ARS | Publication request: Analysis of Salmonella enterica with reduced susceptibility to the 3rd generation cephalosporin, ceftriaxone, isolated from US cattle during 2000-2004

Submitted to: Microbial Drug Resistance
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 14, 2008
Publication Date: December 20, 2008
Citation: Frye, J.G., Fedorka-Cray, P.J., Jackson, C.R., Rose, M. 2008. Analysis of salmonella enterica with reduced susceptibility to the third-generation cephalosporin ceftriaxone isolated from U.S. cattle during 2000-2004. Microbial Drug Resistance. 14(4):251-258.

Interpretive Summary: Salmonella is often the cause of a self-limiting gastroenteritis, however sometimes it may become a more serious infection and require antibiotic treatment. Cephalosporins are the antibiotics of choice when treatment is required. One way these bacteria can become resistant to cephlosporins is by acquiring genes that produce an extended spectrum beta-lactamase (ESBL) enzyme that can breakdown cephalosporin antibiotics thus causing treatment failure. To determine the level of resistance in US animals, Salmonella isolates were collected from cattle at slaughter from 2000-2004 (n = 3985) and screened for decreased susceptibility to cephalosporins and ESBL production. A total of 97 Salmonella isolates exhibited decreased susceptibility or resistance to ceftriaxone. Thirteen serotypes were identified including 58 S. Newport, 14 S. Agona, and 8 S. Typhimurium. None of the isolates were confirmed as ESBL producers. These findings will be applied by medical/veterinary doctors, researchers developing new antimicrobials and regulatory agencies controlling their use.

Technical Abstract: Over the past decade enteric bacteria in Europe, Africa and Asia have become increasingly resistant to cephalosporin antimicrobials. This is largely due to the spread of genes encoding extended-spectrum ß-lactamase (ESBL) enzymes which can inactivate many cephalosporins. Recently these resistance mechanisms have been identified in Salmonella isolated from humans in the US. As cephalosporins are the preferred treatment for serious Salmonella infections, monitoring animal isolates for ESBL production is prudent. During 2000-2004, Salmonella cattle slaughter isolates (n=3,984) were tested and 97 (2.4%) were found to have decreased susceptibility (MIC is great than or equal to 32 µg/ml) to the 3rd generation cephalosporin, ceftriaxone. The majority of these were serotypes Newport (58) and Agona (14) some of which were genetically indistinguishable by PFGE analysis. None of the isolates had an ESBL phenotype; all were susceptible to the 4th generation cephalosporins, cefepime (used in human medicine) and cefquinome (used in veterinary medicine in Europe). PCR and sequence analysis for resistance genes detected the blaCMY-2 gene in 68 isolates and the blaTEM-1 gene in 12 isolates; neither gene encodes an ESBL. These data indicate bovine Salmonella isolates from the US with decreased susceptibility or resistance to ceftriaxone do not exhibit an ESBL phenotype and most contain the blaCMY-2 gene.