Characterization of a multidrug-resistant isolate of Salmonella Paratyphi B from Japan

Sir, Enteric fever, comprising both typhoid and paratyphoid fevers, continues to be a global health problem with an estimated 12–33 million cases occurring worldwide each year.1 Paratyphoid fever is caused by Salmonella Paratyphi A, B and C and multidrug-resistance (MDR), defined as resistance to the three first-line agents chloramphenicol, ampicillin and co-trimoxazole, is an emerging problem in organisms of type B.2–4 The molecular basis for the MDR phenotype of the newly emerged S. Paratyphi B was found to be due to the presence of the Salmonella genomic island 1 (SGI1) of Salmonella Typhimurium DT 104.3,4 During the last two decades, a clone of multidrug-resistant S. Typhimurium phage type DT 104 has spread as one of the most common causes of human salmonellosis in several countries.5 The majority of the DT 104 isolates have an MDR phenotype designated ACSSuT indicating that they are resistant to ampicillin, chloramphenicol, streptomycin, sulphonamides and tetracycline. The ACSSuT phenotype of this strain is encoded by a chromosomal locus of about 12.5 kb carrying all resistance genes in a 43 kb genomic island called SGI1.6 The antibiotic resistance gene cluster of SGI1 contains two class 1 integrons, the chloramphenicol and florfenicol resistance gene, floR, and the tetracycline resistance gene tet(G). One of the class 1 integrons carries the aadA2 gene, which confers resistance to streptomycin and spectinomycin and the other contains the β-lactamase gene blaPSE-1 which confers resistance to ampicillin.6 SGI1 has been identified in strains of S. Paratyphi B in Singapore and Canada.3,4 To date, the multidrug-resistant S. Paratyphi B carrying SGI1 has not been reported in Japan; hence, this study aimed to examine a multidrug-resistant strain of S. Paratyphi B recently isolated in Hiroshima prefecture for the presence of SGI1. From 1990 to 2003, a total of 27 strains of S. Paratyphi B were isolated from patients with gastroenteritis in Hiroshima prefecture. Antimicrobial susceptibility testing was performed on all strains using the disc diffusion method. Only one strain, S. Paratyphi B strain 020405, displayed an MDR profile typical of serotype Typhimurium DT 104, being resistant to ampicillin, chloramphenicol, streptomycin, tetracycline (ACSSuT resistance type) and ciprofloxacin. This strain was isolated from an infant (1 year old) suffering from acute gastroenteritis in 2002 in Hiroshima prefecture. To characterize the molecular basis of this MDR, PCR primers specific for class 1 integrons were used to test this strain for the presence of class 1 integrons (Table 1).7 PCR and DNA sequencing results identified two integrons, 1.2 kb and 1 kb in size, carrying the resistance gene cassettes, pse-1 and aadA2, respectively. These two integrons are characteristic of SGI1 of S. Typhimurium DT 104 type.6 To assess the presence of the entire SGI1 and its location in the chromosome of S. Paratyphi B strain 020405, PCR assays were performed with sets of primers representing all regions of the 44 kb SGI1 element as previously described (Table 1).4 A multidrug-resistant strain of S. Typhimurium DT 104 was used as a positive control. PCR results showed that S. Paratyphi B strain 020405 gave PCR amplicons for all primer sets with the same typical target sizes (Table 1) as the positive control (data not shown). These results indicate that S. Paratyphi B strain 020405 contains the entire SGI1 at the same chromosomal location as in serotype Typhimurium DT 104. Therefore, detection of SGI1 in this strain only and its absence from the old strains of S. Paratyphi B indicate that this strain recently emerged in Japan. Primers for detection of class 1 integrons and various regions of SGI1 Primers for detection of class 1 integrons and various regions of SGI1 This work was supported by a Grant-in-Aid for scientific research to T. S. from the Ministry of Education, Culture, Sports, Science and Technology of Japan. 1Laboratory of Food Microbiology and Hygiene, Graduate School of Biosphere Science, Hiroshima University, Higashi-Hiroshima 739-8528, Japan; 2Division of Biological Science, Hiroshima City Institute of Public Health, Hiroshima 733-8650, Japan