Enterobacterial infections are disorders of the digestive tract and other organ systems produced by a group of gram-negative, rod-shaped bacteria called Enterobacteriaceae. Gram-negative means that the organisms do not retain the violet color of the dye used to make Gram stains. The most troublesome organism in this group is Escherichia coli. Other enterobacteria are species of Salmonella, Shigella, Klebsiella, Enterobacter, Serratia, Proteus, and Yersinia.
Enterobacterial infections can be produced by bacteria that normally live in the human digestive tract without causing serious disease, or by bacteria that enter from the outside. In many cases these infections are nosocomial, which means that they can be acquired in the hospital. Klebsiella and Proteus sometimes cause pneumonia, ear and sinus infections, and urinary tract infections. Enterobacter and Serratia often cause bacterial infection of the blood (bacteremia), particularly in patients with weakened immune systems.
Diarrhea caused by enterobacteria is a common problem in the United States. It is estimated that each person in the general population has an average of 1.5 episodes of diarrhea each year, with higher rates in children, institutionalized people, and Native Americans. This type of enterobacterial infection can range from a minor nuisance to a life-threatening disorder, especially in infants, elderly persons, AIDS patients, and malnourished people. Enterobacterial infections are one of the two leading killers of children in developing countries.
Causes and symptoms
Enterobacterial infections in the digestive tract typically start when the organisms invade the mucous tissues that line the digestive tract. They may be bacteria that are already present in the stomach and intestines, or they may be transmitted by contaminated food and water. It is also possible for enterobacterial infections to spread by person-to-person contact. The usual incubation period is 12-72 hours.
ESCHERICHIA COLI INFECTIONS. E. coli infections cause most of the enterobacterial infections in the United States. The organisms are categorized according to whether they are invasive or noninvasive. Noninvasive types of E. coli include what are called enteropathogenic E. coli, or EPEC, and enterotoxigenic E. coli, or ETEC. EPEC and ETEC types produce a bacterial poison (toxin) in the stomach that interacts with the digestive juices and causes the patient to lose large amounts of water through the intestines.
The invasive types of E. coli are called enterohemorrhagic E. coli, or EHEC, and enteroinvasive E. coli, or EIEC. These subtypes invade the stomach tissues directly, causing tissue destruction and bloody stools. EHEC can produce complications leading to hemolytic-uremic syndrome (HUS), a potentially fatal disorder marked by the destruction of red blood cells and kidney failure. EHEC has become a growing problem in the United States because of outbreaks caused by contaminated food. A particular type of EHEC known as O157:H7 has been identified since 1982 in undercooked hamburgers, unpasteurized milk, and apple juice. Between 2-7% of infections caused by O157:H7 develop into HUS.
The symptoms of enterobacterial infections are sometimes classified according to the type of diarrhea they produce.
WATERY DIARRHEA. Patients infected with ETEC, EPEC, some types of Salmonella, and some types of Shigella develop a watery diarrhea. These infections are located in the small intestine, result from bacterial toxins interacting with digestive juices, do not produce inflammation; and do not usually need treatment with antibiotics.
BLOODY DIARRHEA (DYSENTERY). Bloody diarrhea is sometimes called dysentery. It is produced by EHEC, EIEC, some types of Salmonella, some types of Shigella, and Yersinia. In dysentery, the infection is located in the colon, cells and tissues are destroyed, inflammation is present, and antibiotic therapy is usually required.
NECROTIZING ENTEROCOLITIS (NEC). Necrotizing enterocolitis (NEC) is a disorder that begins in newborn infants shortly after birth. Although NEC is not yet fully understood, it is thought that it results from a bacterial or viral invasion of damaged intestinal tissues. The disease organisms then cause the death (necrosis) of bowel tissue or gangrene of the bowel. NEC is primarily a disease of prematurity; 60-80% of cases occur in high-risk preterm infants. NEC is responsible for 2-5% of cases in newborn intensive care units (NICU). Enterobacteriaceae that have been identified in infants with NEC include Salmonella, E. coli, Klebsiella, and Enterobacter.
The diagnosis of enterobacterial infections is complicated by the fact that viruses, protozoa, and other types of bacteria can also cause diarrhea. In most cases of mild diarrhea, it is not critical to identify the organism because the disorder is self-limiting. Some groups of patients, however, should have stool tests. They include:
- patients with bloody diarrhea,
- patients with watery diarrhea who have become dehydrated,
- patients with watery diarrhea that has lasted longer than three days without decreasing in amount,
- patients with disorders of the immune system.
The patient history is useful for public health reasons as well as helping the doctor determine what type of enterobacterium may be causing the infection. The doctor will ask about the frequency and appearance of the diarrhea as well as other digestive symptoms. If the patient is nauseated and vomiting, the infection is more likely to be located in the small intestine. If the patient is running a fever, a diagnosis of dysentery is more likely. The doctor will also ask if anyone else in the patient's family or workplace is sick. Some types of enterobacteriaceae are more likely to cause group outbreaks than others. Other questions include the patient's food intake over the last few days and whether he or she has recently traveled to countries with typhoid fever or cholera outbreaks.
The most important parts of the physical examination are checking for signs of severe fluid loss and examining the abdomen to rule out typhoid fever. The doctor will look at the inside of the patient's mouth and evaluate the skin for signs of dehydration. The presence of a skin rash and an enlarged spleen suggests typhoid rather than a bacterial infection. If the patient's abdomen hurts when the doctor examines it, a diagnosis of dysentery is more likely.
The most common test that is used to identify the cause of diarrhea is the stool test. Examining a stool sample under a microscope can help to rule out parasitic and protozoal infections. Routine stool cultures, however, cannot be used to identify any of the four types of E. coli that cause intestinal infections. ETEC, EPEC, and EIEC are unusual in the United States and can usually be identified only by specialists in research laboratories. Because of concern about EHEC outbreaks, however, most laboratories in the United States can now screen for O157:H7 with a test that identifies its characteristic toxin. All patients with bloody diarrhea should have a stool sample tested for E. coli O157:H7.
The initial treatment of enterobacterial diarrhea is usually empiric. Empiric means that the doctor treats the patient on the basis of the visible symptoms and professional experience in treating infections, without waiting for laboratory test results. Since the results of stool cultures can take as long as two days, it is important to prevent dehydration. The patient will be given fluids to restore the electrolyte balance and paregoric to relieve abdominal cramping.
Newborn infants and patients with immune system disorders will be given antibiotics intravenously once the organism has been identified. Gentamicin, tobramycin, and amikacin are being used more frequently to treat enterobacterial infections because many of the organisms are becoming resistant to ampicillin and cephalosporin antibiotics.
Alternative treatments for diarrhea are intended to relieve the discomfort of abdominal cramping. Most alternative practitioners advise consulting a medical doctor if the patient has sunken eyes, dry eyes or mouth, or other signs of dehydration.
Herbalists may recommend cloves taken as an infusion or ginger given in drop doses to control intestinal cramps, eliminate gas, and prevent vomiting. Peppermint (Mentha piperita) or chamomile (Matricaria recutita) tea may also ease cramps and intestinal spasms.
Homeopathic practitioners frequently recommend Arsenicum album for diarrhea caused by contaminated food, and Belladonna for diarrhea that comes on suddenly with mucus in the stools. Veratrum album would be given for watery diarrhea, and Podophyllum for diarrhea with few other symptoms.
Dysentery — A type of diarrhea caused by infection and characterized by mucus and blood in the stools.
Empirical treatment — Medical treatment that is given on the basis of the doctor's observations and experience.
Escherichia coli — A type of enterobacterium that is responsible for most cases of severe bacterial diarrhea in the United States.
Hemolytic-uremic syndrome (HUS) — A potentially fatal complication of E. coli infections characterized by kidney failure and destruction of red blood cells.
Necrotizing enterocolitis (NEC) — A disorder in newborns caused by bacterial or viral invasion of vulnerable intestinal tissues.
Nosocomial infections — Infections acquired in hospitals.
Toxin — A poison produced by certain types of bacteria.
The prognosis for most enterobacterial infections is good; most patients recover in about a week or 10 days without needing antibiotics. HUS, on the other hand, has a mortality rate of 3-5% even with intensive care. About a third of the survivors have long-term problems with kidney function, and another 8% develop high blood pressure, seizure disorders, and blindness.
The World Health Organization (WHO) offers the following suggestions for preventing enterobacterial infections, including E. coli O157:H7 dysentery:
- Cook ground beef or hamburgers until the meat is thoroughly done. Juices from the meat should be completely clear, not pink or red. All parts of the meat should reach a temperature of 70°C (158°F) or higher.
- Do not drink unpasteurized milk or use products made from raw milk.
- Wash hands thoroughly and frequently, especially after using the toilet.
- Wash fruits and vegetables carefully, or peel them. Keep all kitchen surfaces and serving utensils clean.
- If drinking water is not known to be safe, boil it or drink bottled water.
- Keep cooked foods separate from raw foods, and avoid touching cooked foods with knives or other utensils that have been used with raw meat.
Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta, GA 30333. (800) 311-3435, (404) 639-3311. http://www.cdc.gov.