RESUMEN
Over the last decade, Salmonella enterica serovar Schwarzengrund has become more prevalent in Asia, Europe, and the US with the simultaneous emergence of multidrug-resistant isolates. As these pathogens are responsible for many sporadic illnesses and chronic complications, as well as outbreaks over many countries, improved surveillance is urgently needed. For 20 years, pulsed-field gel electrophoresis (PFGE) has been the gold standard for determining bacterial relatedness by targeting genome-wide restriction enzyme polymorphisms. Despite its utility, recent studies have reported that PFGE results correlate poorly with that of closely related outbreak strains and clonally dominant endemic strains. Due to these concerns, alternative amplification-based molecular methods for bacterial strain typing have been developed, including clustered regular interspaced short palindromic repeats (CRISPR) and multilocus sequence typing (MLST). Furthermore, as the cost of sequencing continues to decrease, whole genome sequencing (WGS) is poised to replace other molecular strain typing methods. In this study, we assessed the discriminatory power of PFGE, CRISPR, MLST, and WGS methods to differentiate between 23 epidemiologically unrelated S. enterica serovar Schwarzengrund isolates collected over an 18-year period from distinct locations in Taiwan. The discriminatory index (DI) of each method for different isolates was calculated, resulting in values between 0 (not discriminatory) and 1 (highly discriminatory). Our results showed that WGS has the greatest resolution (DI = 0.982) compared to PFGE (DI = 0.938), CRISPR (DI = 0.906), and MLST (DI = 0.463) methods. In conclusion, the WGS typing approach was shown to be the most sensitive for S. enterica serovar Schwarzengrund fingerprinting.
RESUMEN
We tested 137 samples of domestic shucked oysters and 114 samples of imported oysters collected from traditional retail markets and supermarkets during 2010 and 2011 in Taiwan for the presence of Salmonella. We obtained a total of 91 Salmonella isolates, representing nine serotypes, from 80 of the domestic samples. We did not find any Salmonella in the imported oysters. The presence of Salmonella contamination tended to be specific to the area from which the oysters were harvested: the Dongshih area had a significantly higher contamination rate (68.8%) than the Budai (20.0%) and Wanggong (9.1%) areas. In addition, the rate of Salmonella contamination was higher in oysters that were packed or sold with water (P < 0.05). The most commonly identified Salmonella serotypes were Saintpaul (26.4%), Newport (22.0%), and Infantis (13.2%). We screened the isolates for susceptibility to nine antimicrobials and compared them genetically by using PCR for the class 1 integron (int1), tetA, tetB, and blaPSE-1 genes. Eighteen isolates (19.8%) were resistant to at least one antimicrobial agent, and the most frequent resistances were those to tetracycline and oxytetracycline (n = 12, 14.3%).We detected the antimicrobial resistance genes int1, tetA, tetB, and blaPSE-1 in 16.5, 26.4, 6.6, and 22.0% of the isolates, respectively. Eleven of the 18 antimicrobial-resistant isolates contained one or two int1 cassettes, suggesting that the presence of int1 is highly correlated with antimicrobial resistance in Salmonella isolates from oysters. The consumption of oysters is increasing in Taiwan, and information related to Salmonella contamination in oysters is rather limited. Our results indicate that raw oyster consumption from retail markets in Taiwan is associated with a human health hazard owing to Salmonella, including multidrug-resistant Salmonella strains.
