Document Type


Publication Date

Spring 2016


Bacteria | Bacteriology | Biology | Environmental Microbiology and Microbial Ecology | Food Microbiology | Food Science | Integrative Biology | Life Sciences | Medicine and Health Sciences | Microbiology


Barbara May, Biology


Chicken eggs are a major component of American diets, with an average yearly consumption of approximately 250 eggs per person (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 bacterial contamination, 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 also been used as growth-promoters. While this practice frequently improves the overall health and productivity of the flock, it also contributes to the development of bacterial 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, resulting in an increased frequency of bacterial resistance. 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 the egg industry, chickens are often raised under a variety of conditions from industrialized production farms to personal hen houses. The quality of the egg is frequently attributed to its production process, leading producers to advertise production methods like vegetarian food and cage free environment for their chickens. Factors like these are boasted to suggest better health benefits and less pathogen contamination and, furthermore, promote sales. However, these claims have not been thoroughly investigated. In an effort to develop a better understanding of egg contamination during production, this experiment utilized a variety of chicken eggs, including those from commercial, local, and private chicken producers. Within these groups, also included were organic, vegetarian fed, free range, farm fresh, and antibiotic free production methods. Bacterial samples were cultured and isolated from the shell, Albumin (egg white), yolk, and outer shell membrane, and some were later identified using 16S DNA sequencing. In an effort to identify emerging bacterial resistance, the samples were tested for resistance 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.) would affect the diversity of bacterial contaminants and the areas of the egg they would be able to contaminate. Additionally, it was hypothesized that eggs coming from chickens previously 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.