S. Paratyphi B dT+ with the resistance phenotype ApCmSmSpSuTc had been isolated sporadically in various states of Australia since 1997, and initial surveys showed a potential association with ownership of home aquariums (D. Lightfoot, unpub. data). In 2000, multidrug-resistant S. Paratyphi B dT+ with an identical phage type (reaction does not conform [RDNC]), designated here as Aus2, and the same drug-resistance profile (ApCmSmSpSuTc) was isolated from humans with gastroenteritis and from fish tanks in the homes of 2 infected patients (Table). In 2003 and 2004, 13 cases of ApCmSmSpSuTc S. Paratyphi B dT+ were investigated by state and commonwealth health departments, and all were associated with home aquariums containing tropical fish (J. Musto et al., unpub. data). Of these, 11 cases were phage type RDNC Aus3, 1 was phage type 1 var 15, and 1 was phage type 3b var. Water and gravel were collected from the domestic aquariums of 5 patients with RDNC Aus3-type infections, and identical isolates were recovered from each fish tank. Four matched sets of isolates, 2 from 2000 and 2 from 2003, were further examined (Table). One isolate (SRC50) characterized previously (7) was used as a control (Table).

To determine if the resistance phenotype of these strains was due to SGI1 (4,5,7,8,11), polymerase chain reaction (PCR) with primer pairs shown in Figure 1 was used as previously described (7). The left and right junctions of SGI1 with the chromosome and of In104 with SGI1 were present in all cases. Regions containing the gene cassettes were amplified by using standard primers (L1 and R1) in the 5´- and 3´-conserved segments of class 1 integrons. Fragments of 1.0 and 1.2 kb were amplified from all isolates, and digestion of these amplicons with RsaI generated a profile (data not shown) that was indistinguishable from the pattern for the 2 amplicons containing the aadA2 and blaP1 cassettes found in In104 and S. Paratyphi B dT+ isolates SRC49 and SRC50 from 2001 (7). The aadA2 gene cassette was linked to SO26 in the SGI1 backbone, which indicates that it is on the left, as in In104, and the expected 1.8-kb PCR fragment was generated by using primers in groEL and blaP1 (Figure 1), which places the blaP1 cassette on the right. Southern hybridization of Xba I–digested whole-cell DNA with a probe for the floR gene as described previously (7) identified a band of ≈12 kb, which is consistent with an SGI1 structure identical to that reported previously (7,8,11) and the groEL-blaP1 amplicon linked this 12-kb XbaI fragment with the adjacent 4.3-kb XbaI fragment (Figure 1).

Figure 2

Figure 2. Pulsed-field gel electrophoresis (PFGE) and IS200 profiles of Salmonella enterica serovar Paratyphi B dT+ isolates positive for Salmonella genomic island 1. A) PFGE profiles. Xba I–digested whole-cell DNA was separated by...

To obtain further evidence for the identity of the matched human and fish tank isolates, macrorestriction analyses of XbaI–digested whole-cell DNA by pulsed-field gel electrophoresis (PFGE) were performed as previously described (12). Several studies (3–6,13) suggest that S. Paratyphi B dT+ isolates possess considerable genetic heterogeneity. However, the SGI1-containing isolates appear to be homogeneous. The band patterns for all SGI1-containing S. Paratyphi B dT+ were identical from humans and fish tanks with phage type RDNC Aus3 (Figure 2A) and Aus2 (data not shown). IS200 profiles were also analyzed by hybridization of an IS200 probe with Pst I–digested whole-cell DNA as described elsewhere (6). Again, all strains showed identical profiles (Figure 2B and data not shown) that differed by 1 band from profile IP1 recently described (6). Thus, matched isolates from humans and their fish tanks were indistinguishable from each other.

An unusual observation in this study was that isolates with different phage types showed identical PFGE and IS200 profiles, indicating that they represented a clonal cluster. The control strain SRC50 (RDNC) also displayed the same patterns, demonstrating that it also is a member of the same clone. Thus, variation in phage type (Table) appears to have occurred within a single clone. Variation in phage type has also been reported in other studies of multidrug-resistant S. Paratyphi B dT+ strains (4–6), although a number of related but slightly different XbaI PFGE patterns were observed in those studies. This finding suggests that all multidrug-resistant S. Paratyphi B dT+ found globally have a single origin, but that variations, possibly because of acquisition of other temperate phages or plasmids, have arisen over time. However, direct comparisons of strains from different countries will be needed to confirm this hypothesis.

This is the first definitive report showing that ornamental fish tanks are a reservoir for multidrug-resistant S. Paratyphi B dT+ (ApCmSmSpSuTc phenotype) containing SGI1 that causes severe disease in humans, particularly young children. In addition to containing SGI1, the matched isolates from humans and their fish tanks had the same phage type and the same XbaI macrorestriction digest pattern and IS200 profile. These findings identify home aquariums containing tropical fish as the most important, although not necessarily the only, source of multidrug-resistant S. Paratyphi B dT+. The fact that 12%–14% of Australian households have ornamental fish (14) and as many as 12 million American and 1 million Canadian families own domestic aquariums (9), together with the young age of most affected patients, indicate that multidrug-resistant S. Paratyphi B dT+ in home aquariums is a risk factor for Salmonella infection and thus becomes a public health issue.