Enterobacteria is the name given to a family of bacteria to which numerous different species belong. Sometimes they are part of the natural intestinal flora, but they can also cause various diseases.
What are enterobacteria?
Enterobacteriaceae is the collective name for different species of bacteria. They are mainly found in the intestines of humans and animals. Some species of rod-shaped bacteria are responsible for the development of serious diseases such as typhoid fever, the plague or dysentery. Other subforms, which include Escherichia coli, only cause infections when they enter regions of the body where they do not belong. This may involve the urinary tract or the eyes. Enterobacteria are classified in the class of Gammaproteobacteria and belong to the division (Divisio) of Proteobacteria. There they form a separate bacterial family. The term enterobacteria comes from the ancient Greek term “enteron”, which translated means intestine. Thus, numerous enterobacteria are considered typical inhabitants of the intestine. Nevertheless, many free-living bacteria that are not located in the intestine are also considered to belong to the Enterobacteriaceae family.
Occurrence, distribution, and characteristics
A large number of the enterobacteria live in the intestines of humans and animals. However, they can also be found in the environment in water or soil. Among the most important representatives of the Enterobacteriaceae is Escherichia coli, which is also known as the coliform bacterium. Other important genera are Proteus such as Proteus mirabilis and Proteus vulgaris, Klebsiellen such as Klebsiella pneumoniae, Salmonella, Shigella, Cronobacter, Citrobacter, Enterobacter, Erwinia as well as Edwardsiella. Among the most feared representatives are Yersinia, as Yersinia pestis can cause plague. Most enterobacteria are gram-negative anaerobic. They have the appearance of a rod and reach a length between 1 and 5 µm. Their diameter is about 0.5 to 1 µm. Enterobacteriaceae do not have oxidase, which makes them easy to distinguish from other bacteria. Most species are equipped with flagella, which allow them mobility. However, some Enterobacteriaceae genera do not have motility. Enterobacteriaceae are considered gram-negative because their cell wall is composed of several layers of murein as well as a second outer membrane. The metabolism of enterobacteria is facultatively anaerobic. This means that they can break down substances via oxidation in the presence of oxygen. Without oxygen, in turn, fermentation is possible. Two anaerobic metabolic pathways are used to distinguish the individual genera. These are mixed acid fermentation and 2,3-butanediol fermentation. Mixed acid fermentation results in by-products and end products such as acids. These primarily include lactic acid, succinic acid and acetic acid. In contrast, butanediol is absent. In 2,3-butanediol fermentation, fewer quantities of acid result from the fermentation process. Instead, larger amounts of the alcohol 2,3-butanediol are formed. The characteristics of 2,3-butanediol fermentation also include the intermediate product acetoin. Furthermore, there is a significantly higher production of CO2 (gas). Typically, butanediol fermentation occurs in enterobacteria such as Klebsiella, Serratia, Erwinia as well as Enterobacter. In contrast, mixed acid fermentation occurs in Proteus, Escherichia coli and Salmonella. The cell surface of Enterobacteriaceae contains antigens by which they can be identified and subdivided. Thus, there are F, H, K, and O antigens, among others.
Significance and function
Some enterobacteria, such as Escherichia coli, are a natural part of the human intestinal flora. The first strains of bacteria colonize the human body shortly after birth. Until adulthood, countless representatives of the Enterobacteriaceae cavort in the large and small intestine, whereby considerably more bacteria are present in the large intestine than in the small intestine. The intestinal flora plays a significant role in the defense against pathogens. Thus, the microorganisms residing there exert influence on the vitamin supply, support digestion, stimulate intestinal peristalsis and supply the epithelial layer of the intestine with energy.
Diseases and ailments
However, enterobacteria can also cause a variety of diseases. For example, enterotoxic Escherichia coli, Salmonella, and Shigella are often the triggers of diarrheal diseases.In contrast, enterohemorrhagic Escherichia coli (EHEC) and Yersinia can cause inflammation of the intestine (enteritis) accompanied by bloody diarrhea. Diarrhea caused by salmonella is mostly due to spoiled food. Often, vomiting sets in at the same time. Typical sources of infection are pork, beef and poultry as well as mayonnaise, raw eggs and creams. Urinary tract infections are mostly caused by Escherichia coli, Proteus, Serratia, Klebsiella, Morganella, Citrobacter and Providencia. In bladder infections (cystitis), the bacteria rise from the intestine via the urethra into the urinary bladder. About 80 percent of all bladder infections are caused by Escherichia coli. The female sex is particularly affected by the disease. The urethra is shorter in women than in men. For this reason, the bacteria only have to travel a shorter distance. Furthermore, the female urethral opening is located closer to the anus than in the male sex. Bacterial pneumonia is often caused by enterobacteria such as Klebsia. It is caused by the Klebsiella pneumoniae subform. Also of concern are Yersinia species such as Yersinia enterocolitica, Yersinia pseudotuberculosis and Yersinia pestis. They cause inflammation of the large and small intestines (enterocolitis), inflammation of the lymph nodes (lymphadenitis) and plague. This highly contagious disease was feared in ancient times as the “Black Death” because it claimed numerous victims. In modern times, however, the plague occurs only very rarely. It is divided into bubonic plague, pneumonic plague and plague sepsis.