Survival of Salmonella Newport in Oysters

Abstract

Salmonella enterica is a foodborne pathogen of major significance, and as such it has been extensively studied by researchers around the world. However, despite the numerous scientific publications on Salmonella, there are still many gaps in our understanding of its biology. One such gap is in the bacteria's interactions with invertebrate hosts, and in particular, oysters. Nearly 70 million pounds of oysters are consumed in the United States each year, and previous work in the Joens' laboratory found Salmonella in roughly 7% of the market oysters they sampled, with the majority of the isolates being the Newport serovar. The majority of oysters are consumed raw, which makes the presence of Salmonella within oysters a potentially significant food safety problem.To more closely examine the interactions between Salmonella and oysters, the Present Study developed a method to consistently and reproducibly raise oysters in a controlled laboratory environment in order to systematically expose them to enteric bacteria and quantify the amount of surviving bacteria at various time points after the initial exposure. Use of this model system throughout the Present Study led to four main conclusions.The first is that Salmonella enterica serovar Newport is capable of surviving in oysters for at least 60 days, from an average concentration of 3.7x103 CFU/g of oyster meat after 10 days, to over 102 CFU/g of oyster meat after 60 days. The second main conclusion is that the Newport serovar of Salmonella, which was found in such predominance in the earlier Joens' laboratory study, does not appear to have any special adaptations for survival within oysters, as other strains of Newport and other serovars of Salmonella survived equally well within our model. The third main conclusion, based on the results of immunohistochemistry, is that the relationship between Salmonella and oysters is not a transient interaction that is limited to the outside of the oyster's gut epithelium, but involves a long-term colonization inside the oysters' connective tissues. Because the survival of Salmonella in oysters could be of a pathogenic nature, the Present Study knocked out two key type III secretion systems (T3SS) found in two distinct Salmonella pathogenicity islands (SPI-1 and SPI-2) known to be critical for pathogenesis in mammalian hosts and examined their role in the bacteria's ability to survive within oysters. The results revealed that neither the SPI-1 nor the SPI-2 T3SS were necessary for Salmonella's survival in oysters, which led to the final conclusion of the Present Study that the nature of Salmonella's infection of oysters is fundamentally different than the pathogenesis that occurs in mammalian hosts and that further study of the mechanisms of the survival of Salmonella in oysters is needed to better understand the important and interesting relationship between a significant source of food and this common, and occasionally deadly, foodborne pathogen.