Different types of salmonella

Salmonella (non-typhoidal)

The burden of foodborne diseases is substantial: every year almost 1 in 10 people fall ill and 33 million of healthy life years are lost. Foodborne diseases can be severe, especially for young children. Diarrhoeal diseases are the most common illnesses resulting from unsafe food, 550 million people falling ill each year, including 220 million children under the age of 5 years. Salmonella is 1 of the 4 key global causes of diarrhoeal diseases.

Salmonella is a gram negative rods genus belonging to the Enterobacteriaceae family. Within 2 species, Salmonella bongori and Samonella enterica, over 2500 different serotypes or serovars have been identified to date. Salmonella is a ubiquitous and hardy bacteria that can survive several weeks in a dry environment and several months in water.

While all serotypes can cause disease in humans, a few are host-specific and can reside in only one or a few animal species: for example, Salmonella enterica serotype Dublin in cattle and Salmonella enterica serotype Choleraesuis in pigs. When these particular serotypes cause disease in humans, it is often invasive and can be life-threatening. Most serotypes, however, are present in a wide range of hosts. Typically, such serotypes cause gastroenteritis, which is often uncomplicated and does not need treatment, but disease can be severe in the young, the elderly, and patients with weakened immunity. This group features Salmonella enterica serotype Enteritidis and Salmonella enterica serotype Typhimurium, the two most important serotypes of Salmonella transmitted from animals to humans in most parts of the world.

The disease

Salmonellosis is a disease caused by the bacteria Salmonella. It is usually characterized by acute onset of fever, abdominal pain, diarrhoea, nausea and sometimes vomiting.

The onset of disease symptoms occurs 6–72 hours (usually 12–36 hours) after ingestion of Salmonella, and illness lasts 2–7 days.

Symptoms of salmonellosis are relatively mild and patients will make a recovery without specific treatment in most cases. However, in some cases, particularly in children and elderly patients, the associated dehydration can become severe and life-threatening.

Although large Salmonella outbreaks usually attract media attention, 60–80% of all salmonellosis cases are not recognized as part of a known outbreak and are classified as sporadic cases, or are not diagnosed as such at all.

Sources and transmission

  • Salmonella bacteria are widely distributed in domestic and wild animals. They are prevalent in food animals such as poultry, pigs, and cattle; and in pets, including cats, dogs, birds, and reptiles such as turtles.
  • Salmonella can pass through the entire food chain from animal feed, primary production, and all the way to households or food-service establishments and institutions.
  • Salmonellosis in humans is generally contracted through the consumption of contaminated food of animal origin (mainly eggs, meat, poultry, and milk), although other foods, including green vegetables contaminated by manure, have been implicated in its transmission.
  • Person-to-person transmission can also occur through the faecal-oral route.
  • Human cases also occur where individuals have contact with infected animals, including pets. These infected animals often do not show signs of disease.


Treatment in severe cases is electrolyte replacement (to provide electrolytes, such as sodium, potassium and chloride ions, lost through vomiting and diarrhoea) and rehydration.

Routine antimicrobial therapy is not recommended for mild or moderate cases in healthy individuals. This is because antimicrobials may not completely eliminate the bacteria and may select for resistant strains, which subsequently can lead to the drug becoming ineffective. However, health risk groups such as infants, the elderly, and immunocompromised patients may need to receive antimicrobial therapy. Antimicrobials are also administered if the infection spreads from the intestine to other body parts. Because of the global increase of antimicrobial resistance, treatment guidelines should be reviewed on a regular basis taking into account the resistance pattern of the bacteria based on the local surveillance system.

Prevention methods

Prevention requires control measures at all stages of the food chain, from agricultural production, to processing, manufacturing and preparation of foods in both commercial establishments and at home.

Preventive measures for Salmonella in the home are similar to those used against other foodborne bacterial diseases (see recommendations for food handlers below).

The contact between infants/young children and pet animals that may be carrying Salmonella (such as cats, dogs, and turtles) needs careful supervision.

National and regional surveillance systems on foodborne diseases are important means to know and follow the situation of these diseases and also to detect and respond to salmonellosis and other enteric infections in early stages, and thus to prevent them from further spreading.

Recommendations for the public and travellers

The following recommendations will help ensure safety while travelling:

  • Ensure food is properly cooked and still hot when served.
  • Avoid raw milk and products made from raw milk. Drink only pasteurized or boiled milk.
  • Avoid ice unless it is made from safe water.
  • When the safety of drinking water is questionable, boil it or if this is not possible, disinfect it with a reliable, slow-release disinfectant agent (usually available at pharmacies).
  • Wash hands thoroughly and frequently using soap, in particular after contact with pets or farm animals, or after having been to the toilet.
  • Wash fruits and vegetables carefully, particularly if they are eaten raw. If possible, vegetables and fruits should be peeled.
  • A guide on safe food for travellers

Recommendations for food handlers

WHO provides the following guidance for people handling food:

  • Both professional and domestic food handlers should be vigilant while preparing food and should observe hygienic rules of food preparation.
  • Professional food handlers who suffer from fever, diarrhoea, vomiting or visible infected skin lesions should report to their employer immediately.
  • The WHO Five keys to safer food serve as the basis for educational programmes to train food handlers and educate consumers. They are especially important in preventing food poisoning. The five keys to Safer Food are:
    • keep clean
    • separate raw and cooked
    • cook thoroughly
    • keep food at safe temperatures
    • use safe water and raw materials.

  • Five keys to safer food

Recommendations for producers of fruits, vegetables and fish

The WHO Five keys to growing safer fruits and vegetables: promoting health by decreasing microbial contamination and the Five keys to safer aquaculture products to protect public health provide rural workers, including small farmers who grow fresh fruits and vegetables and fish for themselves, their families and for sale in local market with key practices to prevent microbial contamination.

The Five keys to growing safer fruits and vegetables are:

  • Practice good personal hygiene.
  • Protect fields from animal faecal contamination.
  • Use treated faecal waste.
  • Evaluate and manage risks from irrigation water.
  • Keep harvest and storage equipment clean and dry.
  • Five keys to growing safer fruits and vegetables

The Five keys to safer aquaculture products to protect public health are:

  • Practice good personal hygiene.
  • Clean the pond site.
  • Manage water quality.
  • Keep fish healthy.
  • Use clean harvest equipment and containers.
  • Five keys to safer aquaculture products to protect public health

WHO response

In partnership with other stakeholders, WHO is strongly advocating the importance of food safety as an essential element in ensuring access to safe and nutritious diets. WHO is providing policies and recommendations that cover the entire food chain from production to consumption, making use of different types of expertise across different sectors.

WHO is working towards the strengthening of food safety systems in an increasingly globalized world. Setting international food safety standards, enhancing disease surveillance, educating consumers and training food handlers in safe food handling are amongst the most critical interventions in the prevention of foodborne illnesses.

WHO is strengthening the capacities of national and regional laboratories in the surveillance of foodborne pathogens, such as Campylobacter and Salmonella.

  • Global Foodborne Infections Network (GFN)

WHO is also promoting the integrated surveillance of antimicrobial resistance of pathogens in the food chain, collecting samples from humans, food and animals and analysing data across the sectors.

  • WHO Advisory Group on Integrated Surveillance of Antimicrobial Resistance (AGISAR)

WHO, jointly with FAO, is assisting Member States by coordinating international efforts for early detection and response to foodborne disease outbreaks through the network of national authorities in Member States.

  • International Network of Food Safety Authorities (INFOSAN)

WHO also provides scientific assessments as basis for international food standards, guidelines and recommendations developed by the FAO/WHO Codex Alimentarius Commission to prevent foodborne diseases.

  • Codex Alimentarius Commission

OVERVIEW: What every clinician needs to know

Pathogen name and classification

Salmonella spp., which includes the agents of typhoid fever, Salmonella typhi, paratyphi and non-typhoidal serotypes including Salmonella typhimurium and enteritidis, which cause gastroenteritis and focal infections. Prior to 1983 it was believed that these were separate species, hence they each have species names. We now know that these are all part of one species. They are classified as serologic variants based on their antigen variation in lipopolysaccharide and flagella. Though technically incorrect, clinically the species names are still widely used.

What is the best treatment?

  • For typhoid fever, the typical therapy for susceptible strains is an oral fluoroquinolone (ciprofloxicin 500mg or ofloxicin 400mg bid for 5-7 days). Nalidixic resistance has been used as a marker for fluoroquinolone resistance, and higher dose treatment with ciprofloxicin 750mg bid has been successful for these strains. Recently, small numbers of nalidixic acid susceptible, fluoroquinolone resistant strains have been isolated from Africa and Asia, making specific testing for fluoroquinolone resistance essential.

  • A reasonable alternative therapy is oral azithromycin: 1g every day for 5 days or 1g day 1 followed by 6 days of 500mg.

  • Ceftriaxone, cefotaxime, and oral cefixime are also alternatives.

  • Despite susceptibility testing that indicates that S. typhi are susceptible to aminoglycosides and first and second generation cephalosporins, these antibiotics are clinically ineffective and should not be used.

  • Other alternative therapies for susceptible strains include ampicillin 1g qid, amoxicillin 1g TID, chloramphenicol 500mg qid, and trimethoprim-sulfamethoxazole 1 double strength tablet BID for 14-21 days. These agents are inexpensive, but antibiotic resistance and availability limits their use (oral chloramphenicol is not available in the United States).

  • Patients with persistent nausea, vomiting, diarrhea, or altered mental status should be treated with parenteral therapy with a third-generation cephalosporin or fluoroquinolone for at least 10 days or 5 days after fever is resolved.

  • High dose dexamethasone therapy (initial dose 3mg/kg followed by 1mg/kg every 6 hours for 48 hours) should be considered for individuals with shock, obtundation, stupor, or coma.

  • Chronic carriage of S. typhi can be treated with 4-6 weeks of an antibiotic. Antibiotic therapy includes treatment with oral amoxicillin, trimethoprim-sulfamethoxazole, ciprofloxacin, or norfloxacin. Removal of anatomic abnormalities, such as gallstones or kidney stones combined with antibiotic therapy may be required in the less than 20% who fail to have eradication with antibiotic therapy alone.

  • Non-typhoidal Salmonella gastroenteritis is usually self-limited and does not require antibiotic therapy; fluid and electrolyte replacement should be administered. Antimicrobial therapy to prevent bacteremia, which occurs in less than 5% of patients, should be considered for those younger than 3 months of age, those older than 50 years of age who have atherosclerosis because of the risk for endovascular infection, those with prosthetic devices, immunosuppression, arthritis, or other endovascular abnormalities. Such prophylaxis should be continued only during the period a patient with Salmonella gastroenteritis is febrile. Non-typhoidal Salmonella bacteremia requires parenteral therapy and a search for endovascular infection. Individuals with HIV and a first episode of bacteremia should be treated with oral therapy for 6 weeks.

How do patients contract salmonellosis, and how do I prevent spread to other patients?


  • Salmonella typhi and paratyphi can only infect humans and, therefore, can be acquired only from ill individuals or chronic carriers who fecally contaminate food or water. In the United States, all cases are from travelers or outbreaks related to chronic carriers. The greatest risk is in travelers to South-central and Southeast Asia that have the highest incidence of typhoid fever, although Mexico and Haiti represent significant risk. The incidence in the United States is low, with less than 500 cases per year. In contrast, worldwide estimates are approximately 27 million cases of typhoid and paratyphoid fever worldwide. The incidence of typhoid fever increases with poor sanitation and lack of access to quality water supplies. The incidence is greater in endemic regions in children and young adults.

  • The incidence of non-typhoidal Salmonella infections continues to increase worldwide and has doubled during the last 2 decades in the United States. Non-typhoidal Salmonella, of which there are more than 2,000 serotypes, colonize many animals and can be acquired from many food sources, including meat, eggs, poultry, and a variety of fresh and processed foods. Recent outbreaks have included fresh vegetables, frozen dinners, dairy products, peanut butter, and orange juice. The second most common serotype, Salmonella enteritidis, can colonize and penetrate into the developing chicken oviduct and, hence, can be present in the egg yolk of fresh eggs, leading to recommendations to completely cook eggs.

Infection control

  • Hospitalized patients should be maintained on enteric precautions. Transmission to healthcare workers is unusual and low risk, although outbreaks have been reported to be related to contaminated bed linens, noncompliance with barrier precautions, and fecally incontinent institutionalized individuals.

  • Theoretically, with proper dietary precautions, typhoid can be prevented in most travelers. Vaccination is recommended for those traveling to endemic areas at high risk for typhoid fever, including South-central and Southeast Asia, South America, Latin America, Africa, and the Caribbean. Vaccination is not a substitute for good dietary practice, as vaccinated individuals can contract the disease. Two vaccines are commercially available, an oral and a parenteral vaccine are also available. Ty21a is an oral attenuated live vaccine given on sequential days in four doses with a booster after 5 years. It is contraindicated in pregnant women, the immunosuppressed, those on antibiotic therapy, and those younger than 6 years of age. The parenteral Vi polysaccharide vaccine is given IM with a booster every 2 years and is not administered to those younger than 2 years of age because of poor response to this type of vaccine.

  • Anti-infective prophylaxis is not recommended.

What host factors protect against this infection?

  • Both humoral- and cell-mediated immunity are required for protection against Salmonella. Humans with deficiencies in interferon-gamma and IL-12 receptor pathways are particularly susceptible to non-typhoidal Salmonellae. Those who have HIV, have received a transplant, or have a lymphoproliferative disease have increased susceptibility to Salmonellae.

  • HIV-infected individuals, those with low stomach acidity, including those on antacids, residents of nursing homes, neonates, those treated with TNF-alpha antagonists, persons with sickle cell disease (osteomyelitis is a one manifestation), patients who have lymphoproliferative diseases, and transplant recipients are at higher risk. Also, those with underlying TNF/IL-12 receptor defects and neutrophil defects are more susceptible.

  • In non-typhoidal salmonellosis, there is acute neutrophil infiltration of the bowel. This is absent in typhoid fever, and, instead, one sees enlargement of the Peyer’s Patches and recruitment of macrophages and monocytes with lymphoid enlargement and proliferation.

What are the clinical manifestations of infection with Salmonellae?

  • Salmonella typhi and paratyphi cause enteric fever, a syndrome associated with fever and abdominal pain. Non-typhoidal Salmonella cause gastroenteritis. Approximately 5% of patients with gastroenteritis develop bacteremia, which can lead to focal infections, particularly endovascular infections in those with atherosclerotic cardiovascular disease.

What common complications are associated with infection with Salmonellae?

Typhoidal infections can be associated with relapse. The most common complications are intestinal bleeding and perforation. Neurological complications can occur because of meningitis and other poorly understood central nervous system (CNS) phenomena.

Non-typhoidal Salmonella can cause dehydration, and toxic megacolon is a rare complication. Focal infections of the endovascular tissues can result in acute rupture and bleeding.

How should I identify Salmonella species?

  • Typhoid fever is diagnosed by culture of blood, bone marrow, stool, or intestinal secretions. The sensitivity of blood culture alone is low (40-80%). Culture of buffy coat or the lysis centrifugation method for culturing blood can improve sensitivity. Bone-marrow culture and examination should be performed if the diagnosis is strongly suspected. The duodenal string test is also another non-invasive culture method for isolation of S. typhi. Stool cultures in children have higher sensitivity for culture of S. typhi (60%) versus adults (27%). A number of serologic tests for S. typhi have been developed, among which the most widely used is the Widal test, but it is neither sensitive nor specific and is not recommended.

  • Non-typhoidal gastroenteritis is diagnosed by stool culture. Bacteremia and focal infections are diagnosed by blood cultures or direct culture of infected tissue.

  • Salmonellae are Gram-negative bacilli that measure 2-3 by 0.4-0.6 uM in size. They are motile and have typical Enterobacteriaceae characteristics, including fermentation of glucose, reduction of nitrates, and do not produce cytochrome oxidase.

  • Freshly passed stool can be cultured on low selectivity agar medium (MacConkey, deoxycholate) and intermediate selectivity medium (Salmonella-Shigella, xylose-lysine-deoxycholate, or Hektoen) used to screen for enteric pathogens. Increasingly, laboratories use selective chromogenic medium, such as CHROMagar, for primary isolation. In the case of low numbers of organisms, enrichment using tetrathionate and selenite enrichment broths can be used. The organisms can be detected on semi-selective media, such as MacConkey agar, which colorimetrically identify lactose fermentation. Less than 1% of Salmonellae ferment lactose. The differential metabolism of sugars can be used to distinguish Salmonellae serotypes. S. typhi is the only organism that does not produce gas on sugar fermentation. Commercially available polyvalent antisera can be used to identify common serotypes or groups. Because the organism is reportable to public health authorities, it can then be sent to a reference lab for further identification.

  • There are polymerase chain reaction (PCR)-based tests, and some are commercially available; however, none have currently replaced culture methods, and the clinical sensitivity and specificity are not known.

How do Salmonella species cause disease?

  • The pathogen uses intracellular entrance, survival, and replication within the phagosome as a virulence strategy. The replication of bacteria allows innate immune stimulation that leads to inflammation and disease manifestations. The bacteria sense host environments through specific environmental sensors, remodel their surface to resist innate immune killing, and modify the host through delivery of more than 50 virulence proteins using type III secretion systems, which translocate proteins across host membranes to promote bacterial entrance and phagosome remodeling.

  • Reorganization of the host cytoskeleton and delivery of specific effector proteins to the cytoplasm in the intestinal tract stimulates inflammatory responses that lead to neutrophil infiltration and diarrhea in the intestinal tract. In typhoidal Salmonella infection, organisms are predominately intracellular, and this allows for a chronic febrile illness that requires antibiotics with intracellular killing capacity to resolve the infection. Relapsing disease may also occur from intracellular foci.

WHAT’S THE EVIDENCE for specific management and treatment recommendations?

CDC Yellow Book Recommendations for International Travel 2012. 2012. (This is a standard reference from the US Centers for Disease Control and Prevention providing data about health risks for international travelers. It is updated every two years.)

Mandell, GL, Douglas, RG, Bennett, JE, Mandell, Gerald L., Bennett, John E., Dolin, Raphael. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases. 2009. (This print and online reference manual contains detailed information on the diagnosis, treatment and epidemiology of infectious diseases, and it contains more detail on treatment recommendations in the chapter Salmonella Species, Including Salmonella Typhi, 2887-2903.)

Guerrant, RL, Walker, DH, Weller, PF. “Nontyphoidal Salmonellosis”. Tropical Infectious Diseases: Principles, Pathogens and Practice (Expert Consult – Online and Print). 2011. (Contains sections related to treatment, diagnosis and epidemiology, as well as on prevention of infection by the organism.)

What Is Salmonella

Foodborne Salmonella infections cause diarrhea and other symptoms that typically resolve on their own.

Salmonella is a group of bacteria that commonly causes foodborne illness in the United States.

An infection by the bacteria is called salmonellosis (or simply salmonella), and you can get it by consuming contaminated food products including raw poultry, eggs, beef, and sometimes unwashed fruits and vegetables.

Handling pets — particularly reptiles such as snakes, turtles, and lizards — can also spread the bacteria.

Types of Salmonella

Salmonella is divided into two groups:

  • Typhoidal Salmonella, which is made up of bacterial strains that cause typhoid fever or paratyphoid fever, including Salmonella Typhi, Paratyphi A, Paratyphi B, and Paratyphi C
  • Non-typhoidal Salmonella, which includes all other Salmonella strains

In the United States, Salmonella Typhi — the typhoidal Salmonella strain that most commonly causes disease — leads to 5,700 illnesses and 620 hospitalizations each year, according to the Centers for Disease Control and Prevention (CDC).

Most Salmonella Typhi infections seen in the United States originate outside the country.

This bacterial strain causes approximately 21.7 million illnesses each year worldwide, the CDC notes.

Non-typhoidal Salmonella causes about 1.2 million illnesses and 450 deaths each year in the United States. Salmonella Typhimurium and Salmonella Enteritidis are the most common strains.

Salmonella Symptoms

Non-typhoidal Salmonella causes a mild to severe diarrheal illness called acute gastroenteritis.

Symptoms, which typically begin 12 to 72 hours after exposure to the bacteria, can include:

  • Diarrhea, which may be bloody
  • Abdominal cramps
  • Fever
  • Headache
  • Nausea and vomiting
  • Loss of appetite

Typhoid fever causes a sustained fever of 103 to 104 degrees F (39 to 40 degrees C), and sometimes also causes:

  • Weakness
  • Headache
  • Stomach pains
  • Loss of appetite
  • Skin rash

Salmonella infections can cause complications if the bacteria become invasive and affect areas outside of the digestive system, including the bloodstream, bones, joints, central nervous system (brain and spinal cord), or other internal organs.

Salmonella Treatment

Most Salmonella-related gastroenteritis cases resolve within 5 to 7 days without any treatment.

If severe dehydration from diarrhea occurs, administering intravenous fluids may be necessary.

Antibiotic treatment is reserved for severe cases of salmonella, and for infected people with a high risk of complications.

Antibiotic options include:

  • Fluoroquinolones, such as ciprofloxacin
  • Third-generation cephalosporins, such as ceftriaxone, cefepime, or ceftazidime
  • Ampicillin

Salmonella ‘Superbugs’

In recent years, Salmonella “superbugs” — strains of the bacteria that are resistant to the drugs typically used to treat infections — have become a serious concern in the United States, according to the CDC.

About 3 percent of non-typhoidal Salmonella infections appear to be resistant to ceftriaxone, another 3 percent show some resistance to ciprofloxacin, and about 5 percent are resistant to five or more types of drugs, the CDC notes.

Salmonella Typhi is also showing resistance to ceftriaxone, azithromycin, and ciprofloxacin. In fact, 67 percent of Salmonella Typhi infections are drug-resistant, according to the CDC.

Salmonella Infections

  • Larger text sizeLarge text sizeRegular text size

What Is Salmonella?

Salmonella is a kind of bacteria, with many different types. The type responsible for most infections in humans is carried by chickens, cows, pigs, and reptiles (such as turtles, lizards, and iguanas). Another, rarer form — called Salmonella typhi — causes typhoid fever.

What Is Salmonella Infection?

Salmonella infection, or salmonellosis, is a foodborne illness caused by infection with Salmonella bacteria. Most infections spread to people through contaminated food (usually meat, poultry, eggs, or milk).

What Are the Signs & Symptoms of Salmonella Infection?

A Salmonella infection typically causes:

  • nausea and vomiting
  • abdominal cramps
  • diarrhea (sometimes bloody)
  • fever
  • headache

Because many different kinds of illnesses can cause these symptoms, most doctors will take a stool sample to make an accurate diagnosis.

Salmonella infections usually clear up without medical treatment.

How Do People Get Salmonella Infections?

Salmonella bacteria are often found in the feces (poop) of some animals, particularly reptiles. People who have these animals as pets can get salmonellosis if they handle the reptiles and get the bacteria on their hands.

Salmonella can spread to people in foods contaminated by infected animal feces. This can happen when foods such as poultry, eggs, and beef are not cooked enough. Fruit and vegetables can also be contaminated from feces in the soil or water where they’re grown.

Are Salmonella Infections Contagious?

Yes. People with salmonellosis can spread the infection from several days to several weeks after they’ve been infected — even if their symptoms have disappeared or they’ve been treated with antibiotics.

Who Is at Risk for Salmonella Infections?

Not everyone who ingests Salmonella bacteria will become ill. Children, especially infants, are most likely to get sick from it. About 50,000 cases of salmonellosis are reported in the United States each year and about one third of those are in kids 4 years old or younger.

People at risk for more serious complications from a Salmonella infection include those who:

  • are very young, especially babies
  • have problems with their immune systems (such as people with HIV)
  • take cancer-fighting drugs or drugs that affect their immune system
  • have sickle cell disease
  • have an absent or nonfunctioning spleen
  • take chronic stomach acid suppression medicine

In these higher-risk groups, most doctors will treat an infection with antibiotics to prevent it from spreading to other parts of the body. Antibiotics do not appear to help a healthy person whose infection is not severe — and may actually lengthen the amount of time the person will carry the bacteria.

How Are Salmonella Infections Diagnosed?

Because many different illnesses can cause similar symptoms (such as nausea, fever, cramping, and diarrhea), doctors may send a stool (poop) sample to the lab for testing.

A severe Salmonella infection will require more testing to see which specific germ is causing the illness and which antibiotics can be used to treat it.

How Are Salmonella Infections Treated?

If your child has salmonellosis and a healthy immune system, your doctor may let the infection pass without giving any medicines. But any time a child develops a fever, headache, or bloody diarrhea, call the doctor to rule out any other problems.

If your child is infected and has a fever, you may want to give acetaminophen to lower the temperature and relieve cramping. As with any infection that causes diarrhea, it’s important to give your child plenty of liquids to avoid dehydration.

How Long Does a Salmonella Infection Last?

Salmonellosis symptoms can take from 6 to 72 hours to start after someone ingests the bacteria. In most people, the illness lasts for 4 to 7 days after symptoms begin.

Can Salmonella Infections Be Prevented?

Hand washing is a powerful way to guard against Salmonella infections. So teach kids to wash their hands well and often, particularly after trips to the bathroom and before handling food.

Here are some other ways to protect your family from Salmonella infections:

  • Cook food thoroughly. Salmonella bacteria are most commonly found in animal products and can be killed by the heat of cooking. Don’t serve raw or undercooked eggs, poultry, or meat. Microwaving is not a reliable way to kill the bacteria. If you’re pregnant, be especially careful to avoid undercooked foods.
  • Handle eggs carefully. Because Salmonella bacteria can contaminate even intact and disinfected grade A eggs, cook them well and avoid serving poached or sunny-side up eggs (with runny yolks).
  • Avoid foods that might contain raw ingredients. Caesar salad dressing, the Italian dessert tiramisu, homemade ice cream, chocolate mousse, eggnog, cookie dough, and frostings can contain raw eggs. Unpasteurized milk and juices also can be contaminated with Salmonella.
  • Clean cooking surfaces regularly. Keep uncooked meats away from cooked and ready-to-eat foods. Thoroughly wash your hands, cutting boards, counters, and knives after handling uncooked foods.
  • Take care with pets. Avoid contact with the feces of family pets — especially reptiles. Wash your hands well after handling an animal and make sure that no reptiles are permitted to come into contact with a baby. Even healthy reptiles (especially turtles and iguanas) are not safe pets for small children and should not be in the same house as an infant.
  • Don’t cook food for others if you are sick, especially if you have vomiting or diarrhea.
  • Keep food chilled. Don’t leave cooked food out for more than 2 hours after serving (1 hour on a hot day) and store it promptly. Also, keep your refrigerator set to under 40°F (4.4°C).

Reviewed by: Rebecca L. Gill, MD Date reviewed: November 2017

Serotypes and the Importance of Serotyping Salmonella

Medical illustration of non-typhoidal Salmonella

Serotypes are groups within a single species of microorganisms, such as bacteria or viruses, which share distinctive surface structures. For instance, Salmonella bacteria look alike under the microscope but can be separated into many serotypes based on two structures on their surface:

  • The outermost portion of the bacteria’s surface covering, called the O antigen; and
  • A slender threadlike structure, called the H antigen, that is part of the flagella.

The O antigens are distinguished by their different chemical make-up. The H antigens are distinguished by the protein content of the flagella. Each O and H antigen has a unique code number. Scientists determine the serotype based on the distinct combination of O and H antigens.

Different Serotypes

Salmonella have many different serotypes. Some serotypes are only found in one kind of animal or in a single place. Others are found in many different animals and all over the world. Some can cause especially severe illnesses when they infect people; while others cause milder illnesses.

Key Terms

  • The bacteria’s surface are covered with lipopolysaccharide (LPS). The outermost portion of the LPS is the O antigen.
  • Flagella are whip-like tails that bacteria use to move around. Flagella is the whole structure, while the slender threadlike portion of the flagella is called the H antigen.

Some groups of people, such as older adults, people with weakened immune systems, and children under five years old have a higher risk for Salmonella infection. Infections in these groups can be more severe, resulting in long-term health consequences or death.1

More than 2,500 serotypes have been described for Salmonella; but, because they are rare, scientists know very little about most of them. Less than 100 serotypes account for most human infections. What we learn about the more common serotypes can help us better understand illness and the natural history of all the Salmonella strains.

Serotypes and Outbreaks

How do serotyping and serology differ?

Serotyping is a subtyping test based on differences in microbial (e.g., viral or bacterial) surfaces. Serology refers to the antibodies that form because of a viral or bacterial infection. Serotyping is sometimes referred to as serology, but this is technically inaccurate.

Since the 1960s, public health scientists in the US have used serotyping to help find Salmonella outbreaks and track them to their sources. Laboratory experts serotype the Salmonella from infected people. When cases with one serotype increase, they suspect an outbreak and disease detectives start their investigation.

Serotyping has been the core of public health monitoring of Salmonella infections for over 50 years. Now, scientists use DNA testing to further divide each serotype into more subtypes and to detect more outbreaks. With the next generation of sequencing technology, advancements continue as the laboratory can find information about the species, serovar, and subtype of bacteria in just one test. Currently, at least two scientists must generate these three important pieces of information using three separate tests or more.2

Salmonella and Antibiotic Resistance

Resistance to two clinically important drugs, ceftriaxone (a cephem) and ciprofloxacin (a quinolone), has climbed in non-typhoidal Salmonella since 1996. In 2011, about 5% of Salmonella tested by CDC were resistant to five or more types of drugs. 3

CDC has posted a series of interactive graphs that allow users to see the percentage of Salmonella human isolates resistant to various antibiotics tracked, by year, through the National Antimicrobial Resistance Monitoring System (NARMS). This graph includes the option to view all Salmonella isolates or any one of five of the most common serotypes with resistance to antibiotics used to treat Salmonella infections: Enteritidis Cdc-pdf , Typhimurium Cdc-pdf , Newport Cdc-pdf , Heidelberg Cdc-pdf , and I 4,Cdc-pdf,12:i:- .

To search the entire book, enter a term or phrase in the form below
Custom Search
Salmonella and Salmonellosis (page 1)
(This chapter has 5 pages)
© Kenneth Todar, PhD
Salmonella is a Gram-negative facultative rod-shaped bacterium in the same proteobacterial family as Escherichia coli, the family Enterobacteriaceae, trivially known as “enteric” bacteria. Salmonella is nearly as well-studied as E. coli from a structural, biochemical and molecular point of view, and as poorly understood as E. coli from an ecological point of view. Salmonellae live in the intestinal tracts of warm and cold blooded animals. Some species are ubiquitous. Other species are specifically adapted to a particular host. In humans, Salmonella are the cause of two diseases called salmonellosis: enteric fever (typhoid), resulting from bacterial invasion of the bloodstream, and acute gastroenteritis, resulting from a foodborne infection/intoxication.

Discovery of the Typhoid Bacillus

At the beginning of the 19th century, typhoid was defined on the basis of clinical signs and symptoms and pathological (anatomical) changes. However, at this time, all sorts of enteric fevers were characterized as “typhoid”.

In 1880s, the typhoid bacillus was first observed by Eberth in spleen sections and mesenteric lymph nodes from a patient who died from typhoid. Robert Koch confirmed a related finding by Gaffky and succeeded in cultivating the bacterium in 1881. But due to the lack of differential characters, separation of the typhoid bacillus from other enteric bacteria was uncertain.

In 1896, it was demonstrated that the serum from an animal immunized with the typhoid bacillus agglutinated (clumped) the typhoid bacterial cells, and it was shown that the serum of patients afflicted with typhoid likewise agglutinated the typhoid bacillus. Serodiagnosis of typhoid was thus made possible by 1896.

Figure 1. Salmonella typhi, the agent of typhoid. Gram stain. (CDC)

Salmonella Nomenclature

The genus Salmonella is a member of the family Enterobacteriaceae, It is composed of bacteria related to each other both phenotypically and genotypically. Salmonella DNA base composition is 50-52 mol% G+C, similar to that of Escherichia, Shigella, and Citrobacter. The bacteria of the genus Salmonella are also related to each other by DNA sequence. The genera with DNA most closely related to Salmonella are Escherichia, Shigella, and Citrobacter. Similar relationships were found by numerical taxonomy and 16S ssRNA analysis.

Salmonella nomenclature has been controversial since the original taxonomy of the genus was not based on DNA relatedness, rather names were given according to clinical considerations, e.g., Salmonella typhi, Salmonella cholerae-suis, Salmonella abortus-ovis, and so on. When serological analysis was adopted into the Kauffmann-White scheme in 1946, a Salmonella species was defined as “a group of related fermentation phage-type” with the result that each Salmonella serovar was considered as a species. Since the host-specificity suggested by some of these earlier names does not exist (e.g., S. typhi-murium, S. cholerae-suis are in fact ubiquitous), names derived from the geographical origin of the first isolated strain of the newly discovered serovars were next chosen, e.g., S. london, S. panama, S. stanleyville.

Susequently it was found that all Salmonella serovars form a single DNA hybridization group, i.e., a single species composed of seven subspecies, and thenomenclature had to be adapted. To avoid confusion with the familiar names of serovars, the species name Salmonella enterica was proposed with the following names for the subspecies:
enterica I
salamae II
arizonae IIIa
diarizonae IIIb
houtenae IV
bongori V
indica VI
Each subspecies contains various serovars defined by a characteristic antigenic formula.

Since this formal Latin nomenclature may not be clearly understood by physicians and epidemiologists, who are the most familiar with the names given to the most common serovars, the common serovars names are kept for subspecies I strains, which represent more than 99.5% of the Salmonella strains isolated from humans and other warm-blooded animals. The vernacular terminology seems preferred in medical practice, e.g., Salmonella ser. Typhimurium (not italicized) or shorter Salmonella (or S.) Typhimurium.

chapter continued
Next Page

About the author

Leave a Reply

Your email address will not be published. Required fields are marked *