Enterobacteriaceae- Definition, Characteristics, Identification

Enterobacteriaceae is a family of Gram-negative rod-shaped (bacilli) bacteria in the order Enterobacterales of the class Gammaproteobacteria in the phylum Pseudomonadota.

They are commonly found in the intestine of animals, hence got the name “enterobacteria” or “enteric bacteria”. It is the most widely studied bacteria family. It contains several economically important and commensal bacteria; although some of them are pathogenic to humans, plants, and animals and can develop a fatal infection.

Enterobacteriaceae Habitat/Ecology

They are cosmopolitan in distribution; mostly reside in the intestine of animals. However, they are found in soil, decaying vegetation and organic matters, water sources, sewage, and other environmental aspects. Some species have an epiphytic, endophytic, or saprophytic relationship with different species of plants. They are mostly commensals and parasites, while some are saprophytes too. Most of them are mesophilic, hence they are not found in extremely cold and extremely hot environments. They are found scarcely in high salinity, xerophilic (extremely dry), and extreme pH areas.

Enterobacteriaceae Classification

Earlier before 1980, 12 genera with 36 species have been listed in the Enterobacteriaceae family in the Bergey’s Manual of Determinative Bacteriology 8^th edition. By 1994, the family was revised with 30 genera and 107 species in the Bergey’s Manual of Determinative Bacteriology 10^th edition. But recently in 2020, based on combined results of phenotypic properties, DNA sequences and DNA homology, phylogenetic findings, and 16S rRNA gene sequence study results, 68 genera with a total of 355 species have been proposed in this family.

Some of the well-studied genera in this family are:

Enterobacteriaceae Characteristics

Some of the common characteristics of Enterobacteria are:

1. Gram-negative
2. Rod-shaped (usually short bacilli; about 1 – 5 μm) 
3. Facultatively anaerobic
4. Non-sporing
5. Mostly motile with peritrichous flagella, ( most species in the genera Klebsiella and Shigella are non-motile)
6. Catalase positive, oxidase negative
7. Both lactose fermenter and non-fermenter
8. Usually, acid-producing
9. Usually, nitrate-reducing
10. Characteristic antigens are known as enterobacterial common antigens (ECA). The outer membrane (O), flagella (H), and capsule (K) are antigens in most the enterobacteria.

Enterobacteriaceae Pathogenicity

A large number of species in the Enterobacteriaceae family are a part of the normal flora of the intestine of humans and animals.    However, they are frequently reported as a pathogen of plants, insects, and other animals including humans. 

Pectobacterium, Dickeya, Erwinia, Pantoea, Brenneria, Gibbsiella, and Lonsdalea species are pathogenic to plants. They cause diseases like soft rot, wilt, canker, leaf blight, brown rot, blackleg, etc. in several varieties of crops like cotton, cucumber, rice, banana, maize, oak, walnut, apple, papaya, etc.

In animals, although they are normal gut flora, they cause gastrointestinal disorders like diarrhea, dysentery, mammary gland infection, hemorrhages, respiratory tract infections (RTIs), colonitis, etc.

Enterobacteria are the major cause of urinary tract infections (UTIs) in humans. Besides, they cause respiratory tract infections (RTIs) (both lower and upper RTIs), bacteremia, septicemia, wounds and ulcers, gastrointestinal tract infections and disorders, systemic infections in internal organs, etc. Some species are well-known human pathogens, and even newer species in Enterobacteriaceae are emerging as human pathogens.

Some most common human pathogens in this family are:
E. coli, Klebsiella spp., Salmonella spp., Shigella spp., Proteus spp., Morganella spp., Erwinia spp., Serratia marcescens, Citrobacter spp., Yersinia spp.
E. coli, K. pneumoniae, and P. mirabilis account for more than 80% of Enterobacteria species isolated from clinical samples.

Enterobacteriaceae Identification

Like other bacteria, Enterobacteria can be identified by using either molecular diagnosis (PCR, DNA homology, 16s rRNA sequencing) system, biochemical tests, or antigen/antibody detection. The most commonly followed method used by microbiologists is a series of biochemical tests. Routinely followed biochemical tests are:

• IMViC test (Indole, MR–VP, Citrate tests)
• TSI (triple sugar fermentation) test
• Lactose fermentation test
• Urease test
• Motility test
• Hydrogen sulfide (H₂S) formation
• Catalase test
• Oxidase test

Read Also: Enterobacteriaceae Cultural Characteristics

Biochemical Tests for Identification of Common Enterobacteriaceae

Enterobacteriaceae Significances

Positive significances

• Coliform group, especially E. coli, is used as the indicator organism for the determination of the microbial quality of water, beverages, dairy, and other food products.
• Most of the species are the normal flora of the intestine and other parts.
• E. coli is a model organism in several scientific studies.

Negative significances

• Some species are human pathogens. E. coli, Klebsiella spp., Proteus spp., and Salmonella spp. are among the most common human pathogens.
• Enterobacteria are also responsible for several plants, and animal diseases.
• Some are responsible for the spoilage of milk and milk products, foods, vegetables, fruits, juices, and beverages. 

Antibiotic Resistance in Enterobacteriaceae

Enterobacteria are the most commonly encountered pathogenic bacteria in clinical cases and the environment. Hence, they are highly influenced by the intake of antibiotics for disease management or in agriculture and the environment. The scenario of antibiotic resistance among the members of Enterobacteriaceae is worst.

Initially, beta-lactam antibiotics like derivatives of penicillin and the 1^st or 2^nd generation cephalosporins were considered effective against them. However, in the present situation due to the development of resistance against the Beta-Lactam antibiotics, the treatment option is very costly and limited. ESBL (extended-spectrum beta-lactamase), ABL (AmpC beta-lactamase), and other types of beta-lactamase-producing Enterobacteriaceae are responsible for several cases of human infection all around the world. Carbapenems also referred to as the last line of antibiotics, are also slowly losing their effectiveness due to the evolution of carbapenem’s resistance Enterobacteria.  Carbapenems resistance genes are frequently encountered in several pathogenic species in the family.

Drug resistance is increasing rapidly in E. coli, Klebsiella spp., Salmonella spp., Shigella spp., and Citrobacter spp. These resistance genes in these bacteria can be transferred horizontally among other members of Enterobacteriaceae, hence causing the rapid spread of resistance genes.

References

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2. Janda, J. M., & Abbott, S. L. (2021). The Changing Face of the Family Enterobacteriaceae (Order: “Enterobacterales”): New Members, Taxonomic Issues, Geographic Expansion, and New Diseases and Disease Syndromes. Clinical microbiology reviews, 34(2), e00174-20. https://doi.org/10.1128/CMR.00174-20
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