Salmonella enterica Serovar Enteritidis, Japan

To the Editor: Nontyphoidal sal-monellae are the important causative agents of foodborne diseases in Japan and other industrialized countries. Salmonella enterica serovar Enter-itidis has risen to the leading cause of infection among Salmonella spp. since 1989 (1). Emergence of drug-resistant S. Enteritidis has been rarely reported while S. Typhimurium, another serovar of major public health concerns, has been reported to acquire multidrug resistance such as DT104 resistance to ampicillin, chloram-phenicol, streptomycin, sulfonamide, and tetracycline (R-ACSSuT) (2). We previously reported outbreaks caused by strains resistant to ampi-cillin and streptomycin (resistance type R-AS, herein); the strains' reactions against the phages used in bacte-riophage typing did not conform to any known reaction patterns (phage type [PT] RDNC-a, herein, with the following reactions: (for or #3, 5–7, 11–13, 15, and 16 phages; (+++) for #2 phage; opaque lysis [OL] for #4 and 9 phages; <OL for #10 phage; and ambiguous reactions (-/+++) were observed for #1, 8, and 14 phages) (3). To investigate the characters of the R-AS strains more extensively, we surveyed isolates from outbreaks that occurred from 1997 to 2002 for antimicrobial drug susceptibility and bacteriophage typing. S. Enteritidis strains from 899 outbreaks that occurred from 1997 to 2002 were tested. Bacteriophage typing was done according to the Public (4). Antimicrobial drug susceptibility testing was done with a disc diffusion method on Mueller-Hinton II agar (Becton Dickinson Microbiology Systems, Cockeysville, MD) as previously described (5). Antimicrobial drugs used in this study were ampi-Dominant phage types were PT4 (36.9%) and PT1 (26.9%). They have been dominant among outbreak-related strains since 1992 (1). Other types were also identified at certain frequencies. For example, RDNC-a, PT47, PT6, PT6a, and PT21 accounted for 4.4%, 5.3%, 4.0%, 3.2%, and 2.0% of the phage types, respectively. Strains sensitive to all the antimi-crobial drugs tested were the most predominant (55.1%), followed by those resistant to only streptomycin (34.8%). R-AS was the third most predominant, accounting for 4.1%. A correlation existed between drug resistance and phage types in that all the R-AS strains (n = 37) showed RDNC-a in bacteriophage typing, and all the RDNC-a strains (n = 40) were resistant to at least ampicillin including two R-A and one R-AST strains. Since previous studies described the correlation between drug resistance and phage types as a result of acquisition of an R-plasmid (6), we focused on the relationship between RDNC-a and ampicillin resistance. Plasmid profiles analysis of the RDNC-a strains …