isolates were further analyzed by DNA microarray. Methods: Salmonella enterica isolated from healthy animals (cattle, chicken and swine) collected on farm or at slaughter were tested for antimicrobial susceptibility using a semi-automated broth micro-dilution custom panel of antimicrobials (SensititerTM, TREK Diagnostic Systems, Inc., Westlake, OH, USA). AR gene content was determined by analysis with a DNA microarray containing probes for 775 resistance genes. DNA extractions, labeling, hybridization, image and data analysis were done following standard microarray techniques (Corning Inc., Acton, MA, USA). Results: Antimicrobial susceptibilities were determined for 54,395 Salmonella animal isolates collected by NARMS from 1997-2007. Isolates resistant is greater than or equal to 5 classes of antimicrobials averaged 14.4% (n=7,828) over the 11 year period of the study and varied from 3.3% (n=79) in 1997 to 21.1% (n=1020) in 2004. Throughout the time period Salmonella enterica serovar Typhimurium was the most prevalent MDR serotype, therefore a MDR S. Typhimurium isolate was chosen from each animal source (cattle, chicken and swine) for each year of the study (n=33). Microarray analysis detected multiple resistance genes in all of the isolates. An average of 30 resistance genes was detected in the isolates. Predominant genes detected encoded resistance to aminoglycosides (aadA, aadB, and aph(3)-I), beta-lactams (blaAMPC, blaCMY-2, and blaTEM), chloramphenicol (flo and chl), sulfanilamide (sulI), tetracycline (tetA, tetB, and tetR) and mobile elements (intI, tnpA, and tnpR). The genes identified also correlated well with the isolates’ phenotypic susceptibility. Interestingly, over the 11 year study the number of AR genes detected in MDR isolates increased in total while certain core genes remained the same. Conclusions: These data demonstrate that AR genes found in MDR isolates of S. Typhimurium have increased during the study period. This may indicate that the genetic elements responsible for MDR have been accumulating in S. Typhimurium during 1997-2007.