Foodborne Illness Part 3: How Does Salmonella Make Us Sick?
Question: What do turtles, casino brunches and psychoactive herbs have in common? (Feel free to get creative and imagine a particularly eventful outing for a herpetology enthusiast club.)
Answer: all are potential sources of Salmonella infection that have been associated with recent outbreaks in the U.S.. While we mostly think of eggs and poultry products as carriers of Salmonella, pets, livestock, produce, and soil can also spread infection.
Salmonella are gram-negative bacteria and common causes of gastrointestinal illness. Infection typically manifests as severe stomach cramps, fever, and diarrhea that can last several days, though other disease presentations, such as sepsis, can also occur. Many of us are unfortunately familiar with the misery that ensues upon eating spoiled food, but what actually happens inside of our bodies during Salmonella infection? What is it about Salmonella that can make us so sick? Salmonella uses a variety of unique virulence mechanisms to invade our intestinal cells and confuse our immune cells, leading to many of the key symptoms associated with food poisoning.
Salmonella Nomenclature: Species, Serotypes, and Typhoid Disease
Before discussing Salmonella pathogenesis in more detail, it’s important to understand the complex and distinct ways in which different strains of Salmonella are characterized. Though there are only 2 species of Salmonella (S. enterica and S. bongori), there are more than 2,500 serotypes, which are groups of related bacteria with similar antigen presentation, across both species. Salmonella enterica is further broken up into 6 subspecies based upon genetic similarity. Of subspecies I, II, IIIa, IIIb, IV, and VI, only members of subspecies I are human pathogens. Despite the large number of identified strains, fewer than 100 Salmonella strains are suspected to be pathogenic.
Though pathogenic serotypes can be very genetically similar, they can have different abilities to cause severe disease. Some serotypes cause a few days of mostly self-limiting illness, whereas others cause life-threatening complications. It is not well understood how specific serotypes alter host immune response and modulate bacterial virulence.
The serotype is defined by the immunoreactivity of distinct molecular patterns on bacterial cell surfaces, namely the “O antigen” and the “H antigen” which are the primary antigens present in many coliform bacteria. The “O” antigen is part of the lipopolysaccharide layer of the bacterial membrane, whereas the “H” antigen is located on the organism’s motile tail, known as the flagella. Many molecular variations in the “O” and “H” antigens exist, allowing for a wide range of different serotype combinations. The “H “antigen on the flagella can be described as “phase 1” or “phase II,” which refers to which key flagellar genes (fliC and fliB) are being expressed. The gold-standard system for serotyping Salmonella strains is the Kauffman-White scheme, a complex alphanumerical system that separates “O” antigens into alphanumerical categories (ex. A, B, C1), “H phase 1” antigens into a-z and z1-z99 categories, and numbers “H phase 2” antigens from 1-12. However, the Center for Disease Control and Prevention (CDC) and many scientists refer to different serotypes using a shortened, simplified nomenclature, and refer to strains in a “genus-species-serotype” format (e.g., S. enterica serovar typhimurium).
Serotypes can be further divided into typhoid and non-typhoid serotypes, based on their ability to cause typhoid or paratyphoid fever, a type of severe S. enterica infection that spreads from person-to-person. Typhoid and paratyphoid strains are typed using O and H antigens, but also by their Vi (Virulence) antigens. The Vi antigen is a capsular antigen that contributes significantly to virulence, which may partially explain the disparity in disease severity between typhoid and non-typhoid strains. Typhoid Salmonella infections are more likely to be life-threatening, causing high fevers, headaches, constipation or diarrhea, and rose spots—patches of red discoloration on the skin where bacterial emboli are present.
Typhoid fever remains a global public health threat, but it is not common in the United States and other developing countries. The disease can be prevented by the typhoid vaccine. The primary serotypes associated with non-typhoid, foodborne gastroenteritis are S. enterica serovar typhimurium and S. enterica serovar enteridis, which are also the most prevalent S. enterica serotypes found in poultry products. In contrast to typhoid fever, which affects only 5,700 Americans each year, foodborne Salmonella gastroenteritis is estimated to cause 1 million illnesses per year.
Let’s assume you ate a questionable, undercooked omelette for breakfast and accidentally exposed yourself to a pathogenic, non-typhoidal S. enterica serovar typhimurium strain—what happens next? Salmonella prefers to replicate and infect host cells intracellularly. Once Salmonella is ingested, it invades the epithelial cells of the intestine, as well as nearby phagocytic immune cells. Salmonella uses a variety of dynamic techniques to impair and confuse host immune cells, including its ability to induce phagocytosis in certain white blood cells, which allows the organism to gain entry into cells more effectively.
Salmonella undermines non-phagocytic immune cells too, by inducing reactive oxygen species (ROS) production from human neutrophils. This defense mechanism is intended to protect the host by damaging bacterial nucleic acids and proteins. However, Salmonella benefits from ROS production, because it has an arsenal of peroxidases and catalases to help it survive ROS exposure. Other resident gut microbes are less equipped to survive this harsh environment, thus creating a selective advantage for Salmonella.
Once inside the host cell, Salmonella divides rapidly, and can either enclose itself within membrane-bound vacuoles, or as was recently discovered, replicate within the cytosol of cells. Salmonella’s preference to replicate in vacuoles versus in the cytosol possibly depends upon flagellar motility. Cytosolic Salmonella have more active flagella than those within vacuoles, and have the ability to extrude epithelial cells, meaning that infected epithelial cells that form a membrane, such as those within the intestinal lining, can “squeeze” out of their membrane layer and wander, allowing the infected cells to potentially spread to other organ sites. Cytosolic Salmonella also divide very quickly and have the ability to hyper-replicate in intestinal epithelial cells, gallbladder epithelial cells, and polarized epithelial cells that mimic the internal environment of the intestine. This adaptability to new environments may suggest that the cytosolic subpopulations of Salmonella have the ability to leave and survive outside of the intestine, potentially allowing them to spread to other body sites, leading to much more serious and invasive illness.
Fortunately, the human immune system is equipped to fight Salmonella’s invasive and evasive maneuvers. While innate immune cells, such as neutrophils and macrophages, can be fooled by Salmonella’s immune evasive strategies, adaptive immune cells are vital for host defense. When Salmonella is introduced to the GI tract orally, such as through contaminated food or kissing your cute pet hedgehog, Salmonella-specific immune cells, such as CD4+ T cells, are generated to help fight the invading pathogen. These cells later develop into memory cells following clearance of the infection, which provides some long-term protection against Salmonella. But because different serotypes are recognized by immune cell interaction with their unique patterns, unrelated serotypes can cause a subsequent bout of disease.
Other adaptive immune cells, including CD8+ T cells and B cells, also play important roles in fighting off Salmonella infections, with the strongest evidence pointing towards the protective role of CD8+ T cells during later stages of infection. CD8+ T cells are specialized cells that can target intracellular pathogens with the help of B cells, which can serve as antigen-presenting cells that help CD8+ T cells to recognize Salmonella-specific antigens and mount an inflammatory immune response to help clear infection. Production of interferon-gamma, a pro-inflammatory cytokine produced by CD4+ and CD8+ T cells, has demonstrated importance for controlling infection in intestinal epithelia. However, even these savvy adaptive cells aren’t foolproof: Salmonella can suppress T cell and B cell responses by impairing effector cell priming to blunt the necessary inflammatory response.
While the immune system is always hard at work, the best ways to prevent Salmonella infection are through careful handwashing, sanitary food preparation, and proper handling of pets and livestock. If you are traveling abroad, it’s always a good idea to check whether typhoid fever is a potential risk at your destination. Extensively drug-resistant (XDR) typhoid, in particular, is an important issue in certain countries. Pakistan is currently experiencing an XDR typhoid outbreak, which began in 2016 and has sickened 5,200 people, including travelers from the US and UK. This type of typhoid infection is resistant to most antibiotics, making it very difficult to treat. Do your hard-working T cells and B cells a favor, especially in extreme cases such as these – get your recommended vaccinations and use proper hygiene to prevent serious Salmonella infections.