The Enterobacteriaceae are complex and diverse group of microorganisms. They are named for their frequent location in the digestive tracts of mammals - including humans - and other animals, such as insects (Tortora et al. 2007).
However, the presence of these bacteria is not restricted to the animal world, they have also been found as pathogens in plants (Cabello, 2007), soil and even in water (Olivas, 2001).
Escherichia coli
According to technical terminology, they are considered "bacilli", a term that refers to the elongated, straight and thin bar shape of these organisms. In addition, they are gram-negative bacteria, which indicates that their cell wall is thin and with a double membrane rich in different types of lipids (Tortora et al. 2007).
From a clinical point of view, there are certain species of Enterobacteriaceae that cause disease in humans, so they have been exhaustively studied. However, not all are pathogenic.
For example, Escherichia coli is one of the most common inhabitants of the mammalian gut, and certain strains are beneficial. In fact, E.coli is capable of producing vitamins and excluding other harmful microorganisms from the intestine (Blount, 2015).
General characteristics
Enterobacteriaceae are free-living bacteria, they do not form spores and are of intermediate size, measuring from 0.3 to 6.0 µm in length and 0.5 µm in diameter. The optimum temperature for its growth is 37 ° C. They are facultative anaerobic, that is, they can live in environments with oxygen or do without it.
Some have flagella (a projection that resembles a whip and is used for movement), while others have no structures for locomotion and are totally immobile.
In addition to flagella, these bacteria generally have a series of shorter appendages known as fimbriae and pilis. Although the appearance of both resembles a hair, they differ in their functions.
The fimbriae are structures used to adhere to the mucosa, while the sexual pili allow the exchange of genetic material between two organisms, serving as a kind of bridge for this process (Tortora et al. 2007).
While it is true that bacteria do not undergo sexual reproduction, this event allows for the exchange of DNA. This new DNA molecule acquired by the recipient bacteria allows it to develop certain characteristics, such as resistance to a particular antibiotic.
This is known as horizontal gene transfer, is common in most bacteria, and has medically relevant implications.
It is typical of some Enterobacteriaceae to be surrounded by an additional layer composed of polysaccharides. This is called a capsule and has the K antigens (Guerrero et al., 2014).
Classification
The Enterobacteriaceae family consists of about 30 genera and approximately more than 130 species, biogroups, and enteric groups. However, the number may vary slightly depending on the author who established the taxonomic ordering.
The classification of these microorganisms is based on determining the presence or absence of certain key enzymes belonging to different metabolic pathways. Likewise, other principles are included to establish the ordering of the group, such as: serological reactions, susceptibility or resistance to certain antibiotics.
Historically, the taxonomic category of tribe was used in the classification of Enterobacteriaceae. This included the Escherichieae, Edwardsielleae, Salmonelleae, Citrobactereae, Klebsielleae, Proteeae, Yersinieae, and Erwiniaeae tribes.
However, according to different authors, this view is already obsolete and has been discarded. Despite this change, the taxonomy of this group has been the subject of arduous debate (Winn, 2006).
In recent years, DNA sequencing and hybridization techniques have made it possible to establish a more precise classification of the organisms that make up this heterogeneous family.
Within the classification and nomenclature of Enterobacteriaceae, the most prominent genera of the group can be mentioned: Escherichia, Shigella, Klebsiella, Yersinia, Enterobacter, Serratia, Hafnia, Proteus, Morganella, Providencia, Citrobacter, Edwardsiella and Salmonella.
Biochemical tests
Biochemical tests are essential in the laboratory to identify pathogens in humans as well as in soil and food. The response of microorganisms to different biochemical reactions yields a characteristic that helps their typing.
The most important characteristics of the metabolism of this family of bacteria include:
-The ability to reduce nitrates to nitrites, a process called denitrification (there are some exceptions such as Pantoea agglomerans, Serratia and Yersinia).
-Ability to ferment glucose.
-Negativity to the oxidase test, positive to the catalase test, and neither pectate nor alginate liquefied (Gragera, 2002; Cullimore, 2010; Guerrero et al., 2014).
- Likewise, some of the pathogenic Enterobacteriaceae do not ferment lactose.
Among the most common tests for the identification of these microorganisms are: production of acetyl-methyl-carbinol, test of methyl red, production of indole, use of sodium citrate, production of sulfuric acid, hydrolysis of gelatin, hydrolysis of urea and fermentation of glucose, lactose, mannitol, sucrose, adonitol, sorbitol, arabinose, among other carbohydrates (Winn, 2006; Cabello, 2007).
The tests that are considered to have the greatest power to discern between the identity of bacteria are: indole production, lysine decarboxylase, H2S and ornithine decarboxylase (García, 2014).
epidemiology
Enterobacteriaceae are the causative agents of various pathologies. Among the most common are urinary tract infections, pneumonia, septicemia, and meningitis. Although the production of the infection depends mainly on the state of the patient's immune system.
Among the genera of medically important Enterobacteriaceae, the most relevant are:
-Salmonella: it is transmitted by contaminated food or water and causes fever, diarrhea and vomiting.
-Klebsiella: it is associated with urinary tract infections, diarrhea and abscesses and rhinitis.
-Enterobacter: it is associated with meningitis and sepsis.
Serratia: it causes pneumonia, endocarditis and sepsis.
Some genera of Proteus cause gastroenteritis.
Citrobacter causes urinary and respiratory tract infections in sick patients.
Treatments
Treatment for these bacterial pathogens is quite complex and depends on a wide variety of factors, such as the initial situation of the patient and the symptoms that this manifests.
Enterobacteriaceae, which are harmful agents, are generally sensitive to certain antibiotics such as: quinolones, ampicillin, cephalosporins, amoxicillin-clavulanate, cotrimoxazole and some are susceptible to tetracycline.
It should be noted that the indiscriminate use of antibiotics increases the frequency of bacteria resistant to them. This is considered a delicate global health problem and, logically, hinders the allocation of treatment.
For example, the fact that some Enterobacteriaceae are resistant to carbapenemases greatly hinders treatments, and the simplest viable solution is to apply a treatment that combines several antibiotics (Falagas et al., 2013), such as tigecycline and colistin (Guerrero et al., 2014).
Recent research suggests the use of aminoglycosides, polymyxins, fosfomycin, and temocillin (Van Duin, 2013).
References
- Blount, ZD (2015). The natural history of model organisms: The unexhausted potential of E. coli. Elife, 4, e05826.
- Cabello, RR (2007). Human microbiology and parasitology. Etiological bases of infectious and parasitic diseases. Pan American Medical Ed
- Cullimore, DR (2010). Practical atlas for bacterial identification. CRC Press.
- Falagas, ME, Lourida, P., Poulikakos, P., Rafailidis, PI, & Tansarli, GS (2013). Antibiotic treatment of infections due to carbapenem-resistant Enterobacteriaceae: systematic evaluation of the available evidence. Antimicrobial agents and chemotherapy, AAC-01222.
- García, P., & Mendoza, A. (2014). Traditional and high-resolution biochemical tests for manual identification of Enterobacteriaceae. Latin American Clinical Biochemical Acta, 48 (2), 249-254.
- Gragera, BA (2002). Enterobacterial infections. Medicine-Accredited Continuing Medical Education Program, 8 (64), 3385-3397.
- Guerrero, PP, Sánchez, FG, Saborido, DG, & Lozano, IG (2014). Enterobacterial infections. Medicine-Accredited Continuing Medical Education Program, 11 (55), 3276-3282.
- Olivas, E. (2001). Basic Microbiology Laboratory Manual. Sports Training Program. UACJ.
- Tortora, GJ, Funke, BR, & Case, CL (2007). Introduction to microbiology. Panamerican Medical Ed.
- Van Duin, D., Kaye, KS, Neuner, EA, & Bonomo, RA (2013). Carbapenem-resistant Enterobacteriaceae: a review of treatment and outcomes. Diagnostic microbiology and infectious disease, 75 (2), 115-120.
- Winn, WC (2006). Koneman's color atlas and textbook of diagnostic microbiology. Lippincott williams & wilkins.