Enterobacteriaceae- Definition, Characteristics, Identification

Enterobacteriaceae is a large and diverse family of bacteria that includes many well-known pathogens, such as Escherichia coli, Salmonella, Shigella, and Yersinia. These bacteria are Gram-negative, meaning that they have a thin cell wall that does not retain the purple dye in the Gram stain technique. They are also rod-shaped (bacilli) and usually have flagella that enable them to move. Most of them are facultatively anaerobic, which means that they can grow with or without oxygen. They can also ferment sugars to produce acids and gases, and they can reduce nitrate to nitrite.

Enterobacteriaceae are widely distributed in nature and can be found in soil, water, plants, and animals. Some of them are part of the normal flora of the intestine of humans and other animals, where they help with digestion and vitamin synthesis. However, some of them can also cause infections and diseases in humans and animals, especially when they enter other parts of the body or when they acquire antibiotic resistance or virulence factors. Some of the common diseases caused by Enterobacteriaceae are diarrhea, dysentery, urinary tract infections, septicemia, wound infections, and plague.

Enterobacteriaceae are classified into different genera based on their phenotypic and genotypic characteristics. Some of the criteria used for classification are the presence or absence of certain enzymes (such as indole, urease, oxidase), the ability to ferment different sugars (such as lactose, sucrose, mannitol), the production of gas from glucose fermentation, the motility, and the antigenic structure (such as O, H, and K antigens). Currently, there are more than 30 genera and over 100 species in the Enterobacteriaceae family.

In this article, we will discuss the habitat and ecology, classification, characteristics, pathogenicity, and identification of Enterobacteriaceae.

Enterobacteriaceae are widely distributed in nature and can be found in various habitats and environments. They are mostly associated with the intestinal tract of animals, where they form part of the normal flora or cause infections. However, they are also present in soil, water, plants, and other organic materials.

Some of the factors that influence the habitat and ecology of Enterobacteriaceae are:

• Temperature: Most Enterobacteriaceae are mesophilic, meaning they grow best at moderate temperatures (around 37°C). Some species can tolerate higher or lower temperatures, but they are not common in extreme environments such as hot springs or polar regions.
• Oxygen: Enterobacteriaceae are facultatively anaerobic, meaning they can grow with or without oxygen. They have a respiratory system that allows them to use different electron acceptors, such as nitrate, nitrite, fumarate, or sulfate. They can also ferment sugars and produce acids and gases. Some species can switch between aerobic and anaerobic metabolism depending on the availability of oxygen.
• pH: Enterobacteriaceae prefer neutral to slightly alkaline pH (around 7.0 to 8.0). They can tolerate some acidity, but they are not found in highly acidic environments such as the stomach or vinegar. They can also adapt to changes in pH by regulating their internal pH or producing buffers.
• Salinity: Enterobacteriaceae are not halophilic, meaning they do not require high salt concentrations for growth. They can grow in low to moderate salinity (up to 5% NaCl), but they are inhibited by high salinity (above 10% NaCl). They are found in freshwater, seawater, and brackish water sources.
• Moisture: Enterobacteriaceae need water for growth and survival. They are not xerophilic, meaning they do not tolerate dry conditions. They can survive in moist environments such as soil, decaying vegetation, or organic matter. Some species can form biofilms or capsules that protect them from desiccation.
• Hosts: Enterobacteriaceae have a wide range of hosts and interactions with other organisms. They can be commensals, parasites, pathogens, symbionts, epiphytes, endophytes, or saprophytes. They can colonize the intestinal tract of humans and animals (such as mammals, birds, reptiles, fish, insects) and cause diseases such as diarrhea, dysentery, septicemia, urinary tract infections, wound infections, etc. They can also infect plants (such as crops, fruits, vegetables) and cause diseases such as soft rot, wilt, blight, canker, etc. Some species have beneficial relationships with plants (such as nitrogen fixation) or insects (such as endosymbiosis).

Enterobacteriaceae are versatile and adaptable bacteria that can thrive in different habitats and environments. They play important roles in the ecology and health of humans and animals. However, they can also pose serious threats to public health and agriculture due to their pathogenicity and antibiotic resistance.

Enterobacteriaceae is a large and diverse family of Gram-negative rod-shaped bacteria that belongs to the order Enterobacterales of the class Gammaproteobacteria in the phylum Pseudomonadota. The family was first proposed by Rahn in 1936, and since then it has undergone several revisions based on phenotypic, genotypic, and phylogenetic studies.

The current classification of Enterobacteriaceae includes 68 genera and 355 species, according to a recent study by Adeolu et al. (2020) that combined multiple sources of evidence, such as DNA sequences, DNA homology, 16S rRNA gene sequences, and phenotypic properties. Some of the well-known genera in this family are:

• Escherichia
• Salmonella
• Shigella
• Klebsiella
• Enterobacter
• Citrobacter
• Proteus
• Yersinia
• Serratia
• Erwinia

The classification of Enterobacteriaceae is still a subject of debate, as new species and genera are being discovered and reclassified based on molecular methods. Some genera, such as Cronobacter, Raoultella, and Rosenbergiella, were previously considered as members of other genera, such as Enterobacter, Klebsiella, and Citrobacter, respectively. Moreover, some species within the same genus may have different ecological niches, pathogenicity, and antibiotic resistance profiles.

One of the main challenges in the classification of Enterobacteriaceae is the high level of genetic diversity and horizontal gene transfer among its members. This can result in incongruence between different molecular markers and phenotypic characteristics. For example, some species of Enterobacteriaceae may have different flagellar antigens (H antigens) depending on the environmental conditions or host interactions. Therefore, a polyphasic approach that integrates multiple sources of information is recommended for the accurate identification and classification of Enterobacteriaceae.

Enterobacteriaceae are a large and diverse family of Gram-negative rod-shaped bacteria that share some common features. Some of the general characteristics of Enterobacteriaceae are:

• They are facultatively anaerobic, which means they can grow in the presence or absence of oxygen.
• They are non-sporing, which means they do not form endospores to survive harsh conditions.
• They are mostly motile with peritrichous flagella, which are hair-like structures that surround the cell and enable movement. However, some species in the genera Klebsiella and Shigella are non-motile.
• They are catalase positive and oxidase negative, which means they can produce the enzyme catalase to break down hydrogen peroxide, but they cannot produce the enzyme oxidase to reduce oxygen.
• They can reduce nitrate to nitrite, which is a chemical reaction that indicates the presence of nitrate reductase enzyme.
• They can produce acid from glucose fermentation, which is a metabolic process that converts glucose into organic acids and gases.
• They have characteristic antigens that are used for identification and classification. These include enterobacterial common antigens (ECA), which are present in all members of the family, and outer membrane (O), flagella (H), and capsule (K) antigens, which vary among different species and strains.

In addition to these common features, Enterobacteriaceae also have some specific characteristics that distinguish them from other bacteria. Some of these are:

• They can be divided into lactose fermenters and non-lactose fermenters based on their ability to utilize lactose as a carbon source. Lactose fermenters produce acid and gas from lactose, while non-lactose fermenters do not. Examples of lactose fermenters are Escherichia coli, Enterobacter, and Klebsiella, while examples of non-lactose fermenters are Salmonella, Shigella, and Proteus.
• They can be classified into different serogroups based on their O, H, and K antigens. Serogroups are groups of bacteria that share the same antigenic structure and can elicit a specific immune response. For example, E. coli O157:H7 is a serogroup of E. coli that has O157 as its O antigen and H7 as its H antigen.
• They can produce different types of toxins that contribute to their pathogenicity. Toxins are substances that can harm the host cells or tissues by interfering with their normal functions. For example, Shiga toxin is a toxin produced by some strains of Shigella and E. coli that can damage the intestinal lining and cause bloody diarrhea.
• They can acquire different types of antibiotic resistance mechanisms that make them difficult to treat. Antibiotic resistance is the ability of bacteria to survive or grow in the presence of antibiotics that would normally kill them or inhibit their growth. For example, some Enterobacteriaceae can produce beta-lactamases, which are enzymes that break down beta-lactam antibiotics such as penicillins and cephalosporins.

These are some of the main characteristics of Enterobacteriaceae that make them an important group of bacteria in microbiology, medicine, and agriculture.

Enterobacteriaceae are a diverse group of bacteria that can cause various infections in plants, animals, and humans. Some species are part of the normal flora of the intestine, while others are opportunistic or professional pathogens that can invade different sites of the body.

Enterobacteriaceae can cause plant diseases such as soft rot, wilt, canker, leaf blight, brown rot, blackleg, and others. These diseases affect many crops such as cotton, cucumber, rice, banana, maize, oak, walnut, apple, papaya, etc. Some of the plant pathogens in this family are Pectobacterium, Dickeya, Erwinia, Pantoea, Brenneria, Gibbsiella, and Lonsdalea.

In animals and humans, Enterobacteriaceae can cause gastrointestinal disorders such as diarrhea, dysentery, colitis, and hemorrhages. They can also cause urinary tract infections (UTIs), respiratory tract infections (RTIs), bacteremia, septicemia, wounds and ulcers, systemic infections in internal organs, and other conditions. Some of the most common animal and human pathogens in this family are Escherichia coli, Klebsiella spp., Salmonella spp., Shigella spp., Proteus spp., Morganella spp., Erwinia spp., Serratia marcescens, Citrobacter spp., and Yersinia spp. E. coli, K. pneumoniae, and P. mirabilis account for more than 80% of Enterobacteriaceae isolates from clinical samples.

The pathogenicity of Enterobacteriaceae depends on several factors such as the host susceptibility, the bacterial virulence factors, and the antimicrobial resistance mechanisms. Enterobacteriaceae have various virulence factors that enable them to adhere to host cells, invade tissues, evade host defenses, produce toxins and enzymes, and form biofilms. Some of these factors are pili (fimbriae), flagella (motility), capsules (antiphagocytic), lipopolysaccharide (LPS) (endotoxin), siderophores (iron acquisition), plasmids (gene transfer), secretion systems (effector delivery), and toxins (cytotoxicity).

Enterobacteriaceae are also notorious for their ability to acquire resistance to multiple antimicrobial agents through various mechanisms such as mutation, gene transfer (plasmids or transposons), enzyme production (β-lactamases), membrane impermeability (porin loss or modification), efflux pumps (active drug extrusion), and target modification (ribosomal or DNA gyrase alteration). Some of the resistance genes are located on mobile genetic elements that can be transferred between different species of Enterobacteriaceae or even to other bacteria. This poses a serious threat to public health as it limits the treatment options and increases the risk of treatment failure and mortality.

Enterobacteriaceae are among the most common causes of nosocomial outbreaks and infections associated with healthcare settings. They are also emerging as community-acquired pathogens that can affect healthy individuals. Therefore, it is important to identify them accurately and monitor their susceptibility patterns to guide appropriate therapy and infection control measures.

Enterobacteriaceae are a large and diverse family of Gram-negative rod-shaped bacteria that can be found in various habitats and cause various infections in humans and animals. To identify the different genera and species of Enterobacteriaceae, microbiologists use a combination of molecular, biochemical, and immunological methods.

Molecular methods

Molecular methods are based on the analysis of the genetic material of the bacteria, such as DNA or RNA. These methods can provide accurate and rapid identification of Enterobacteriaceae, especially for new or rare species that may not be detected by conventional methods. Some examples of molecular methods are:

• Polymerase chain reaction (PCR): This technique amplifies a specific region of DNA using primers and enzymes. The amplified DNA can then be detected by gel electrophoresis, hybridization, or sequencing. PCR can target genes that encode for specific enzymes, antigens, or resistance factors of Enterobacteriaceae.
• DNA homology: This technique compares the degree of similarity between the DNA sequences of different bacteria. The higher the similarity, the more closely related they are. DNA homology can be measured by DNA-DNA hybridization, restriction fragment length polymorphism (RFLP), or multilocus sequence typing (MLST).
• 16S rRNA sequencing: This technique sequences the 16S ribosomal RNA gene, which is present in all bacteria and has conserved and variable regions. The variable regions can be used to distinguish different genera and species of Enterobacteriaceae. 16S rRNA sequencing can also reveal the phylogenetic relationships among Enterobacteriaceae.

Biochemical methods

Biochemical methods are based on the detection of the metabolic activities of the bacteria, such as enzyme production, carbohydrate fermentation, or gas formation. These methods can provide presumptive identification of Enterobacteriaceae based on their characteristic biochemical profiles. Some examples of biochemical methods are:

• IMViC test: This test consists of four reactions: indole production from tryptophan, methyl red test for mixed acid fermentation, Voges-Proskauer test for 2,3-butanediol fermentation, and citrate utilization as a sole carbon source. Different combinations of these reactions can differentiate between Escherichia coli, Enterobacter aerogenes, Klebsiella pneumoniae, and other Enterobacteriaceae.
• TSI test: This test uses a triple sugar iron agar that contains glucose, lactose, sucrose, iron salts, and phenol red indicator. The bacteria are inoculated into a slant and a butt of the agar and incubated for 24 hours. The results are interpreted based on the color change of the slant and the butt (red or yellow), the gas production (cracks or bubbles), and the hydrogen sulfide production (black precipitate). The TSI test can differentiate between lactose fermenters and non-fermenters, glucose fermenters and non-fermenters, and hydrogen sulfide producers and non-producers among Enterobacteriaceae.
• Commercial kits: Many commercial kit systems are available to identify Enterobacteriaceae. Two of these commercial systems are the Enterotube and the API20E. Even though these systems are miniaturized and compact, they still test for bacterial enzymes to identify the bacteria. The Enterotube contains 12 compartments with different substrates that test for 15 biochemical reactions. The API20E contains 20 microtubes with different substrates that test for 21 biochemical reactions. The results are recorded as positive or negative and compared with a code book or a computer database to obtain an identification.

Immunological methods

Immunological methods are based on the detection of the antigens or antibodies of the bacteria using specific reagents or kits. These methods can provide serological identification of Enterobacteriaceae based on their characteristic antigens. Some examples of immunological methods are:

• Agglutination tests: These tests use latex particles or bacterial cells coated with antibodies that react with specific antigens on the surface of Enterobacteriaceae. The reaction results in visible clumping or agglutination that indicates a positive result. Agglutination tests can detect O (somatic), H (flagellar), or K (capsular) antigens of Enterobacteriaceae.
• ELISA tests: These tests use enzyme-linked antibodies that bind to specific antigens on the surface or in the culture supernatant of Enterobacteriaceae. The bound antibodies are then detected by adding a substrate that produces a color change in the presence of the enzyme. ELISA tests can detect O (somatic), H (flagellar), K1 (capsular), or Shiga toxin antigens of Enterobacteriaceae.

In summary, identification of Enterobacteriaceae can be achieved by using various methods that target different aspects of their biology. Molecular methods can provide accurate and rapid identification based on genetic analysis; biochemical methods can provide presumptive identification based on metabolic profiles; and immunological methods can provide serological identification based on antigenic properties.

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