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The gram negative bacteria Salmonella typically cause diarrhea and sometimes cause a more serious infection, typhoid fever.

  • People are usually infected when they eat contaminated food, such as undercooked chicken or eggs.
  • The bacteria usually infect the digestive tract but can travel through the bloodstream and infect other parts of the body.
  • People have nausea and crampy abdominal pain, followed by watery diarrhea, fever, and vomiting.
  • Identifying the bacteria in a sample, usually of stool, confirms the diagnosis.

Lost fluids are replaced.

  • Antibiotics are usually not helpful for people who have Salmonella intestinal infections, but antibiotics are helpful for people who are at risk of or have bacteremia.

Salmonella bacteria cause several types of infection. Most often, these bacteria cause gastroenteritis, but they sometimes cause typhoid fever, a more serious infection.

There are over 2,500 different types of Salmonella bacteria.

Some Salmonella reside only in people. Other species of Salmonella normally reside in the digestive tract of many wild and domestic animals, such as cattle, sheep, pigs, fowl, and reptiles (including snakes, lizards, and turtles). Many of these can cause infections in people.

Salmonella bacteria are excreted in the feces of infected animals and people, leading to contamination. In the United States during the 1970s, many infections were spread by pet turtles, so their sale was prohibited, resulting in fewer infections. Recently, the legal and illegal sale of pet reptiles has increased. Up to 90% of pet reptiles and amphibians, such as aquatic frogs, are infected with Salmonella.

People are infected usually by eating undercooked poultry or eggs but sometimes by eating undercooked beef and pork, unpasteurized dairy products, or contaminated seafood or fresh produce. Salmonella bacteria can infect the ovaries of hens and thus infect the egg before the egg is laid. Other foods may be contaminated by animal feces (for example, in slaughterhouses) or by infected food handlers who do not adequately wash their hands after using a toilet. People can also become infected if they drink contaminated water. Other reported sources of infection include infected pet turtles and reptiles and contaminated marijuana.

Because stomach acid tends to destroy Salmonella, a large number of these bacteria must be consumed for infection to develop, unless people have a deficiency of stomach acid. Such a deficiency may occur in

  • Children under 1 year old
  • Older people
  • People taking antacids or drugs that inhibit stomach acid production, including histamine-2 (H2) blockers (such as famotidine) or proton pump inhibitors (such as omeprazole)

Salmonella bacteria cause inflammation of the intestine (gastroenteritis) and thus are a common cause of diarrhea.

Symptoms of Salmonella Infections

When the intestine is infected, symptoms usually start 12 to 48 hours after the bacteria are ingested. Nausea and crampy abdominal pain occur, soon followed by watery diarrhea, fever, and vomiting. Salmonella symptoms resolve in 1 to 4 days. Occasionally, symptoms are more severe and last a long time.

Long after symptoms are gone, a few people continue to excrete the bacteria in their stool. Such people are called carriers.

About 10 to 30% of adults develop reactive arthritis weeks to months after diarrhea stops. This disorder causes pain and swelling, usually in the hips, knees, and Achilles tendon (which connects the heel bone and calf muscle).

Other symptoms may develop if bacteremia develops and infection spreads. For example, if a bone is infected, the area over it is often tender or painful. If a heart valve is infected, people may feel short of breath. If the aorta is infected, the back and abdomen may be painful.

People usually recover well. Exceptions are people who had a disorder, particularly one that weakens the immune system, before the Salmonella infection or who have a complication due to the infection.

Diagnosis of Salmonella Infections

Culture of stool, pus, blood, or a sample from the rectum

To diagnose a Salmonella infection, doctors take a sample of stool, pus, or blood or use a swab to obtain a sample from the rectum. The sample is sent to a laboratory where bacteria, if present, can be grown (cultured). Identifying the bacteria in the sample confirms the diagnosis.

Bacteria are also tested to see which antibiotics are effective (a process called susceptibility).



Cholera is an acute diarrheal illness caused by infection of the intestine with Vibrio cholerae bacteria. People can get sick when they swallow food or water contaminated with cholera bacteria. The infection is often mild or without symptoms, but can sometimes be severe and life-threatening.

About 1 in 10 people with cholera will experience severe symptoms, which, in the early stages, include:

  • profuse watery diarrhea, sometimes described as “rice-water stools”
  • vomiting
  • thirst
  • leg cramps
  • restlessness or irritability

Health care provider should look for signs of dehydration when examining a patient with profuse watery diarrhea. These include:

  • rapid heart rate
  • loss of skin elasticity
  • dry mucous membranes
  • low blood pressure

People with severe cholera can develop severe dehydration, which can lead to kidney failure. If left untreated , severe dehydration can lead to shock, coma, and death within hours.

The profuse diarrhea produced by cholera patients contains large amounts of the infectious Vibrio cholerae germ that can infect others if swallowed. This can happen when the bacteria get on food or into water.

To prevent the bacteria from spreading, all feces (human waste) from sick persons should be thrown away carefully  to ensure it does not contaminate anything nearby.

People caring for cholera patients must wash their hand thoroughly after touching anything that might be contaminated with patients’ feces (poop).

When cholera patients are treated quickly, they usually recover without long-term consequences. Cholera patients do not typically become carriers of the cholera bacteria after they recover, but they get sick if exposed again.



Shigella infection (shigellosis) is an intestinal infection caused by a family of bacteria known as shigella. The main sign of shigella infection is diarrhea, which often is bloody.

Shigella is very contagious. People get infected with shigella when they come in contact with and swallow small amounts of bacteria from the stool of a person who is infected with shigella. For example, this can happen in a child care setting when staff members don’t wash their hands well enough after changing diapers or helping toddlers with toilet training. Shigella bacteria can also be passed in infected food or by drinking or swimming in unsafe water.

Children under age 5 are most likely to get shigella infection, but it can occur at any age. A mild case usually clears up on its own within a week. When treatment is needed, doctors generally prescribe antibiotics.


Signs and symptoms of shigella infection usually begin a day or two after contact with shigella. But it may take up to a week to develop. Signs and symptoms may include:

  • Diarrhea (often containing blood or mucus)
  • Stomach pain or cramps
  • Fever
  • Nausea or vomiting

Symptoms generally last for about five to seven days. In some cases, symptoms may last longer. Some people have no symptoms after they’ve been infected with shigella. However, their feces may still be contagious up to a few weeks.



Pertussis is a highly communicable disease occurring mostly in children and adolescents and caused by the gram-negative bacterium Bordetella pertussis. Symptoms are initially those of nonspecific upper respiratory infection followed by paroxysmal or spasmodic coughing that usually ends in a prolonged, high-pitched, crowing inspiration (the whoop). Diagnosis is by nasopharyngeal culture, polymerase chain reaction testing, and serologic assays. Treatment is with macrolide antibiotics.

Pertussis is endemic throughout the world. Its incidence in the US cycles every 3 to 5 years. Pertussis occurs only in humans; there are no animal reservoirs.

Transmission is mainly via droplets of respiratory secretions that contain B. pertussis (a small, nonmotile, gram-negative coccobacillus) from infected patients, particularly during the catarrhal and early paroxysmal stages. The infection is highly contagious and causes disease in ≥ 80% of close contacts. Transmission by contact with contaminated articles is rare. Patients are usually not infectious after the 3rd week of the paroxysmal phase.


Pertussis is a vaccine-preventable childhood disease that is increasing in incidence. In the US, the case rate in the 1980s was at an all-time low of about 1/100,000 population, which, by 2014, increased to about 10/100,000. The 2019 surveillance report by the Centers for Disease Control and Prevention (CDC) reported an incidence of 5.7/100,000 (1). The increase since the 1980s is due to

  • Immunity waning in previously vaccinated adolescents and adults
  • Parents refusing to vaccinate their children

Respiratory complications, including asphyxia in infants, are most common. Otitis media occurs frequently. Bronchopneumonia (common among older people) may be fatal at any age.

Seizures are common among infants but are rare in older children.

Hemorrhage into the brain, eyes, skin, and mucous membranes can result from severe paroxysms and consequent anoxia. Cerebral hemorrhage, cerebral edema, and toxic encephalitis may result in spastic paralysis, intellectual disability, or other neurologic disorders.



Diphtheria is a contagious, sometimes fatal infection of the upper respiratory tract caused by the gram positive, rod-shaped bacteria Corynebacterium diphtheriae. Some types of Corynebacterium diphtheriae release a potent toxin that can damage the heart, kidneys, and nervous system.

  • Diphtheria is caused by a bacterial infection that is now rare in developed countries.
  • Typical symptoms include a sore throat, general feeling of illness, and fever, sometimes with swollen lymph nodes, and a tough, gray pseudo membrane forms in the throat.
  • The diagnosis is based on symptoms, particularly a sore throat and the pseudo membrane, and the results of a culture.
  • Vaccination can help prevent this infection.
  • People are hospitalized and given antibiotics to eliminate the infection.

Symptoms of Diphtheria

  • The illness typically begins a few days (an average of 5 days) after exposure to the bacteria. Diphtheria symptoms then begin over a few days, with sore throat, painful swallowing, hoarseness, a general feeling of illness (malaise), and a low-grade fever (about 100.4 to 102° F, or 38 to 38.9° C). Children may also have a fast heart rate, nausea, vomiting, chills, and a headache. The lymph nodes in the neck may swell (called bull neck). The inflammation may make the throat swell, narrowing the airway and making breathing extremely difficult.
  • pseudo membrane forms near the tonsils or other parts of the throat. This membrane is a tough, gray sheet of material made by the bacteria. It is composed of dead white blood cells, bacteria, and other substances. The pseudo membrane narrows the airway. The roof of the mouth may be paralyzed. When inhaling, the pseudo membrane may cause people to make a noisy gasping sound. Also, the pseudo membrane may extend into the windpipe or airway or suddenly become detached and block the airway completely. As a result, people may be unable to breathe.


  • The toxin produced by certain types of diphtheria bacteria sometimes affects certain nerves, particularly those to the muscles of the face, throat, arms, and legs, causing symptoms such as difficulty swallowing or moving the eyes, arms, or legs. The diaphragm (the most important muscle used for inhaling) may become paralyzed, sometimes causing respiratory failure. These symptoms take weeks to resolve. Toxin effects on nerves can result in rapid heart rate, an abnormal heart rhythm, and low blood pressure. The bacterial toxin may also cause inflammation of the heart muscle (myocarditis), sometimes leading to abnormal heart rhythms, heart failure, and death.
  • Severe infection can also damage the kidneys.
  • If diphtheria affects only the skin, it causes what look like scrapes (abrasions) and sores that vary in appearance. These sores appear on the arms and legs and resemble other skin disorders, such as eczema, psoriasis, and impetigo. A few people have open sores that do not heal. Sores may be painful and red and may ooze.



The causative organisms of human brucellosis are B. abortus (from cattle), B. melitiness (from sheep and goats), and B. suis (from hogs). B. canis (from dogs) has caused sporadic infections. Generally, B. melitensis and B. suis are more pathogenic than other Brucella species.

The most common sources of infection are farm animals and raw dairy products. Deer, bison, horses, moose, caribou, hares, chickens, and desert rats may also be infected; humans can acquire the infection from these animals as well.

Brucellosis is acquired by

  • Direct contact with secretions and excretions of infected animals
  • Ingesting undercooked meat, raw milk, or milk products containing viable organisms
  • Inhaling aerosolized infectious material
  • Rarely, person-to-person transmission

Most prevalent in rural areas, brucellosis is an occupational disease of meatpackers, veterinarians, hunters, farmers, livestock producers, and microbiology laboratory technicians. Brucellosis is rare in the US, Europe, and Canada, but cases occur in the Middle East, Mediterranean regions, Mexico, and Central America and in travelers to these areas.

Because very few organisms (perhaps as few as 10 to 100) may cause infection via aerosol exposure, Brucella species are potential agents of biological terrorism.

Patients with acute, uncomplicated brucellosis usually recover in 2 to 3 weeks, even without treatment. Some go on to subacute, intermittent, or chronic disease.