Genetic Basis of Antimicrobial Drug Resistance in Clinical Isolates of Salmonella enterica Serotype Hadar from a Spanish Region

Abstract

The genetic bases of antimicrobial drug resistance (R) of 79 Salmonella enterica serotype Hadar clinical isolates (recovered during 1995–2001 in a Spanish region) was investigated. The isolates showed a limited genomic variation, as demonstrated by PFGE analysis using XbaI (three profiles, S ≥ 0.77) and BlnI (seven profiles, S ≥ 0.49; with 95% of the isolates falling into two clusters, S ≥ 0.75). Thirteen R-profiles, ranging from susceptible to multidrug resistant, were recognized. All susceptible isolates (14%) were recovered before or during 1998, when multidrug resistance (MDR) was still uncommon (20% from 1995–1998). In later years, the percentage of MDR increased considerably (92% in 2001). Resistance to nalidixic acid, tetracycline, streptomycin and ampicillin-cefalotin, encoded by gyrA–Asp87/Asn, tet(A), strA/B, and bla _TEM genes, respectively, were the most common, appearing together in 38% of the isolates. In all tetracycline- and streptomycin-resistant isolates, strA/B and tet(A) were chromosomally located, whereas bla _TEM was plasmid-born. Five different bla _TEM plasmids (pUO-ShR1 to pUO-ShR5, of about 9.4, 23, 30, 45, and 95 kb, respectively) were identified. pUO-ShR3 and pUO-ShR5 harbored additional R-genes: [dfrA1] and [acc(3)IV-strA/B], respectively. pUO-Sh2, pUO-Sh3, pUO-ShR4, and pUO-Sh5 were self-transferable, and the latter could also mobilize pUOShR1. The reported data constitute a useful background for further epidemiological studies of MDR in S. Hadar.