Chromosomal transferable multidrug resistance genes of Salmonella enterica serovar infantis.

*Corresponding author: Mailing address: Infectious Disease Reserach Division, Hyogo Prefectural Institute of Public Health and Environmental Sciences, Arata-cho 2-1-29, Hyogo-ku, Kobe 652-0032, Japan. Fax: +81-78-531-7080 In Japan, Salmonella enterica serovar Infantis, together with S. enterica serovar Typhimurium, has been one of the major causes of Salmonella infections during the past decades, though in 1989 S. enterica serovar Enteritidis suddenly emerged and its epidemic continues (1-3). While rare among S. Enteritidis (3), multidrug resistance (MDR) was frequent among S. Infantis and S. Typhimurium (1,2). MDR genes were transferable along with or without a class 1 integron among them (1,2) and also among enterohemorrhagic Escherichia coli (2). The present communication shows the chromosomal localization of MDR genes in S. Infantis. Among 32 S. Infantis isolates (Inf01 Inf32) from 19922003 in Hyogo Prefecture, 23 isolates were implicated in three food poisoning outbreaks and 15 sporadic infections and were of human sources (see the legend to Fig. 1). The remaining nine isolates were from other than humans, one from causative food in an outbreak (Inf25), six from chickens/eggs (Inf03, Inf04, Inf05, Inf07, Inf22, and Inf31), and two from the environment (Inf06 and Inf30). They had no mutual epidemiological relations except between Inf08 and Inf10 from an epidemic in 1996, between Inf15 and Inf17 in an epidemic in 1999, and between Inf23 and Inf25 in an epidemic in 2001. All the isolates were tested for drug sensitivities by the disk method (Becton Dickinson Microbiology Systems, Cockeysville, Md., USA) (1,2) and by the dilution method. For the dilution method, lactose (0.5%) MacConkey agar (Lac-MAC) plates were used for testing sensitivities to kanamycin (Km, 25 μg/ml), streptomycin (Sm, 12.5, 25, and 50 μg/ml), and tetracycline (Tc, 25 μg/ml), and MuellerHinton (MH) plates for testing sensitivities to trimethoprim (Tm, 25 μg/ml) and sulfamethoxazole (Su, 125 μg/ml). Twelve isolates had MDR that could be classified into three groups. Eight isolates (Inf07, Inf13, Inf14, Inf18, Inf27, Inf28, Inf29, and Inf32) were resistant to Km, Sm, Tc, Tm, and Su,