Today we will explore the fundamentals of salmonella, what it is, how it can be classified, its microbiology, metabolism, and how infection occurs.
Salmonella first came to our understanding in the 1800s when the bacteriologist Karl Eberth noticed it in the Peyer’s patches and spleen of patients that had contracted typhoid.
It was only 4 years later when Georg Gaffky was able to successfully grow the pathogen into a pure culture.
It was initially thought to be the causative agent of hog cholera, and the name salmonella was not used until around the 1900s.
What Is Salmonella?
Salmonella can be considered a group of gram-negative bacteria that is most well known to cause food poisoning and also gastrointestinal tract infections.
Depending on the strain all depends on how the treatment is delivered.
Some salmonella infections can be treated fairly effectively with antibiotics, however some strains of the bacteria have been shown to resist antibiotics and can therefore be considered deadly to the human body.
This is why so much care and attention is put into preparing food such as raw chicken and eggs, and it is recommended that it is cooked thoroughly to avoid contracting salmonella.
The- two major species of salmonella are S. bongori and S. enterica.
Classification Of Salmonella
If we Now explore the genus of salmonella, the first thing to identify it is that it is actually closely related to E. Coli bacteria and research suggests that it may have diverged from this bacteria around 150 million years ago.
Due to this fact, it has therefore adapted over time and studies have shown that it can be found in several niches within the environment.
There are several methods of classification of salmonella, and whilst this is the case the fact is that no single method or approach can be agreed on unanimously.
According to the Center for Disease Control (CDC), The following list of the most recent classifications which have been on the recommendation by the World Health Organization (WHO):
- Domain Is the bacteria as bacteria, and salmonella is prokaryotes that contain a simple cell structure however they do lack membrane-bound organelles.
- Order enterobacterial are gram-negative rods (Bacillus) that move by using flagella. These do not form endospores or microcysts.
- Family Enterobacteriaceae is the only family in Order enter antibacteriales and this group is composed of rod-shaped gram-negative bacteria.
- Genus is salmonella
- Species that have already been described are S. bongori and S. enterica.
- Subspecies S.bongori is one that has a single subspecies. This can be referred to as Subspecies V.
Within this classification, there are also subspecies of Salmonella enterica which are the following:
- Enterica I
- Salamae II
- Arizona IIIa
- Arizona IIIb
- houtenae IV
- Indica VI
And in addition to the listed subspecies, You can also find various serotypes of salmonella that have been suggested to be in the range of 2202 around about 4400 serotypes or serovar.
One final point to make on classification is that serotype is based on cell surface antigens.
Serotypes (Using The Kauffman Classification)
When we refer to serotypes, epidemiologic classification of organisms can be based on a number of factors which include the morphology, structure, as well as mode of reproduction, and ecology to name a few.
Based on these determining factors, it is, therefore, possible to identify species in which the organisms can be classified.
There is a species of microorganisms that you commonly know such as bacteria, via all curry similarities that means that they can be grouped a species, although their surface antigens may vary and therefore a subspecies level classification needs to be introduced.
This is commonly referred to as a serotype, in which a group of organisms within a species have the same type as a number of surface antigens.
Now if we take a look at the salmonella serotypes, bacteria for these have been shown to possess three types of antigen.
These are known as antigen H (flagella antigen), antigen O (somatic antigen), and Vi (capsular). These Three antigens will play an integral role when it comes to the grouping of serotyping which will be explored now.
Antigen O is composed of lipopolysaccharides and occurs on the outer membrane which is typically determined by the sugar sequence.
Antigen H includes proteins that are found on the flagella of the bacteria. When considering Antigen H, the antigen occurs as either phase one or phase two, and in some cases, both will occur.
While occurring in one of these types, exhaustion has been found that organisms have been shown to change from one phase to the other.
Studies continue and as of now have identified over 18 serovars within this particular classification.
Antigen VI is typically found in a few serovars and is known as a superficial antigen. This means it overlies the o-antigen.
Because of this fact, It is an additional antigen and can be found in such organisms as Salmonella typhi and Salmonella paratyphi C. In these, it plays an important role in confirming serotype determination.
Salmonella is a well-known bacteria that includes several syndromes and can cause several symptoms. Among the gastroenteritis, enteric fever, septicemia, focal infection, and can be an asymptomatic carrier state.
One thing to note is that any stereotype can produce any of the syndromes listed above. For general purposes, it is worth noting that more serious infections occur in infants and adults that are over the age of 50.
Also, Individuals with debilitating illnesses may be prone to more serious infections.
The most common way salmonella enters the body is through contaminated food and the subsequent ingestion of it.
Another column method that is not as well publicized is that person-to-person spreading of salmonella can occur.
In order to be fully pathogenic, salmonellae must have a variety of attributes which are called virulence factors which include the following:
- The ability to invade cells
- a complete lipopolysaccharide coat
- the ability to replicate intracellularly
- the elaboration of toxins
Once salmonella has been ingested the organisms will colonize the gastrointestinal tract, namely the ileum and colon, as well as invade the intestinal epithelium and proliferate within the epithelium and lymphoid follicles.
This is done via binding to specific receptors within the epithelial cell surface. Following this is invasion which occurs by the organism inducing the enterocyte membrane and stimulating endocytosis of the organisms.
One thing to note about this invasion is that it is dependent on the rearrangement of the cell cytoskeleton and research likely suggests that it involves increases in cellular inositol phosphate and calcium.
Attachment and invasion can be looked at from a genetic control perspective and involves multiple genes in both chromosomes and plasmids.
Once it has Invaded the epithelium, this is the moment the organism multiplies intracellularly and then spreads to lymph nodes and throughout the body via circulation.
The reticuloendothelial system combines and will do its work to control the spread of the bacteria.
However, depending on the serotype as well as the host’s defense, an infection may spread to the liver, spleen, gallbladder, phones, and other organs.
The good news is most serovars are eradicated properly within extraintestinal sites and the most common infection of salmonella which is gastroenteritis, remains confined to the gastrointestinal tract, mainly the intestine.
Evolution And Its Ecological Niche
As per the ongoing scientific research, salmonella has evolved from E Coli through genetic alterations for well over 150 million years. This has resulted in the alteration of the genetic pathogen ecology.
As there are 2300 genetically and phenotypically diverse serotypes, this particular bacteria has been changed to be capable of infecting a plethora of hosts and not just humans.
Salmonella, like many bacteria, has evolved to adapt and survive in a wide variety of habitats and environments, which means that they are referred to as environmental salmonellae.
Here is a list of some of the ecological niches in which you will find different species of salmonella:
Certain species can survive in water, such as S. enteritidis.
You can find many serotypes found in lakes, rivers, and small streams, and when we take into consideration their lifespan, this will ultimately depend on a number of factors which include the temperature of the water, how much oxygen is present, whether there is a high amount of contamination from things such as animal faces or carcasses, and one other factor is if there is any other competing Flora.
Some species have been shown to reproduce in warm temperatures over colder climates, along with contamination via animal faces which have the potential to provide a rich source of nutrients.
In this particular example, you may find certain species such as frogs compact as carriers, which act as a secondary role in the spread of the bacteria.
Birds And Wild Animals
Speaking of spreading salmonella via hosts, birds and wild animals are other common carriers of salmonella and the level of salmonella being carried will be determined by the species of bird.
One example is that pigeons can carry around 17% of salmonella, whereas pigeons that are breeding have been shown to carry high levels of the bacteria.
In comparison, wild ducks typically carry much fewer levels than other species of birds.
With transmission via Birds, most of the transfer of bacteria is simply via a vector role, in which bacteria is simply spread from one side to the next rather than transmission via one species to the next.
So rather than a human picking up salmonella from a bird, it’s more likely the bird transferred the salmonella to a surface in which the humans contaminated themselves.
Something to also note is that wild animals at even zoo animals can be sources of exotic and rare serovars.
Similar to birds, the level of bacteria carried will largely depend on the type of animal and we have a chat in which they live.
For example, birds and wild animals will have a much higher rate of carrying than reptiles.
This spreads to domestic animals that work in an agricultural capacity and can easily be a contributing Factor to contamination.
Especially when you take into consideration the fact that they are around human food a lot of the time, and so common animal types such as poultry are a large source of salmonella.
Some of the most popular species can be found in animal feeds, dairy food, as well as aquatic Flora.
You can find serovar such as S. paratyphi B To thrive and multiply in sewage around about 10 degrees Celsius.
Some studies suggested that bacteria in these types of conditions are not attributed to an animal source, and instead live freely in this particular habitat.
If sewage ever gets into healthy bodies such as rivers, streams, lakes, seas, and soil; There’s no denying salmonella can rapidly spread and multiply.
Because of this, we can begin to see how Salmonella can spread via animals which send and act as a carrier to spread to other areas, in which humans can then become susceptible and then become contaminated.
Other Van host specificity there are several factors at play that show an important role in The successful infection of a host. These are the following:
Similar to many gram-negative bacteria, salmonella species like Salmonella typhi will produce endotoxin which is a toxic substance produced when an outer membrane of the organism is fundamentally disrupted.
This process will increase the chance of salmonella infection as well as inflammation at the site of infection.
There are various Strains that have been shown to process a capsule as the outermost envelope.
This capsule formation is one of the most important roles in the overall survival of a bacteria, and the reason for this is that they are really difficult to remove.
This has resulted in the resistance to certain treatments and the association with prolonged infection.
Other than capsule formations, salmonella has also been shown to produce outer membrane proteins that enable them to be survived in macrophages.
Other than capsules that have the propensity to protect the salmonella and increase the chances of survival, some strains of salmonella produce adhesives that enable bacteria to remain stuck to surfaces of the affected site.
These are known as asfimbrtials (and non-fimbrials.)
Adhesives reinforce making the bacterial strain more enhanced.
Flagella enables salmonella to move through the intestinal Nikos from one site to another.
Because of its sporadic nature, salmonella is notoriously difficult to always diagnose and around 60 to 80% of cases can go undiagnosed.
In 2010, around 93.8 million cases of gastroenteritis were due to a salmonella infection, and the estimated number of deaths due to salmonella was approximately 155000.
2014 data suggested that children under 4 were around 32 2 to to 80 times more likely to have a salmonella infection, as well as those who are more susceptible are pregnant women, winning the, and those with deficient immune systems.
The epidemiology of typhoid type fever typically involves person-to-person spread as these organisms lack a significant animal Reservoir. Fever and non-typhoidal salmonella, two factors I have big significance.
One of those factors is that around 3% of those infected with salmonella typhi can become carriers, and this carrier state lasts for many weeks, and in some cases may even last year’s. However one thing to note is that children rarely become typhoid carriers.
This makes salmonella a major public health problem.
Diagnosing salmonella will require bacteriology isolation of the organisms. Laboratory identification of salmonella is achieved by biochemical tests and serologic testing is carried out.
This can be tested via types of blood, feces, or another type of specimen. Biochemical reactions of suspicious colonies can be determined using triple sugar iron agar and lysine iron agar.
Salmonella is what are known as facultative anaerobes which are capable of fermenting on sugars, mannitol,, and sorbitol.
In terms of Characteristics, the majority of salmonella bacteria will typically fall into one of these categories:
- Has the capability to grow aerobically or anaerobically meaning that it can grow in the presence of oxygen. What makes them tricky is that whilst they are are are capable of using oxygen for respiration, they can also survive anaerobically via anaerobic respiration. This is carried out by fermenting organic compounds, however, the fermentative pathway is the final electron acceptor in this process.
- Typically prefer using oxygen for a greater yield of energy with respiration. However recent studies have shown that when salmonella is in the presence of fermentable substances, a good number of bacteria will prefer fermentation. In this instance, sugar has been shown to repress respiratory enzymes while minimizing respiration in the absence of sugar and other non-fermentable substances, an increase of enzymes from a respiratory perspective will enhance respiration
- When comparing anaerobiosis; the breakdown rate of sugar is much smaller.
If there’s one thing we can be certain about, it’s that salmonella is notoriously difficult to eradicate from the environment.
However, there are many ways to reduce the likelihood of infection from a control perspective. One of the most ways we become infected is through salmonella harbored in poultry and livestock.
Limiting salmonella in these animals would go a long way to reducing total human exposure across the globe.
One way this is carried out is by treating Animal Feeds that aim to kill the route of the salmonella before the animal is distributed, which has shown a marked reduction in total salmonella cases.
This method is seen in such countries as Denmark.
Along with treating the feed of animals, you can also look at the slaughtering process and reducing cross-contamination via animal carcasses, and looking at hygiene practices with handling them can show a marked improvement in the reduction of salmonella.
There are also some less common methods that are considered more controversial.
For example, radiation of poultry produce contamination is an uncommon method that is not yet widely accepted, though the fact remains this technology would indeed vastly decrease the rate of salmonella.
From a virology perspective, vaccines are available for things such as typhoid fever and may prove to be somewhat effective, especially in those with susceptible populations like young children and adults over 50.
Although no vaccine is available for non-typhoidal salmonella. Research continues into this area of infection and understanding the mechanism of immunity will continue to be studied.
Prevention And Treatment
As we’ve already established, there are a number of ways that salmonella can enter the human body as well as spread in different habitats.
Keeping the process as simple as possible, let’s assume that salmonella infects the body as a result of either drinking contaminated water, eating uncooked poultry or meat, or consuming contaminated foods such as vegetables.
The simplest solution for the prevention of salmonella is to practice good food preparation and cooking.
Ensuring that all foods are washed before cooking as well as sufficiently cooking foods such as meat, poultry, and eggs, will help to eradicate any potential bacteria.
Another good practice is to ensure that you wash your hands before any food preparation or cooking.
We also discussed that animals have the potential to spread salmonella.
And whilst the majority of people will not be handling animals in a zoo or stroking wild ducks or pigeons, care is still needed with animals, especially people who own pets.
Ensuring ample hygiene is carried out which will include washing hands with water and soap and being mindful of keeping hands clean will be a useful preventative measure against salmonella.
One thing to bear in mind is if you have children in the house. The data also shows that maintaining a healthy routine of washing their hands will ensure that salmonella infection is prevented.
If, however, the infection does occur then one of the most common methods of treatment is the use of antibiotics as well as anti-motility drugs.
Along with following a course of drugs, it is recommended that replacing fluids and electrolytes be a key component of the treatment process.
Another common drug used in those that suffer cramping with salmonella is loperamide. However, one thing tonight with this drug is that it can have problematic side effects such as diarrhea.
Understanding salmonella and how it affects the human body continues to be a researched topic that progresses overtime.
As we learn more about salmonella and protective mechanisms that help to reduce total infection in humans, we hope to see someone’s love to be decreased over time.
All hope is not lost if a human contracts salmonella, and depending on the population type and the severity of the condition, will depend on treatment options and how long it will take to heal with a 100% recovery.
Frequently Asked Questions
How Is Typhoid Fever Caused?
Typhoid fever is typically caused by salmonella invading the bloodstream in its typhoidal form which will then spread throughout the body and can invade many of the body’s organs.
With this, it can secrete endotoxin in its septic form. Because of this it can be a life-threatening condition that can lead to septic shock which will require intensive care and the administration of antibiotics.
It is important to seek professional help should you suspect someone has typhoid fever.
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