Enterococcus
Overview
 
Enterococcus is a genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of lactic acid bacteria
Lactic acid bacteria
The lactic acid bacteria comprise a clade of Gram-positive, low-GC, acid-tolerant, generally non-sporulating, non-respiring rod or cocci that are associated by their common metabolic and physiological characteristics. These bacteria, usually found in decomposing plants and lactic products, produce...

 of the phylum Firmicutes
Firmicutes
The Firmicutes are a phylum of bacteria, most of which have Gram-positive cell wall structure. A few, however, such as Megasphaera, Pectinatus, Selenomonas and Zymophilus, have a porous pseudo-outer-membrane that causes them to stain Gram-negative...

. Enterococci are Gram-positive
Gram-positive
Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...

 cocci that often occur in pairs (diplococci
Diplococcus
A diplococcus is a round bacterium that typically occurs in the form of two joined cells. Examples are Streptococcus pneumoniae, Moraxella catarrhalis, Neisseria gonorrhoeae and Neisseria meningitidis...

) or short chains, and are difficult to distinguish from streptococci
Streptococcus
Streptococcus is a genus of spherical Gram-positive bacteria belonging to the phylum Firmicutes and the lactic acid bacteria group. Cellular division occurs along a single axis in these bacteria, and thus they grow in chains or pairs, hence the name — from Greek στρεπτος streptos, meaning...

 on physical characteristics alone. Two species are common commensal organisms in the intestines of humans: E. faecalis
Enterococcus faecalis
Enterococcus faecalis – formerly classified as part of the Group D Streptococcus system – is a Gram-positive, commensal bacterium inhabiting the gastrointestinal tracts of humans and other mammals. It is among the main constituents of some probiotic food supplements. Like other species in the genus...

(90-95%) and E. faecium
Enterococcus faecium
Enterococcus faecium is a Gram-positive, alpha hemolytic or nonhemolytic bacterium in the genus Enterococcus. It can be commensal in the human intestine, but it may also be pathogenic, causing diseases such as neonatal meningitis.Vancomycin-resistant E. faecium is often referred to as VRE.Some...

(5-10%). There are rare clusters of infections with other species including E.
Encyclopedia
Enterococcus is a genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of lactic acid bacteria
Lactic acid bacteria
The lactic acid bacteria comprise a clade of Gram-positive, low-GC, acid-tolerant, generally non-sporulating, non-respiring rod or cocci that are associated by their common metabolic and physiological characteristics. These bacteria, usually found in decomposing plants and lactic products, produce...

 of the phylum Firmicutes
Firmicutes
The Firmicutes are a phylum of bacteria, most of which have Gram-positive cell wall structure. A few, however, such as Megasphaera, Pectinatus, Selenomonas and Zymophilus, have a porous pseudo-outer-membrane that causes them to stain Gram-negative...

. Enterococci are Gram-positive
Gram-positive
Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...

 cocci that often occur in pairs (diplococci
Diplococcus
A diplococcus is a round bacterium that typically occurs in the form of two joined cells. Examples are Streptococcus pneumoniae, Moraxella catarrhalis, Neisseria gonorrhoeae and Neisseria meningitidis...

) or short chains, and are difficult to distinguish from streptococci
Streptococcus
Streptococcus is a genus of spherical Gram-positive bacteria belonging to the phylum Firmicutes and the lactic acid bacteria group. Cellular division occurs along a single axis in these bacteria, and thus they grow in chains or pairs, hence the name — from Greek στρεπτος streptos, meaning...

 on physical characteristics alone. Two species are common commensal organisms in the intestines of humans: E. faecalis
Enterococcus faecalis
Enterococcus faecalis – formerly classified as part of the Group D Streptococcus system – is a Gram-positive, commensal bacterium inhabiting the gastrointestinal tracts of humans and other mammals. It is among the main constituents of some probiotic food supplements. Like other species in the genus...

(90-95%) and E. faecium
Enterococcus faecium
Enterococcus faecium is a Gram-positive, alpha hemolytic or nonhemolytic bacterium in the genus Enterococcus. It can be commensal in the human intestine, but it may also be pathogenic, causing diseases such as neonatal meningitis.Vancomycin-resistant E. faecium is often referred to as VRE.Some...

(5-10%). There are rare clusters of infections with other species including E. casseliflavus, E. gallinarum
Enterococcus gallinarum
Enterococcus gallinarum is a species of Enterococcus.E. gallinarum demonstrates an inherent, low-level resistance to vancomycin....

, and E. raffinosus.

Physiology and classification

Enterococci are facultative anaerobic organism
Facultative anaerobic organism
A facultative anaerobic organism is an organism, usually a bacterium, that makes ATP by aerobic respiration if oxygen is present but is also capable of switching to fermentation...

s, i.e., they are capable of cellular respiration
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...

 in both oxygen-rich and oxygen-poor environments. Though they are not capable of forming spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...

s, enterococci are tolerant of a wide range of environmental conditions: extreme temperature (10-45°C), pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 (4.5-10.0) and high sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...

 concentrations.

Enterococci typically exhibit gamma-hemolysis
Hemolysis
Hemolysis —from the Greek meaning "blood" and meaning a "loosing", "setting free" or "releasing"—is the rupturing of erythrocytes and the release of their contents into surrounding fluid...

 on sheep's blood agar.

History

Members of the genus Enterococcus were classified as Group D Streptococcus
Streptococcus
Streptococcus is a genus of spherical Gram-positive bacteria belonging to the phylum Firmicutes and the lactic acid bacteria group. Cellular division occurs along a single axis in these bacteria, and thus they grow in chains or pairs, hence the name — from Greek στρεπτος streptos, meaning...

until 1984, when genomic DNA analysis indicated a separate genus classification would be appropriate.

Pathology

Important clinical infections caused by Enterococcus include urinary tract infection
Urinary tract infection
A urinary tract infection is a bacterial infection that affects any part of the urinary tract. Symptoms include frequent feeling and/or need to urinate, pain during urination, and cloudy urine. The main causal agent is Escherichia coli...

s, bacteremia
Bacteremia
Bacteremia is the presence of bacteria in the blood. The blood is normally a sterile environment, so the detection of bacteria in the blood is always abnormal....

, bacterial endocarditis, diverticulitis
Diverticulitis
Diverticulitis is a common digestive disease particularly found in the large intestine. Diverticulitis develops from diverticulosis, which involves the formation of pouches on the outside of the colon...

, and meningitis
Meningitis
Meningitis is inflammation of the protective membranes covering the brain and spinal cord, known collectively as the meninges. The inflammation may be caused by infection with viruses, bacteria, or other microorganisms, and less commonly by certain drugs...

. Sensitive strains of these bacteria can be treated with ampicillin
Ampicillin
Ampicillin is a beta-lactam antibiotic that has been used extensively to treat bacterial infections since 1961. Until the introduction of ampicillin by the British company Beecham, penicillin therapies had only been effective against Gram-positive organisms such as staphylococci and streptococci...

 and vancomycin
Vancomycin
Vancomycin INN is a glycopeptide antibiotic used in the prophylaxis and treatment of infections caused by Gram-positive bacteria. It has traditionally been reserved as a drug of "last resort", used only after treatment with other antibiotics had failed, although the emergence of...

.

From a medical standpoint, an important feature of this genus is the high level of intrinsic antibiotic resistance
Antibiotic resistance
Antibiotic resistance is a type of drug resistance where a microorganism is able to survive exposure to an antibiotic. While a spontaneous or induced genetic mutation in bacteria may confer resistance to antimicrobial drugs, genes that confer resistance can be transferred between bacteria in a...

. Some enterococci are intrinsically resistant to β-lactam-based antibiotics (some penicillin
Penicillin
Penicillin is a group of antibiotics derived from Penicillium fungi. They include penicillin G, procaine penicillin, benzathine penicillin, and penicillin V....

s and virtually all cephalosporin
Cephalosporin
The cephalosporins are a class of β-lactam antibiotics originally derived from Acremonium, which was previously known as "Cephalosporium".Together with cephamycins they constitute a subgroup of β-lactam antibiotics called cephems.-Medical use:...

s), as well as many aminoglycoside
Aminoglycoside
An aminoglycoside is a molecule or a portion of a molecule composed of amino-modifiedsugars.Several aminoglycosides function as antibiotics that are effective against certain types of bacteria...

s. In the last two decades, particularly virulent strains of Enterococcus that are resistant to vancomycin (vancomycin-resistant Enterococcus
Vancomycin-Resistant Enterococcus
Vancomycin-resistant Enterococcus, or vancomycin-resistant enterococci , are bacterial strains of the genus Enterococcus that are resistant to the antibiotic vancomycin. To become VRE, vancomycin-sensitive enterococci typically obtain new DNA in the form of plasmids or transposons which encode...

, or VRE) have emerged in nosocomial infections of hospitalized patients, especially in the US. Other developed countries, such as the UK, have been spared this epidemic, and, in 2005, Singapore managed to halt an epidemic of VRE. VRE may be treated with quinupristin/dalfopristin
Dalfopristin
Dalfopristin is a streptogramin antibiotic derived from pristinamycin IIA. The combination quinupristin/dalfopristin, in a weight-to-weight ratio of 30% quinupristin to 70% dalfopristin, is used to treat infections by staphylococci and by vancomycin-resistant Enterococcus faecium....

 (Synercid) with response rates of approximately 70%.

Enterococcal meningitis is a rare complication of neurosurgery. It often requires treatment with intravenous or intrathecal
Intrathecal
Intrathecal is an adjective that refers to something introduced into or occurring in the space under the arachnoid membrane of the brain or spinal cord...

 vancomycin, yet it is debatable as to whether its use has any impact on outcome: the removal of any neurological devices is a crucial part of the management of these infections.

Water quality

In bodies of water, the acceptable level of contamination is very low, for example in the state of Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...

, with among the strictest tolerances in the United States, the limit for water off its beaches is 7 colony-forming units per 100 ml of water, above which the state may post warnings to stay out of the ocean. In 2004, Enterococcus spp. took the place of fecal coliform as the new federal standard for water quality at public beaches. It is believed to provide a higher correlation than fecal coliform with many of the human pathogens often found in city sewage.
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