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Biology | Food Science | Life Sciences


Barbara May, Biology


Chicken eggs are a major component of American diets, with an average yearly consumption of approximately 250 eggs per person (according to estimates made by the American Humane Society). While highly nutritious, eggs are also one of the leading causes of food poisoning and food borne illness in the United States. Eggs may become contaminated by a number of different types of bacteria during production, including Salmonella, a group of bacteria that, according to the CDC, causes more than 1.2 million cases of food borne illness in the United States every year. In an effort to decrease the frequency of food contamination with bacteria like Salmonella, many food producers have begun to treat their livestock and poultry with antibiotics, as a method of preventing and treating illness within the population. In some cases, antibiotics have even been used as growth-promoters. While this practice frequently improves the overall health and productivity of the flock, it also contributes to a phenomenon in which bacteria develop a resistance to antibiotics (Singer, Hofacre Avian Diseases). This phenomenon has been observed and studied with the emergence of methicillin-resistant Staphylococcus aureus (MRSA), a pathogen commonly affecting humans. According to the National Institute of Health, MRSA has developed as a result of bacterial adaptation due to repeated administration of antibiotics. As antibiotics commonly used to treat S. aureus increase in the environment, those bacteria that are randomly resistant to antibiotics persist and, as a result, the frequency of bacterial resistance increases. As the use of antibiotics in egg production increases, antibiotic-resistant strains of Salmonella and other bacteria are likely to emerge, contributing to increased food borne illness and decreased ability to treat infections.

In an effort to develop a better understanding of egg contamination during production, this experiment utilized a variety of types of chicken eggs, including those from commercial producers and local, private producers. These types included eggs with a variety of labels, such as organic, vegetarian fed, free range, farm fresh, and antibiotic free eggs. Bacterial samples were cultured and isolated from the shell, albumen (egg white), yolk, and outer shell membrane, and were identified using 16S DNA sequencing. In an effort to identify emerging bacterial resistance, the samples were tested for resistance (using the Kirby-Bauer method) to antibiotics and cleaners that are commonly used in egg production and are approved by the USDA for use on laying hens. It was hypothesized that differences in production (free range vs. caged, organic vs. non-organic, vegetarian fed vs. normal feed, etc.) may have some effect on the variety of bacterial contaminants and the areas of the egg they would be able to contaminate. Additionally, it was hypothesized that eggs that were more exposed to antimicrobials and antibiotics would exhibit more resistance. Finally, the experiment was expected to reveal trends in the types and strains of bacteria are able to penetrate various membranes within the egg.