|Ph.D Student||Kalily Emmanuel|
|Subject||Characterization of Salmonella Enterica Serovar Senftenberg|
Resistance to Linalool and its Impact on
Survival in Basil
|Department||Department of Biotechnology and Food Engineering||Supervisor||Professor Sima Yaron|
|Full Thesis text|
With the progressive increase in the number of human infection outbreaks associated with consumption of raw fruits, vegetables and leafy greens, fresh produce has emerged as an important vehicle for the transmission of enteric pathogens such as Salmonella to humans. A recent outbreak of Salmonella enterica serovar Senftenberg, which occurred at 2007 in Europe and the US, was traced to the consumption of fresh basil leaves. This outbreak raises concerns since basil produces a variety of antimicrobial agents in its essential oil (EO). This research aims at investigating whether S. Senftenberg had developed resistance to basil oil ingredients, exploring the impact of such a resistance on basil plant colonization and examination of the molecular mechanisms which contribute to this resistance.
Results revealed that S. Senftenberg (a clinical isolate from the 2007 basil outbreak) is more tolerant to basil oil and its principal constituents: linalool, estragole and eugenol, compared to S. Typhimurium, one of the most prevalent human pathogens. Moreover, irrigation of basil plants with water solutions harboring the two Salmonella serovars showed significantly higher counts in favor of S. Senftenberg on pre- and post- harvested basil. Collectively, the positive correlation between S. Senftenberg heightened resistance and better survival implies of its adaptation to the basil habitat.
Furthermore, cryogenic-transmission electron microscopy (Cryo-TEM) analysis together with permeability tests of linalool treated cells showed that linalool’s antimicrobial activity is achieved through disruption of the membrane. Screening of random transposon-insertion mutants for altered response to linalool indicated that linalool’s resistance is attributed to the following mechanisms: (i) active efflux; (ii) inhibition of porin production; (iii) alternation in fatty acid composition; (iv) aggregate formation; and (v) perhaps changes in lipopolysaccharide (LPS) profile.
In addition, following sequential exposure to linalool both S. Senftenberg and S. Typhimurium were able to improve their tolerance to linalool and basil oil with minimum inhibitory concentration (MIC) increments of up to 4- and 16-folds, respectively. Surprisingly, adaptation to linalool also conferred cross-protection to several clinically used antibiotics with a MICs range of up to 32-folds.
Overall, these findings suggest that bacterial adaptation to EOs can occur providing the emergence/selection of multidrug resistant bacteria that could colonize new niches in our surroundings with S. Senftenberg serving as one of the very first examples.