Less frequent Salmonella serovars as a reservoir of antimicrobial resistance

Abstract

Sir, Salmonella enterica serovars Enteritidis and Typhimurium are the most frequent Salmonella serovars isolated from humans. However, a significant proportion of human Salmonella isolates belong to other serovars, for which antimicrobial susceptibility patterns are rarely reported. In Denmark, the frequency of isolation of these less frequent Salmonella serovars from human clinical samples has increased in recent years. They represented 30% of all human Salmonella isolates, which corresponded to an incidence of 10.5 per 100 000 inhabitants in 2005. From 2003 to 2005, 530 isolates belonging to these Salmonella serovars were tested for their antimicrobial susceptibility by broth microdilution (Sensititre; Trek Diagnostic Systems Ltd, East Grinstead, UK) at the Statens Serum Institut. These included 106 Salmonella Virchow, 95 Salmonella Newport, 91 Salmonella Stanley, 80 Salmonella Dublin, 53 Salmonella Hadar, 38 Salmonella Saintpaul, 32 Salmonella Derby, 20 Salmonella Uganda and 15 Salmonella Anatum obtained from sporadic clinical infections, isolated at the 16 clinical microbiology laboratories in Denmark. The MICs for these 530 human clinical Salmonella isolates were determined for ampicillin (1–32 mg/L), cefalotin (4–32 mg/L), chloramphenicol (2–64 mg/L), ciprofloxacin (0.03–4 mg/L), gentamicin (1–32 mg/L), nalidixic acid (8–64 mg/L), sulfamethoxazole (64–1024 mg/L), streptomycin (4–64 mg/L), tetracycline (2–32 mg/L) and trimethoprim (4–32 mg/L). The CLSI (formerly NCCLS) breakpoints were used. For ciprofloxacin, a breakpoint of 0.125 mg/L was also used, and for streptomycin, a breakpoint of 32 mg/L was used. Escherichia coli ATCC 25922 was used for quality control. The number of tested isolates and the percentages of resistance are reported in Table 1. Isolates belonging to some serovars, e.g. Salmonella Dublin, Salmonella Saintpaul and Salmonella Derby, were susceptible to most antimicrobials tested, whereas isolates belonging to other serovars showed much higher frequencies of resistance, e.g. Salmonella Virchow, Salmonella Newport and Salmonella Stanley. A high prevalence of multidrug resistance (resistance to four or more antimicrobial agents) was observed among isolates belonging to the following serovars: Salmonella Hadar (30%), Salmonella Virchow (25%), Salmonella Stanley (22%) and Salmonella Newport (19%). Resistance to ampicillin, chloramphenicol, streptomycin, sulfamethoxazole and tetracycline was present in Salmonella Newport (15%), Salmonella Stanley (8%) and Salmonella Virchow (1%). Resistance to these five antimicrobials could indicate the presence of the multidrug-resistant region Salmonella genomic island 1 (SGI1). This region is well described and common in Salmonella Typhimurium phage type DT104. It has also been found in other Salmonella serovars, e.g. Salmonella Newport. Of the tested Salmonella Hadar and Salmonella Virchow isolates, 87% and 80%, respectively, were resistant to nalidixic acid. However, these isolates were not resistant to ciprofloxacin when the CLSI breakpoint (4 mg/L) was used. When using the EUCAST breakpoint for ciprofloxacin (0.125 mg/L), a good consistency was observed between nalidixic acid and ciprofloxacin resistance frequencies, even though minor differences were