Estudio de la resistencia a quinolonas y cefalosporinas en cepas del género salmonella aisladas de casos clínicos humanos

Resumen

Salmonella spp. is an important pathogen of humans and animals, causing enteric fever and gastroenteritis. About 1.3 billion cases of illness and 3 million deaths are reported annually due to non-typhoidal salmonellosis in humans. Usually salmonellosis is a self-limiting disease not requiring treatment, but in the case of elderly people and children or if the microorganism causes invasive illness, antimicrobial therapy may be needed. In a collection of Salmonella enterica isolated from human clinical cases, in Extremadura, the minimal inhibitory concentration to 23 antimicrobians was determined and their clonal relations were established by PFGE. The presence of class 1 integrons and gene cassettes, and the main mechanisms of resistance to quinolones and cephalosporins were investigated. The activity of ß-lactamases was also measured in the collection of isolates. Class 1 integrons were associated to multiresistance and 5 different combinations of gene cassettes were found codifying resistance to streptomycin and spectinomycin, ß-lactams and trimethoprim. Resistance to quinolones was associated to S. Enteritidis, being mutations in the QRDR of gyrA and parC the mains mechanisms of resistance found. No mutations were found in the QRDR of gyrB and parE. Mutations in gyrA were found at Asp87 and Ser83 and in parC only the mutation at Thr57 was found. Isolates with simultaneous mutations at gyrA and parC were also found. PMQR was not disseminated in the isolates since only one qnrS1 gene was found in one strain of S. Typhimurium DT104B. Conjugation experiments showed that the plasmid containing the qnrS1 gene was transferable by conjugation. Non-classical resistance to fluoroquinolones (resistance to ciprofloxacin and sensitivity to nalidixic acid) was detected among a group of isolates. The most frequent cephalosporinase detected among isolates of the Salmonella spp. collection was OXA, which expression strongly correlated to resistance to cefquinome, ceftiofur and cefotaxime. Beside this, the cephalosporinases TEM, CMY and SHV had a limited relevance on cephalosporin resistance since their presences were not widely distributed among isolates of the Salmonella collection. The highest activity was detected in isolates carrying blaCMY genes, an enzyme that was only slightly inhibited by clavulanate. The major beta-lactamase enzyme found, blaOXA, was expressed to lower levels than that of the second most frequently found enzyme, blaTEM, whilst blaSHV, was expressed to intermediate levels. The expression of beta-lactamase enzymes in Salmonella isolates conferred different phenotypes of resistance to cephalosporins, including antimicrobials like ceftiofur and cefquinome which are exclusively used in Veterinary. The gene blaCTX-M, nowadays considered as epidemic, was not found in the collection. The analysis of genotypes and phenotypes of the Salmonella enterica isolates expressing ß-lactamase enzymes might indicate that strains from humans could have been originated in animals.