Submitted to: Microbial Drug Resistance
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/10/2010
Publication Date: 9/1/2010
Citation: Lugo-Melchor, Y., Quinones, B., Amezquita-Lopez, B.A., Leon-Felix, J., Garcia-Estrada, R., Chaidez, C. 2010. Characterization of tetracycline resistance in Salmonella enterica strains recovered from irrigation water in the Culiacan Valley, Mexico. Microbial Drug Resistance. 6(3):185-190. Interpretive Summary: Tetracyclines are broad-spectrum agents, exhibiting activity against a wide range of gram-positive and gram-negative bacteria. These antibiotics are currently used for therapy and prophylactics for human infections, for the prevention and control of bacterial infections in veterinary medicine and as animal growth promoters. Tetracyclines are also used in aquaculture to control infections and sprayed onto fruit trees and other plants to treat infection by phytopathogenic bacteria. As a consequence of the widespread use of tetracyclines, the emergence and spread of tetracycline-resistant bacterial pathogens, among them the foodborne pathogen Salmonella enterica, has become a serious health hazard worldwide. Salmonella spp. is one of the most important pathogens responsible for gastrointestinal infections in humans. The vast majority of cases of salmonellosis are associated with consumption of contaminated poultry, meat, eggs, and fresh produce. The increase of Salmonella strains showing resistance against antibiotics has resulted in limiting the effective treatment of human infections. Different tet genes have been described to be responsible for conferring resistance to tetracyclines in Salmonella. The most frequent types of tet genes belong to classes A, B, C, D and G. The genes tet(B), tet(C) and tet(D) were detected on the chromosomes of S. enterica bacteria of different serotypes, including Typhimurium, Enteritidis, Hadar, Saintpaul and Cholereaesuis. The tet(G) gene has been identified in Salmonella genomic island 1 located within the S. Typhimurium DT104 chromosome. The tet gene of class A is one of the most widespread among gram-negative bacteria and has been found on plasmids as well as on the chromosome. As previously shown in Salmonella strains from various sources, the tet(A) gene is often part of a small non-conjugative transposon called Tn1721 and in a truncated variant of this transposon. Given that there has been an increased incidence of resistance to tetracyclines in Salmonella spp. of human and animal origins worldwide, additional information on the mechanisms responsible for this antibiotic resistance is still needed. Recent studies resulted in the isolation of several Salmonella strains with different serotypes, showing antimicrobial resistance. These Salmonella strains were recovered from irrigation water in the Culiacan Valley, an important agricultural region in Mexico for horticultural crops that are exported to the United States. The initial characterization studies demonstrated that the S. enterica strains with the serovar Typhimurium displayed some resistance to tetracycline. To dissect the molecular mechanisms that are conferring the resistance to tetracycline, the present study identified the gene tet(A) in the transposon Tn1721 on the chromosomes and plasmids in strains of S. enterica serovar Typhimurium, isolated in Mexico’s Culiacan Valley during the years 2004 and 2006.
Technical Abstract: Salmonella enterica is one of the most important pathogens responsible for gastrointestinal infections in humans. The increase of S. enterica strains showing resistance against antibiotics has resulted in limiting the effective treatment of human infections. The present study characterized the resistance to tetracycline in S. enterica serovar Typhimurium strains, recovered from irrigation water in distinct regions in the Culiacan Valley, an important agricultural region in Mexico for horticultural crops that are exported to the United States. Analysis of the genomic diversity by PFGE typing showed that the S. Typhimurium strains were grouped into distinct genotypic clusters, indicating genomic diversity among the strains examined. The PCR and DNA sequencing analysis demonstrated that the tet(A) gene was found on the chromosome as well as plasmid and was located within a truncated version of transposon Tn1721. The comparative analysis of the tet(A) gene sequence in S. Typhimurium strains identified high sequence similarity to the tet determinant of plasmid RP1, which is homologous to the tet gene in Tn1721. These findings from our study showed that the tet(A) was prevalent among the tetracycline resistant S. Typhimurium strains, isolated from irrigation water used for growing fresh fruits and vegetables.