A total of 133 Salmonella isolates recovered from retail meats purchased in the United States and the People's Republic of China were assayed for antimicrobial susceptibility, the presence of integrons and antimicrobial resistance genes, and horizontal transfer of characterized antimicrobial resistance determinants via conjugation. Seventy-three (82%) of these Salmonella isolates were resistant to at least one
antimicrobial agent. Resistance to the following
antibiotics was common among the United States isolates:
tetracycline (68% of the isolates were resistant),
streptomycin (61%),
sulfamethoxazole (42%), and
ampicillin (29%). Eight Salmonella isolates (6%) were resistant to
ceftriaxone. Fourteen isolates (11%) from the People's Republic of China were resistant to
nalidixic acid and displayed decreased susceptibility to
ciprofloxacin. A total of 19 different antimicrobial resistance genes were identified in 30 multidrug-resistant Salmonella isolates. The
bla(CMY-2) gene, encoding a class A
AmpC beta-lactamase, was detected in all 10 Salmonella isolates resistant to extended-spectrum
beta-lactams. Resistance to
ampicillin was most often associated with a TEM-1 family
beta-lactamase gene. Six
aminoglycoside resistance genes, aadA1, aadA2, aacC2, Kn, aph(3)-IIa, and aac(3)-IVa, were commonly present in the Salmonella isolates. Sixteen (54%) of 30 Salmonella isolates tested had integrons ranging in size from 0.75 to 2.7 kb. Conjugation studies demonstrated that there was plasmid-mediated transfer of genes encoding CMY-2 and TEM-1-like
beta-lactamases. These data indicate that Salmonella isolates recovered from retail raw meats are commonly resistant to multiple antimicrobials, including those used for treating
salmonellosis, such as
ceftriaxone. Genes conferring antimicrobial resistance in Salmonella are often carried on integrons and plasmids and could be transmitted through conjugation. These mobile
DNA elements have likely played an important role in transmission and dissemination of antimicrobial resistance determinants among Salmonella strains.