Salmonella enterica: foodborne illness

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[Figure 1]Commercial Poultry Farm.[1].

The bacteria Salmonella is commonly associated with food poisoning in countries all over the world. There are two species of Salmonella: S. enterica and S. bongori. However, the species that most people refer to when they talk about Salmonella is S. enterica. This species is divided into a subset comprising of serovars. In S. enterica alone, there are over 2,000 serovars of bacteria. The most common cause of food poisoning is associated with the serovar S. Enteritidis. These outbreaks are attributed to contaminated poultry, chicken eggs, and products that contain eggs. Because of the high demand for poultry and poultry products, farms can become highly unsanitary. For example, chicken farms contain thousands up on thousands of chickens all living in the same area. Dead chickens are left for days before they are removed from the others. If one chicken contains the Salmonella bacteria, the organism can spread rapidly through thousands of others. When these chickens are processed and sent to stores all over the country, a mass Salmonella outbreak can occur. When humans contract this disease, they can expel some of the Salmonella bacteria in their feces. In areas with poor sanitation, these bacteria can get into the water system where they can infect another person or animal and begin a whole new cycle of transmission.

Poultry eggs can contain Salmonella bacteria both on the surface as well as inside. After hens lay eggs, they sit on top of the to keep them warm. While sitting, fecal matter can transfer from the chicken to the surface of the egg. If the chicken is infected with Salmonella, the eggs must be properly disinfected in order to prevent disease. This is a larger problem in developing countries rather than the US or UK. The more common method of transmittance in the US is attributed to eggs infected with Salmonella on the inside. Salmonella Enteritidis can infect the ovaries of a hen and thus contaminate the egg before the shell forms. Because the bacterium silently infects the hen’s ovaries, the hen does not show any signs of an illness. Therefore, seemingly healthy hens transmit this disease.

In the United States alone, Salmonella is responsible for 1.4 million infections, 15,000 hospitalization, and 400 deaths per year. These statistics are attributed to all serovars of Salmonella enterica not solely Enteritidis.

Pathogenesis

[Figure 1]Salmonella enterica serovar Enteritidis.[2].

Salmonella Enteritidis is a rod-shaped, gram-negative, proteobacteria that is non-motile. These facultative anaerobes are well adapted to survive in condition with our without oxygen, allowing them to live in diverse environments. However, Salmonella enteritidis are usually found within animal hosts where they can be transmitted easily through food processing or fecal matter in areas with poor sanitation.

There are 27 phage types differentiated within the serovar Enteritidis. The current dominant phage type worldwide is PT4. However PT8 are also commonly found in the US and UK. PT4, PT8, and PT13a comprise the majority of Enteritidis infections caused worldwide.

The disease that Salmonella Enteritidis causes is known as a zoonotic disease, meaning that it transmits from animal to human. Salmonella Enteritidis begins when a person ingests a food item infected with a high concentration of the bacteria. In healthy adults, the acidity of the stomach can kill the bacteria if they are present in a low concentration. However, if a large amount of the bacteria are ingested, they have a higher probability of surviving and reproducing. Once in the stomach, the bacteria can migrate to the liver or spleen. Here, the bacteria replicate and move back into the intestines. S. Enteritidis require glucose to survive and use glucose fermentation to produce energy. This method of metabolism releases gas as a bi-product that then builds up in the victim’s intestinal system causing irritation. The bacteria reproduce extremely quickly causing illness due to the sheer number of foreign bacteria present. At the peak of infection, there can be up to one billion Salmonella bacteria present per gram of feces.

Research: A Case Study

A recent case study published in 2011 researched a Salmonella Enteritidis outbreak that occured in Singapore in 2007. This outbreak, reportedly causing illness in 15 people, could be traced back to cream cakes consumed at a birthday party. After this, various cases were reported in other areas of Singapore as well. These too were eventually traced back to the consumption of cream cakes from various food stores. All of these cream cakes were found to be made at a well known bakery.

A study was undertaken to examine all aspects of this food-poisoning outbreak. Victims of 20 cases were reported and tested for the presence of Salmonella Enteritidis in their stool. 176 food handlers and staff from the bakery, as well as 232 staff members from the retail outlet were tested for the presence Salmonella Enteritidis in their stool. Salmonella isolates were taken from the stool and grown on blood agar plates. Serotyping was used to determine the serovar of Salmonella and was in fact determined to be Enteritidis. Antibiotic susceptibility was tested using the disk diffusion method on Mueller Hinton agar. Phage typing was performed on the isolates that showed that the majority of phages present were phage type 1 and 13. Ribotyping, also known as molecular typing, was done to determine the genetic relatedness between the isolated strains. Figure 3 shows the results of this test. Although there are two different phage types, they are almost identical genetically.

Of the 20 reported cases of food poisoning, 14 tested positive for Salmonella Enteritidis. 6/176 food handlers and staff from the bakery and 4/232 staff members from the retail outlets tested positive for Salmonella Enteritidis.

One of the main ingredients for the butter cream icing of the cakes was egg whites. These egg whites were separated manually from the egg yokes. Before these egg whites were adding to the icing mixture, they were not pasteurized or heated to a high temperature, in order to ensure the killing of existing bacteria. This is hypothesized to be the origin of contamination. Before refrigerated trucks distributed the cakes to different retail outlet stores, the cakes sat at room temperature, uncovered, for at least two hours. This could also contribute to the multiplication of <Salmonella Enteritidis bacteria causing a high concentration to be ingested in a small amount of food.

Symptoms, Diagnosis, and Prevention

The most common symptoms of Salmonella Enteritidis include fever, diarrhea, vomiting, abdominal cramps, muscle aches, and headache. These symptoms generally last between 4-7 days. Healthy individuals can usually rid themselves of this bacteria on their own; however, children, elderly people, and those with compromised immune systems may require additional treatment.

Although these symptoms are an easy way to detect food poisoning, there are also clinical ways of testing for the presence of bacteria and more specifically Salmonella Enteritidis. A complete blood count can be done to look for anemia. Anemia, or a low iron content in red blood cells, can be a sign of bacterial infection. Iron is needed as an oxygen carrier in the blood; however, bacteria also require iron as a nutrient and can therefore deplete the blood's iron concentration. A low white blood count can also be a sign of bacterial infection. The liver function may be slightly elevated due to the fact that the bacteria are reproducing in the liver or spleen. More specifically, a stool culture may be done to determine if the bacterial infection is in fact Salmonella Enteritidis. Once isolated from the stool, PCR amplification can be used to determine the strain present. Serotyping can be performed in order to identify the serovar of Salmonella enterica. This method involves immunological tests to detect specific bacterial proteins present in the sample.

The most effective way to prevent Salmonella infections is to thoroughly cook all meats. The United States Department of Agriculture has also published guidelines to prevention. These include washing your hands and cooking surfaces often as well as washing hands, cooking utensils, and cooking surfaces with hot, soapy water before handling any food. Because red meat, poultry, and seafood are more susceptible to carrying Salmonella, they also recommend separating these foods from other food in your refrigerator. Cooking at proper temperatures and refrigeration are important steps in preventing Salmonella outbreaks.

Treatment

For those cases that require medical treatment, the most common method is through the use of antibiotics. There are a multitude of antibiotics available for treatment of Salmonella Enteritidis such as ampicillin, amoxicillin, ciprofloxacin, or trimethoprim/sulfamethozazole (TMP-SMX). However, the rising use of antibiotics to promote the growth of feed chickens has caused a rise in antibiotic resistance. Therefore, a combination of antibiotics may be required to completely rid the body of the bacteria.

Because S. Enteritidis causes gastroenteritis, or intestinal irritation, one of the body's most effective methods to ridding the body of bacteria is through feces. While diarrhea rids the body of the bacteria, it also causes the problem of excessive water loss. Oral hydration is effective, but it may not be enough. Some patients require intravenous hydration in order to ensure that the organs are getting enough water.

Conclusion

While Salmonella rarely causes death in areas where sanitary precautions are taken, it is unsettling to know that Salmonella Enteritidis can silently infect eggs. This characteristic makes it increasingly common as a foodborne pathogen in developed countries. The exact mechanism of this infection is not well understood and is an important area of research for Departments of Health around the globe. While we rarely think of people dying from food poisoning, bacterial infections can be quite dangerous. As bacteria become more resistant to antibiotics, scientists must find other methods of combatting the rapid reproduction of these microbes. The battle between bacterial resistance and antibiotic production is a rising problem that will only get worse as bacteria continue to naturally select against antibiotics. Therefore, it is important to learn as much as possible about bacterial metabolism in order to keep bacterial populations under control.

References

[Sample reference] Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "Palaeococcus ferrophilus gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". International Journal of Systematic and Evolutionary Microbiology. 2000. Volume 50. p. 489-500.

Edited by student of Joan Slonczewski for BIOL 238 Microbiology, 2009, Kenyon College.