Salmonella and tomatoes

Outbreak information linking fresh tomato fruit to illnesses is reviewed in this chapter. While tomato fruit appear to support substantial proliferation of certain serovars of Salmonella enterica, detection of this pathogen in tomato plants prior to harvest is rare, and reports of Salmonella existence in tomato fruit still attached to field-grown plants are virtually non-existent. The bacterium is sensitive to UV and can be outcompeted by the native phytomicrobiota, which may explain its absence in field-grown crops. However, the persistence of certain serovars in fields and ponds of certain production areas is noted. Together with evidence of bacteria becoming internalized in tomato fruit during crop development likely through natural apertures, the presence of S. enterica in and around production fields suggests that an unusual weather event could lead to Salmonella contamination of fruit prior to harvest. The bacterium appears physiologically adaptive toward proliferation in tomato fruit. Once inside tomatoes, Salmonella is capable of sensing the availability of nutrients and physiological state of the fruit and differentially regulates specific genes. However, because Salmonella is an efficient nutrient scavenger, removal of multiple metabolic and regulatory genes was required to reduce its fitness within the fruit. Plants do not appear to recognize human enterics as pathogens, and their defenses treat them as endophytes

Laboratory activity to teach about the proliferation of salmonella in vegetables

We designed a three-week laboratory experience that can complement any Microbiology teaching laboratory to expand students’ knowledge of the ecology of human enteric pathogens outside of their animal hosts. Through their participation in this laboratory activity, students learned that vegetative and reproductive plant parts could be a natural habitat for enteric bacteria such as non-typhoidal strains of Salmonella enterica. This field was recently brought to the forefront of the scientific and public interest by outbreaks of human illness linked to the consumption of fresh fruits and vegetables. Students were encouraged to develop their own testable hypotheses to compare proliferation of Salmonella enterica sv Typhimurium LT2 in different vegetables: cherry and regular size tomatoes, onions, lettuce, yellow and red bell peppers (Escherichia coli can be substituted for BSL1 laboratories). Upon completion of the laboratory experience, students were able to: 1) Develop testable hypotheses addressing the ability of a human pathogen Salmonella enterica to colonize and proliferate in vegetables; 2) Determine that different vegetables support the growth of Salmonella to a different extent; 3) Conduct statistical analysis and identify any significant differences. The teaching-learning process was assessed with a pre/post test, with an average increase in content understanding from ~15% to 85%. We also measured students’ proficiency while conducting specific technical tasks, revealing no major difficulties while conducting the experiments. Students indicated satisfaction with organization and content of the practices. 100% of the students agreed that the exercises improved their knowledge of this subject

Laboratory activity to teach about the proliferation of salmonella in vegetables

We designed a three-week laboratory experience that can complement any Microbiology teaching laboratory to expand students’ knowledge of the ecology of human enteric pathogens outside of their animal hosts. Through their participation in this laboratory activity, students learned that vegetative and reproductive plant parts could be a natural habitat for enteric bacteria such as non-typhoidal strains of Salmonella enterica. This field was recently brought to the forefront of the scientific and public interest by outbreaks of human illness linked to the consumption of fresh fruits and vegetables. Students were encouraged to develop their own testable hypotheses to compare proliferation of Salmonella enterica sv Typhimurium LT2 in different vegetables: cherry and regular size tomatoes, onions, lettuce, yellow and red bell peppers (Escherichia coli can be substituted for BSL1 laboratories). Upon completion of the laboratory experience, students were able to: 1) Develop testable hypotheses addressing the ability of a human pathogen Salmonella enterica to colonize and proliferate in vegetables; 2) Determine that different vegetables support the growth of Salmonella to a different extent; 3) Conduct statistical analysis and identify any significant differences. The teaching-learning process was assessed with a pre/post test, with an average increase in content understanding from ~15% to 85%. We also measured students’ proficiency while conducting specific technical tasks, revealing no major difficulties while conducting the experiments. Students indicated satisfaction with organization and content of the practices. 100% of the students agreed that the exercises improved their knowledge of this subject

Fitness of antibiotic-resistant bacteria in the environment: a laboratory activity

In this laboratory experiment, we propose an opportunity for students to broaden their understanding of the ecology of antibiotic-resistant and sensitive waterborne bacteria. Antibiotics can be found in rivers or soil as a consequence of agricultural practices or as a result of human use. Concentrations of antibiotics in the environment may range from a few ng to μg L-1. Such concentrations can affect the selection and fitness of resistant bacteria. In this laboratory activity, students learn how to set up a fitness experiment by using an isogenic pair of antibiotic-resistant and sensitive bacteria in the presence or absence of selective pressure. Microcosms were generated by using filtered river water containing populations of resistant and sensitive bacteria. Competition of both populations was measured in the presence or absence of antibiotics. Students appreciated the use of microcosms for in vitro experiments and the extent to which the fitness of resistant and sensitive bacteria changed in the presence and/or absence of a selective pressure in river water. Student learning was measured by using different types of assessments: multiple-choice, true/false, fill in the blanks, laboratory skills observations, and laboratory reports. After the laboratory activity, the percentage of correct answers significantly rose from ~20% to ~85%. Laboratory skills were also evaluated during the exercises, showing no major issues during the experiment. Students showed proficiency in analyzing the complexity of fitness data by reaching a mean of 5.57 (standard error 0.57) over a maximum score of 7 points