Fate of mesophilic aerobic bacteria and Salmonella enterica on the surface of eggs as affected by chicken feces, storage temperature, and relative humidity

Highlights

• Microbial levels of eggshells from traditional market and supermarket were compared.

• Behaviors of microbes on eggshells as affected by storage conditions were studied.

• Eggshells from traditional market had more microbes than those from supermarket.

• Populations of Salmonella on eggshell surfaces decreased under all tested conditions.

• Survival of Salmonella increased when stored with feces at low temperature and RH.

Abstract

We compared the microbiological quality of chicken eggshells obtained from a traditional wholesale market and a modern supermarket. We also determined the survival and growth characteristics of naturally occurring mesophilic aerobic bacteria (MAB) and artificially inoculated Salmonella enterica on eggshells under various environmental conditions (presence of chicken feces, temperature [4, 12, or 25 °C], and relative humidity [RH; 43 or 85%]). The populations of MAB, coliforms, and molds and yeasts on eggshells purchased from a traditional wholesale market were significantly (P ≤ 0.05) higher than those from a modern supermarket. In the second study, when we stored uninoculated eggs under various storage conditions, the population of MAB on eggshells (4.7–4.9 log CFU/egg) remained constant for 21 days, regardless of storage conditions. However, when eggshells were inoculated with S. enterica and stored under the same conditions, populations of the pathogen decreased significantly (P ≤ 0.05) under all tested conditions. Survival of S. enterica increased significantly (P ≤ 0.05) in the presence of feces, at low temperatures, and at low RH. These observations will be of value when predicting the behavior of microorganisms on eggshells and selecting storage conditions that reduce the populations of S. enterica on eggshells during distribution.

Introduction

Salmonella is one of the most common foodborne pathogens (CDC, 2014, HPA, 2012, KMFDS, 2014a) and causes a disease characterized by fever, abdominal pain, diarrhea, nausea, and sometimes vomiting (WHO, 2013). Based on estimates of the number of foodborne illnesses in the United States, nontyphoidal Salmonella is the second most common pathogen, causing 11% of foodborne illnesses, following norovirus, and is the leading cause of hospitalization (35%) and death (28%) (Scallan et al., 2011). Outbreaks of Salmonella infections are associated with consumption of various foods, including poultry, red meats, pork, vegetables, and fruits that can be contaminated with the pathogen. Eggs are the most common single food associated with outbreaks of salmonellosis (CDC, 2006, EFSA, 2009, Jackson et al., 2013, Moffatt and Musto, 2013). A multi-state outbreak of 1939 cases of Salmonella enterica subsp. enterica serovar Enteritidis (S. Enteritidis) infection occurred in 11 states of the United States in 2010, and eggshells were suspected as the source of infection (CDC, 2010). Organic shell eggs were recalled by the U.S. Food and Drug Administration due to contamination with S. Enteritidis (FDA, 2014). At least six people were infected.

Eggs have high nutritional value and provide high-quality proteins as well as fatty acids, iron, phosphorus, minerals, and vitamins (FAO, 2003, Seuss-Baum, 2007). Per capita consumption of eggs is increasing (FAOSTAT, 2014); thus, outbreaks associated with eggs may also become more common. Generally, there are two possible routes of contamination of eggs with Salmonella. One is through colonization of Salmonella in the reproductive organs of the hen. Colonized Salmonella spreads into the yolk, yolk membranes, albumen, shell membrane, and on the surface of eggshells before oviposition (vertical transmission) (Keller et al., 1995, Miyamoto et al., 1997, Shivaprasad et al., 1990). Another possible route is when eggshells are exposed to feces containing Salmonella or a contaminated environment, such as a nest or hatchery. Salmonella on the eggshell penetrates into the egg contents during or after oviposition (horizontal transmission) (De Reu et al., 2006, Gantois et al., 2009, Miyamoto et al., 1998). When eggshells are contaminated by horizontal transmission, chicken feces and other moist organic materials can protect Salmonella against inactivation and promote growth by providing nutrients (Gantois et al., 2009, Howard et al., 2012). The predominant route of egg contamination remains unclear but Wang and Slavik (1998) demonstrated that horizontal transmission appears to play a major role.

Governmental public health regulations for cleaning and storing eggs vary among countries. In the United States, commercially available eggs must be washed using sanitizers meeting certain standards and must be distributed at refrigeration temperature (less than 45 °F [7.2 °C]) (USDA, 2011). In contrast, in the European Union, washing eggs is not permitted and eggs are not refrigerated (European Commission, 2008). In the Republic of Korea, egg washing and conditions for the distribution are not critically regulated by the Korean Ministry of Food and Drug Safety (KMFDS, 2014b), although it is recommended that eggs be distributed at low temperatures (0–15 °C). In reality, eggs washed with water and unwashed eggs are distributed at ambient as well as refrigerated temperatures in Korea. In general, unwashed eggs stored at ambient temperatures (20 ± 5 °C) are sold at traditional wholesale markets and washed eggs stored under refrigeration temperatures (4–10 °C) are sold at modern supermarkets.

Numerous reports have described the survival of Salmonella in egg contents (Chen and Thesmar, 2008, Gast et al., 2010, Humphrey and Whitehead, 1993). However, only a few reports have described survival and growth patterns of microorganisms (naturally occurring mesophilic aerobic bacteria [MAB] or S. enterica) on the surface of eggshells as affected by storage conditions. One objective of this study was to compare the microbiological quality of the eggshell surfaces of commercial eggs distributed in a traditional wholesale market and a modern supermarket. A second objective was to investigate the survival and growth patterns of naturally occurring MAB and artificially inoculated S. enterica on eggshells when eggs were exposed to various environmental conditions (the presence of chicken feces, temperature [4, 12, or 25 °C], and relative humidity [RH; 43 or 85%]).