Clonal dissemination of the multi-drug resistant Salmonella enterica serovar Braenderup, but not the serovar Bareilly, of prevalent serogroup C1 Salmonella from Taiwan

Abstract

Background: Nontyphoidal Salmonella is the main cause of human salmonellosis. In order to study the prevalent serogroups and serovars of clinical isolates in Taiwan, 8931 Salmonellae isolates were collected from 19 medical centers and district hospitals throughout the country from 2004 to 2007. The pulsed-field eletrophoresis types (PFGE) and antibiotic resistance profiles of Salmonella enterica serovars Bareilly (S. Bareilly) and Braenderup (S. Braenderup) were compared, and multi-drug resistance (MDR) plasmids were characterized.

Results: Over 95% of human salmonellosis in Taiwan was caused by five Salmonella serogroups: B, C1, C2-C3, D1, and E1. S. Typhymurium, S. Enteritidis, S. Stanley and S. Newport were the four most prevalent serovars, accounting for about 64% of isolates. While only one or two major serovars from four of the most prevalent serogroups were represented, four predominant serovars were found in serogroup C1 Salmonellae. The prevalence was decreasing for S. Choleraeuis and S. Braenderup, and S. Virchow and increasing for S. Bareilly. S. Braenderup mainly caused gastroenteritis in children; in contrast, S. Bareiley infected children and elderly people. Both serovars differed by XbaI-PFGE patterns. Almost all S. Bareilly isolates were susceptible to antibiotics of interest, while all lacked plasmids and belonged to one clone. Two distinct major clones in S. Braenderup were cluster A, mainly including MDR isolates with large MDR plasmid from North Taiwan, and cluster B, mainly containing susceptible isolates without R plasmid from South Taiwan. In cluster A, there were two types of conjugative R plasmids with sizes ranging from 75 to 130 kb. Type 1 plasmids consisted of replicons F1A/F1B, blaTEM, IS26, and a class 1 integron with the genes dfrA12-orfF-aadA2-qacEDelta1-sulI. Type 2 plasmids belonged to incompatibility group IncI, contained tnpA-blaCMY-2-blc-sugE genetic structures and lacked both IS26 and class 1 integrons. Although type 2 plasmids showed higher conjugation capability, type 1 plasmids were the predominant plasmid.

Conclusions: Serogroups B, C1, C2-C3, D1, and E1 of Salmonella caused over 95% of human salmonellosis. Two prevalent serovars within serogroup C1, S. Bareilly and cluster B of S. Braenderup, were clonal and drug-susceptible. However, cluster A of S. Braenderup was MDR and probably derived from susceptible isolates by acquiring one of two distinct conjugative R plasmids.

Figure 1

Dendrograms were constructed by PFGE-XbaI patterns to determine the genotypes for S. Braenderup (A) and S. Bareilly (B) with corresponding information including the number and size of plasmids, PFGE subtypes, antimicrobial resistance patterns and collection location of each isolate. The dendrograms were generated by the unweighted pair group method with arithmetic mean (UPGMA) using the Dice-predicted similarity value of two patterns. The BioNumerics version 4.5 statistics program was used with settings of 1.0% optimization and 0.7% tolerance. Symbols of black square and white square represent resistant and susceptible respectively. Plasmids were separated into four groups by size. Ex, 1, 1, 1, 3 indicates that this strain harbored 6 plasmids, one is >90 kb, one is from >50 to <90 kb, one is from >6.6 to <50 kb, and three are <6.6 kb.

Figure 2

HindIII-digested RFLP profiles of ampicillin resistance plasmids in S. Braenderup isolates. M1: HindIII-digested lambda DNA size marker. M2: 1 kb size marker.

Figure 3

PCR amplification of IS26 and IS26-associated DNA fragments. (A) Primer design. Symbols of arrow and dashed arrow represent IS26in primers and IS26out primers, respectively. (B) PCR products amplified by IS26in primers. (C) PCR products amplified by IS26out primers. M1: 100-bp size marker. N: negative control. M2: 1-kb size marker.