ENTEROCOCCUS: CHARACTERISTICS, MORPHOLOGY, TAXONOMY, PATHOGENESIS - BIOLOGY - 2024

Enterococcus is one of the four genera of the Enterococcaceae family of bacteria, belonging to the order Lactobacillales, class Bacilli of the phylum Firmicutes. This genus groups a great diversity of gram-positive bacteria, with an ovoid shape that do not form spores. At least 34 species are recognized in this genus.

Bacteria of the genus Enterococcus are part of the intestinal flora of humans. However, it is an opportunistic pathogen, increasingly implicated in nosocomial or hospital infections.

Enterococcus faecalis. By United States Department of Agriculture, via Wikimedia Commons

Enterococcus faecalis is the most frequently isolated species in medical materials (80–90%) followed by Enterococcus faecium (8–16%). Bacteria of this genus have also been isolated from food, plants, soil, and surface waters, but their presence in these environments is believed to be associated with fecal contamination.

Enterococci are extremely hardy organisms, capable of living in extreme environments. They can grow at temperatures ranging from 10 to 45 ºC. They support hypotonic, hypertonic, acidic or alkaline environments and can grow in atmospheres with or without oxygen as they are facultative anaerobes. They are very resistant to dehydration.

Some species of enterococci can create antibiotic resistance, making them a public health problem. The World Health Organization lists Enterococcus faecium on a list of pathogens with critical priority for research and development of new antibiotics, due to its worrying resistance to vancomycin.

Enterococcus have been used as probiotics in food and feed, however this use is controversial because they are potential pathogens linked to human diseases and because of the risks of transferring antimicrobial resistance and virulence genes to human strains.

General characteristics

Metabolisms

Bacteria of the genus Enterococcus are facultatively anaerobic, with a preference towards anaerobic atmospheres.

Physiologically they are mostly catalase negative, although some strains reveal pseudocatalase activity when grown in blood-containing media. Hemolytic activity is variable and depends largely on the species.

The optimal growth temperature for most species is between 35 and 37 ° C, although many species can grow between 42 and 45 ° C and very slowly at 10 ° C. They are able to survive at 60ºC for 30 minutes.

They are chemoganotrophic, with generally complex nutrient requirements. These bacteria can obtain their energy from the oxidation of reduced inorganic compounds such as ammonia, elemental sulfur, hydrogen, iron ions, nitrite and sulfur. Therefore, they can get all their cellular carbon from carbon dioxide, and they can grow without any organic compounds and without light.

Bacteria of the genus Enterococcus have a fermentative metabolism, being able to ferment a wide variety of substrates. The main route of energy production is the homofermentative formation of lactic acid mainly from glucose. Under aerobic conditions, glucose is metabolized to acetic acid, acetoin, and CO ₂.

Some species are dependent on CO ₂ (carbophilic).

Morphology

Bacteria of the genus Enterococcus are ovoid-shaped cells and can measure from 0.6 to 2.0 microns by 0.6 to 2.5 microns. They are sessile but some strains may have short flagella that give them some mobility.

The cells occur singly or in pairs, sometimes in short chains, often elongated in the direction of the chain. Depending on the species, strain, and culture conditions, daughter cells can be separated, so that the culture appears to be composed of single cells and pairs of dividing cells when viewed by phase contrast microscopy.

In other cases, the daughter cells can remain attached to each other, thus showing chains of cells.

Taxonomy

Members of the genus Enterococcus were classified within the genus Streptococcus until 1984, when the results of genomic DNA analysis indicated that a separate classification of the genus would be appropriate.

Subsequently, the existence of groups within the species of the genus have been established, which associate species with similar phenotypic characteristics, which are very difficult to differentiate from each other.

Some of them may have 99.8% similar gene sequences. However, these can be identified by DNA-DNA similarity determinations and by some molecular methods.

Pathogeny

Enterococcus have a low pathogenic potential in healthy people, however, they constitute opportunistic pathogens in elderly patients, infants and immunosuppressed people.

Despite their low pathogenicity, Enterococcus are increasingly implicated in nosocomial or hospital infections. Thus, these bacteria have been considered among the main causes of nosocomial infections, being responsible for more than 10% of infections acquired in hospitals.

The pathogenicity of Enterococcus bacteria is mediated by their high adhesion capacity to host cells and their subsequent invasion into tissues, by their high degree of resistance to adverse conditions and finally, by their potential to create resistance to antibiotics and Virulence factors.

Human infections

Bacteria of the genus Enterococcus have been implicated in human infections mainly in the urinary tract, blood, heart and wounds, although with less frequency they have been isolated in infections of the respiratory tract, central nervous system, otitis, sinusitis, septic arthritis, endophthalmitis and burns.

These bacteria have also been identified as a cause of infections in poultry and other animal species, specifically in septicemia, osteomyelitis, and endocarditis.

Resistance

Enterococci are inherently resistant to chloramphenicol, tetracyclines, macrolides, lincosamides, streptogramins, quinolones, aminoglycosides, β-lactams, and glycopeptides.

These bacteria acquire resistance to antibiotics through extrachromosomal DNA elements (plasmids, transposons). Resistance to vancomycin is a serious problem, particularly in hospital settings, as this is the most potent antibiotic, used as a last resort to treat bacterial infections that do not respond to any other antibiotic.

Treatment for infections caused by Enterococcus bacteria depends on the sensitivity of the strains. Thus it is possible to treat some susceptible strains with ampicillin, penicillin and vancomycin.

Nitrofurantoin can also be used, even in cases of vancomycin resistance, to treat urinary tract infections.

Uses in food

Enterococcus are lactic acid bacteria, which is why they have been used in the food industry as fermenters and as probiotics in animals and humans. However, its use in food is controversial due to the pathogenic qualities of these bacteria.

These foods are given as a treatment for diarrhea, irritable bowel syndrome, to lower cholesterol levels, or to improve the host's immune system.

In animals, these probiotics are used primarily to treat or prevent diarrhea, for immune stimulation, or to enhance growth.

From the point of view of food microbiology, the safety of bacteria used as probiotics must be guaranteed. Data on the main strains in use thus far indicate that they are safe.

References

1. Devriese L., Baele M., Butaye P. (2006). The Genus Enterococcus. In: Dworkin M., Falkow S., Rosenberg E., Schleifer KH., Stackebrandt E. (eds) The Prokaryotes. Springer, New York, NY.
2. Díaz Pérez, M., Rodríguez Martínez, CC & Zhurbenko, R. (2010) Fundamental aspects of the genus Enterococcus as a highly important pathogen today. Cuban Journal of Hygiene and Epidemiology. 48 (2) 147-161.
3. Vos, P., Garrity, G., Jones, D., Krieg, NR, Ludwig, W., Rainey, FA, Schleifer, K.-H., Whitman, W. (2009). Bergey's Manual of Systematic Bacteriology: Volume 3: The Firmicutes. USES.
4. Wikipedia. (2018, October 1). Enterococcus. In Wikipedia, The Free Encyclopedia. Retrieved 03:14, October 2, 2018, from
5. Ferreira Araújo, T. & Fortes Ferreira, CL 2013. The genus Enterococcus as probiotic: safety concerns. Brazilian Archives of Biology and Technology, 56 (3): 457-466.