Growth of Salmonella Bacteria in Salsa vs. Sour Cream Chip Dip by Chris B., Jack B., and Katie R.
Background:
Salmonella are gram negative, anaerobic bacteria (1). Salmonella bacteria are one of the leading causes of food borne illnesses in the United States (2). It causes approximately 1.4 million infections every year. Most cases of salmonella infections come from the ingestion of contaminated food like eggs, dairy, and meat, although, many other types of foods can be infected (1). In 2006, the most common foods in which outbreak related cases were attributed were poultry (21%), leafy vegetables (17%), and fruits/nuts (16%) (3). Risk factors for salmonella food poisoning include gastric hypoacidity, recent use of antibiotics, extremes of age, and a variety of immunosuppressive conditions (1). Salmonella enterica is a common cause of gasteroenteritis. Human infections are commonly caused by ingestion of food that has been contaminated by animal feces (4).
Materials and Methods:
First Time:
A pure broth was inoculated with an unknown amount of salmonella bacteria and was placed in an incubator for 18 hours to be sure that there were lots of salmonella bacteria to start with.
18 hours later, 2 tablespoons of salsa was placed in a sterile beaker with a foil top and 2 tablespoons of French onion chip dip was placed in another. French onion chip dip and salsa were spread onto EMB plates as controls to be sure that there was no salmonella bacteria in them before the experiment began. 0.2mL of salmonella broth was mixed into each of the experimental beakers. At the same time, a serial dilution was performed on the salmonella broth to see how much bacteria was in the 0.2mL of broth added to the dips. Salsa was picked up from the salsa experimental beaker with an inoculating loop and streaked out on an EMB plate. French onion chip dip was also picked up from the French onion experimental beaker and streaked out on an EMB plate. The experimental beakers were left in a room at room temperature for the duration of the experiment. Samples of French onion chip dip and salsa were taken from the experimental beakers and streaked with an inoculating loop onto EBM plates every hour for 8 hours. All of the EMB plates were placed in an incubator overnight and the numbers of colonies were counted the next day.
Second Time:
A pure broth was inoculated with an unknown amount of salmonella bacteria and was placed in an incubator for 18 hours to be sure that there were lots of salmonella bacteria to start with.
18 hours later, a jar of salsa and container of French onion sour cream chip dip were opened and samples of each were plated on EMB plates as controls to be sure that there was no salmonella bacteria in them before the experiment began. 0.1mL of the salmonella broth was mixed into one 14oz jar of salsa and another 0.1mL of the broth was mixed into a 12oz container of French onion chip dip. A serial dilution was performed on the salmonella broth while 1tsp samples of French onion chip dip and salsa were plated in EMB agar using a pour plate technique. The chip dip and salsa were left out at room temperature during the experiment. 1tsp samples of chip dip and salsa were plated in EMB agar every hour for 8 hours. All of the EMB plates were placed in an incubator overnight and the numbers of colonies were counted the next day.
Results:
Two trials were performed for the experiment. The first trial was not very successful because the serial dilution was executed incorrectly, it needed to be more diluted. Another fault was discovered on the agar plates. All of the plates had lawns of bacteria, making it difficult to conclude anything. If the ratio between the salmonella and dip had been different there would not have been lawns of bacteria. Trial two was slightly changed to compensate for the mistakes made in trial one.
The trial two serial dilution turned out quite nicely. After counting the bacterial colonies from the serial dilution, there were 43,500,000,000 colonies on the 108 plate and 265,000,000,000 on the 109 plate. However, plating the dip contaminated with Salmonella proved to be a little more difficult. The contaminated dip was placed in a sterile plate and EMB agar was poured over top. The EMB had to be re-melted from solid to liquid form before every use. The salsa spread out nicely, but the French onion dip stayed clumped together when the agar was poured over top. Another problem the results unveiled was that there were no bacteria on the plates with EMB. It was concluded that by microwaving the EMB and then pouring it over the dips killed the bacteria inside.
Discussion
The first time that the experiment was done the serial dilution was performed incorrectly. The ratio between the bacteria and sterile broth was incorrect. Also another problem that we had was that the ratio to dip and Salmonella was also incorrect as there was not enough dip and there was too much Salmonella for the little amount of dip.
The second time the experiment was done the agar that was used was to hot. A new technique was used which was the pour technique. The agar needed to be microwaved so that it was melted and then it was poured on the plate. It was not able to cool long enough which resulted in the death of the bacteria.
A good idea for next time the experiment is done would be keeping the agar in a 50-Celsius degree water bath so that the agar does not kill the bacteria when poured into the plate. Make sure all serial dilutions are done correctly and have the right ratio of bacteria to dip. When further research was done into pour plate technique it was found that the agar should be kept at 45-Celsius. A good idea for future experiments would be to contrast other foods to determine the best environment for Salmonella to grow. Suggestions for contrasting foods are poultry, fish, and beef as they are most widely know for carrying Salmonella (3).
Sources:
1.) Cianflone F. (2008). Salmonellosis and the GI Tract: More than Just Peanut Butter. Curr Gastroenterol Rep. 10(4): 424-431
2.) Olsen EV, Gibbins CS, Grayson JK. (2009). Real-time FRET PRC essay for Salmonella enterica serotype detection in food. Military Medicine. 174(9): 983-90
3.) Centers for Disease Control and Prevention. (2009) Surveillance for foodborne disease outbreaks – United States. MMWR: Morbidity & Mortality Weekly Report. 58(22): 609-15
4.) Aarestrup F, Hendriksen R. (2007). International Spread of Multidrug –resistant Salmonella Schwarzengrund in Food Products. Emerging Infectious Diseases. 13(5): 726-31
5.) Frankhauser, D. 2005. Pour Plate Technique for Bacterial Enumeration. University of Cincinnati Clermont College.
