Salmonella Outbreaks: A Story of Antibiotic Resistance

Salmonella: An Overview

So, what is Salmonella? You might have heard of the term related to an illness, and that it is related to outbreaks of foods. But, what actually is it? Salmonella is a bacteria. They are gram-negative, facultative anaerobic chemoheterotrophs, with a bacillus shape. Gram-negative means that they have a thin cell wall made up of a material called peptidoglycan. Facultative anaerobe means that the bacteria can utilize oxygen if it is present, and if it is not, they still can function. Chemoheterophs means that they get their energy from chemical sources, and cannot produce their own carbon, looking to other sources such as from eating other organisms. Finally, bacillus refers to the shape of their bodies, which is rod-shaped.

So what do these bacteria have to do with foodborne illness outbreaks? Well, humans can become infected by Salmonella, typically through eating and handling contaminated foods, or through contact with animals. When humans become infected with Salmonella, they typically begin to develop symptoms within 12-72 hours of exposure. Some of these symptoms, according to the CDC are diarrhea, fever, and abdominal cramps. The illness typically lasts from 4 days to a week and does not usually require any treatment. However, both younger and older patients (younger than 5 and older than 65), and those with compromised immune systems may have a more severe illness. While death is rare, it still may occur if patients do not receive proper antibiotic treatments. Some sign of more advanced cases includes fevers of over 102 degrees, dehydration, bloody stool, and continued diarrhea with no signs of improvement.

Chicken Salad 2018 Salmonella Outbreak

2018 Salmonella Outbreak

The above map shows an example of a 2018 Salmonella outbreak. It affected 8 states, and 265 people, as depicted in the bar graph to the left. This particular outbreak was found to have likely occurred due to contaminated chicken salads, distributed by Triple T Specialty Meats located in Iowa, to Fareway Grocery stores in Illinois, Iowa, Minnesota, Nebraska, and South Dakota. The reason this was deemed to be the source of the outbreak is due to the fact that of the 265 identified infected people, there were 222 people interviewed, with 194 of them indicating they ate the chicken salads before becoming sick. There is a buffer around the distributor, depicting about 300 miles of travel by truck, which is a possible mode of transportation taken to deliver the contaminated meats. 300 miles is only half of the average distance a truck driver travels in a day, and the buffer still includes a majority of the affected states, providing further evidence of Triple T Specialty Meats being a likely source of the outbreak. A buffer for a 5-hour drive can be toggled on, as well, for a different view. This is a little under half of the average truck driver's driving time per day, still including almost each affected state. Investigators in Iowa eventually tested two chicken salad samples found in Iowa Fareway stores, finding Salmonella in both.

Antibiotic Resistance

During the investigative process, strains of Salmonella were isolated from a combined 127 food and personnel sources. Antibiotic-resistant genes were identified in 15 of these samples. Antibiotic resistance is something that raises alarm in this case, as antibiotics are the first line of offense against bacterial infections. One of the antibiotics resistance was found for is ampicillin. The way ampicillin works, in the most simple terms, is by inhibiting an enzyme needed by bacteria in order to synthesize their cell walls. Without this, the cells will die off. However, before we can talk about why antibiotic resistance is an issue, let's talk about how bacteria confer antibiotic resistance in the first place.

Antibacterial resistant genes often occur naturally, or are developed by bacteria through a process of random mutations, though it is important to keep in mind that the bacteria do not choose to develop these traits simply because it is best for them. When bacteria with these genes are subject to antibiotics, they are then put into a position where there is high selective pressure for them. Those that have this gene are more likely to go on to reproduce, passing them on, as those without the gene will die off when subject to antibiotics. When this happens, the frequency of antibiotic-resistant bacteria will increase. Another way for this frequency to increase is through horizontal gene transfer, when bacteria can give these genes to the bacteria around them through a variety of methods, without having to first reproduce.

What's the big deal?

The main takeaway here is that while antibiotic-resistant genes often are a random mutation, this mutated gene will often increase in frequency in the presence of antibiotics due to natural selection. Those with the trait will survive to go on to reproduce more than those who do not have it. As mentioned before, antibiotics are used to treat Salmonella cases that otherwise could have lead to death. If a person who requires antibiotics is infected with an antibiotic-resistant strain of Salmonella, this could be life or death. You may be thinking, if using antibiotics leads to antibiotic resistance, what can we do to prevent this?

Well, we live in an age where antibiotics and antibacterial substances are quite common whether it be medicine, or possibly even antibacterial soap sitting right next to your sink. It is important that we use antibiotics only when necessary, and as prescribed by trained professionals. If you are prescribed antibiotics for infections, make sure to take it for as long as your doctor advises, even if you begin to feel better, in order to make sure the antibiotic can fully run its course and kill off bacteria before they can begin to build resistance. It is time to ditch antibacterial soap, as well, due to recent studies depicting that bacteria exposed to it were often much more likely to survive being subject to antibiotics. It is already a well-known fact that plain soap works just as well as antibacterial soap, as well. If we do our parts in order to minimize antibiotic usage, we can help to keep antibiotic resistance frequency down!

Relation to Geography

As seen in the map above, geography can be an absolutely essential tool in the process of following and investigating foodborne illness outbreaks, and other types of disease. It can be used to map out the path the illness took, and be used to further understand and pinpoint a source of the outbreak as a supplement to other data sources. This was seen through the famous example of John Snow and his efforts during a cholera outbreak. Mapping can help us further understand how both bacteria and illness travel not only from person to person, but through communities, overseas, and all around the planet. It can also help us determine trends and possible future paths of the illness to be utilized in a preventative measure. Geography is a field that can be expanded into so many different lanes, and its applications are only limited by the human imagination.

Works Cited

Gamez, Tony. Instant messenger interview. 10 Dec. 2020.

"Salmonella Infections Linked to Chicken Salad, 2018." Centers for Disease Control and Prevention, US Department of Health and Human Services, 16 Apr. 2018, www.cdc.gov/salmonella/typhimurium-02-18/index.html.

Shmerling, Robert H. "Are Antibacterial Products with Triclosan Fueling Bacterial Resistance?" Harvard Health Publishing, Harvard University, 6 Aug. 2019, www.health.harvard.edu/blog/are-antibacterial-products-with-triclosan-fueling-bacterial-resistance-2019080617473.