''Acinetobacter'' is a
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of
gram-negative
Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wa ...
bacteria belonging to the wider class of
Gammaproteobacteria. ''Acinetobacter'' species are
oxidase-negative, exhibit
twitching motility
Twitching motility is a form of crawling bacterial motility used to move over surfaces. Twitching is mediated by the activity of hair-like filaments called type IV pili which extend from the cell's exterior, bind to surrounding solid substrates a ...
, and occur in pairs under magnification.
They are important
soil organisms
Soil biology is the study of microbial and faunal activity and ecology in soil.
Soil life, soil biota, soil fauna, or edaphon is a collective term that encompasses all organisms that spend a significant portion of their life cycle within a soil ...
, where they contribute to the
mineralization of, for example,
aromatic compounds
Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The parent member of aromatic compounds is benzene. The word "aromatic" originates from the past groupin ...
. ''Acinetobacter'' species are a key source of infection in debilitated patients in the hospital, in particular the species ''
Acinetobacter baumannii
''Acinetobacter baumannii'' is a typically short, almost round, rod-shaped ( coccobacillus) Gram-negative bacterium. It is named after the bacteriologist Paul Baumann. It can be an opportunistic pathogen in humans, affecting people with compromi ...
''.
Description
Species of the genus ''Acinetobacter'' are
strictly aerobic,
nonfermentative,
Gram-negative
Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wa ...
bacilli
Bacilli is a taxonomic class of bacteria that includes two orders, Bacillales and Lactobacillales, which contain several well-known pathogens such as ''Bacillus anthracis'' (the cause of anthrax). ''Bacilli'' are almost exclusively gram-positi ...
. They show mostly a
coccobacillary morphology on nonselective agar. Rods predominate in fluid media, especially during early growth.
The morphology of ''Acinetobacter'' species can be quite variable in Gram-stained human clinical specimens, and cannot be used to differentiate ''Acinetobacter'' from other common causes of infection.
Most strains of ''Acinetobacter'', except some of the ''A. lwoffii'' strain, grow well on
MacConkey agar
MacConkey agar is a selective and differential culture medium for bacteria. It is designed to selectively isolate Gram-negative and enteric (normally found in the intestinal tract) bacteria and differentiate them based on lactose fermentation. ...
(without salt). Although officially classified as not lactose-fermenting, they are often partially lactose-fermenting when grown on MacConkey agar. They are
oxidase
In biochemistry, an oxidase is an enzyme that catalyzes oxidation-reduction reactions, especially one involving dioxygen (O2) as the electron acceptor. In reactions involving donation of a hydrogen atom, oxygen is reduced to water (H2O) or hydro ...
-negative, catalase-positive, indole-negative,
nonmotile, and usually
nitrate-negative.
Bacteria of the genus ''Acinetobacter'' are known to form intracellular inclusions of
polyhydroxyalkanoates
Polyhydroxyalkanoates or PHAs are polyesters produced in nature by numerous microorganisms, including through bacterial fermentation of sugars or lipids. When produced by bacteria they serve as both a source of energy and as a carbon store. M ...
under certain environmental conditions (e.g. lack of elements such as phosphorus, nitrogen, or oxygen combined with an excessive supply of carbon sources).
Etymology
''Acinetobacter'' is a compound word from scientific Greek
+ κίνητο + βακτηρ(ία) meaning nonmotile rod. The first element ''acineto-'' appears as a somewhat
baroque rendering of the Greek
morpheme
A morpheme is the smallest meaningful constituent of a linguistic expression. The field of linguistic study dedicated to morphemes is called morphology.
In English, morphemes are often but not necessarily words. Morphemes that stand alone are ...
ακίνητο-, commonly
transliterated
Transliteration is a type of conversion of a text from one script to another that involves swapping letters (thus ''trans-'' + '' liter-'') in predictable ways, such as Greek → , Cyrillic → , Greek → the digraph , Armenian → or L ...
in English is ''akineto-'', but actually stems from the French ''cinetique'' and was adopted directly into English.
Taxonomy
The genus ''Acinetobacter'' comprises 38 validly named species.
Identification
Identification of ''Acinetobacter'' species is complicated by lack of standard identification techniques. Initially, identification was based on phenotypic characteristics such as growth temperature,
colony morphology
In microbiology, colonial morphology refers to the visual appearance of bacterial or fungal colonies on an agar plate. Examining colonial morphology is the first step in the identification of an unknown microbe. The systematic assessment of the c ...
, growth medium, carbon sources, gelatin hydrolysis, glucose fermentation, among others. This method allowed identification of ''A. calcoaceticus–A. baumannii'' complex by the formation of smooth, rounded, mucoid colonies at 37 °C. Closely related species could not be differentiated and individual species such as ''A. baumannii'' and ''Acinetobacter'' genomic species 3 could not be positively identified phenotypically.
Because routine identification in the clinical microbiology laboratory is not yet possible, ''Acinetobacter'' isolates are divided and grouped into three main complexes:
* ''Acinetobacter calcoaceticus-baumannii complex'': glucose-oxidising nonhemolytic (''A. baumannii'' can be identified by OXA-51 typing)
* ''Acinetobacter lwoffii'': glucose-negative nonhemolytic
* ''Acinetobacter haemolyticus'':
hemolytic
Hemolysis or haemolysis (), also known by several other names, is the rupturing (lysis) of red blood cells (erythrocytes) and the release of their contents (cytoplasm) into surrounding fluid (e.g. blood plasma). Hemolysis may occur in vivo o ...
Different species of bacteria in this genus can be identified using fluorescence-lactose-denitrification to find the amount of acid produced by
metabolism
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run c ...
of
glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...
. The other reliable identification test at genus level is chromosomal DNA transformation assay. In this assay, a naturally competent tryptophan
auxotrophic mutant of ''Acinetobacter baylyi'' (BD4 trpE27) is transformed with the total DNA of a putative ''Acinetobacter'' isolate and the transformation mixture is plated on a brain heart infusion agar. The growth is then harvested after incubation for 24 h at 30 °C, plating on an ''Acinetobacter'' minimal agar (AMA), and incubating at 30 °C for 108 h. Growth on the AMA indicates a positive transformation assay and confirms the isolate as a member of the genus ''Acinetobacter''. ''E. coli'' HB101 and ''A. calcoaceticus'' MTCC1921T can be used as the negative and positive controls, respectively.
Some of the molecular methods used in species identification are repetitive extragenic palindromic sequence-based PCR, ribotyping, pulsed field gel electrophoresis (PFGE), random amplified polymorphic DNA, amplified fragment length polymorphism (AFLP), restriction and sequence analysis of tRNA and 16S-23S rRNA gene spacers and amplified 16S ribosomal DNA restriction analysis (ARDRA). PFGE, AFLP, and ARDRA are validated common methods in use today because of their discriminative ability. However, most recent methods include multilocus sequence typing and multilocus PCR and electrospray ionization mass spectrometry, which are based on amplification of highly conserved housekeeping genes and can be used to study the genetic relatedness between different isolates.
Habitat
''Acinetobacter'' species are widely distributed in nature, and commonly occur in
soil
Soil, also commonly referred to as earth or dirt
Dirt is an unclean matter, especially when in contact with a person's clothes, skin, or possessions. In such cases, they are said to become dirty.
Common types of dirt include:
* Debri ...
and water.
Their ability to survive on moist and dry surfaces, as well as to survive exposure to various common disinfectants, allows some ''Acinetobacter'' species to survive in a hospital environment.
[ Furthermore, ''Acinetobacter'' species can grow at a broad range of temperatures, allowing them to survive in a broad array of environments.][
]
Clinical significance
''Acinetobacter'' is frequently isolated in nosocomial infection
A hospital-acquired infection, also known as a nosocomial infection (from the Greek , meaning "hospital"), is an infection that is acquired in a hospital or other health care facility. To emphasize both hospital and nonhospital settings, it is so ...
s, and is especially prevalent in intensive care unit
220px, Intensive care unit
An intensive care unit (ICU), also known as an intensive therapy unit or intensive treatment unit (ITU) or critical care unit (CCU), is a special department of a hospital or health care facility that provides intensi ...
s, where both sporadic cases and epidemic
An epidemic (from Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of patients among a given population within an area in a short period of time.
Epidemics of infectious ...
and endemic
Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found else ...
occurrences are common. ''A. baumannii'' is a frequent cause of hospital-acquired pneumonia
Hospital-acquired pneumonia (HAP) or nosocomial pneumonia refers to any pneumonia contracted by a patient in a hospital at least 48–72 hours after being admitted. It is thus distinguished from community-acquired pneumonia. It is usually caused by ...
, especially of late-onset, ventilator-associated pneumonia
Ventilator-associated pneumonia (VAP) is a type of lung infection that occurs in people who are on mechanical ventilation breathing machines in hospitals. As such, VAP typically affects critically ill persons that are in an intensive care unit (I ...
. It can cause various other infections, including skin and wound infections, bacteremia, and meningitis, but ''A. lwoffi'' is mostly responsible for the latter.
Of the ''Acinetobacter'', ''A. baumannii'' is the greatest cause of human disease, having been implicated in a number of hospital-acquired infections such as bacteremia, urinary tract infections (UTIs), secondary meningitis, infective endocarditis, and wound and burn infections. In particular, ''A. baumannii'' is frequently isolated as the cause of hospital-acquired pneumonia among patients admitted to the intensive care unit
220px, Intensive care unit
An intensive care unit (ICU), also known as an intensive therapy unit or intensive treatment unit (ITU) or critical care unit (CCU), is a special department of a hospital or health care facility that provides intensi ...
. Risk factors include long-term intubation and tracheal or lung aspiration. In most cases of ventilator-associated pneumonia, the equipment used for artificial ventilation such as endotracheal tubes or bronchoscopes serve as the source of infection and result in the colonization of the lower respiratory tract by ''A. baumannii''. In some cases, the bacteria can go on to enter the bloodstream, resulting in bacteremia with mortality rates ranging from 32% to 52%. UTIs caused by ''A. baumannii'' appear to be associated with continuous catheterization, as well as antibiotic therapy. ''A. baumannii'' has also been reported to infect skin and soft tissue in traumatic injuries and postsurgical wounds. ''A. baumannii'' commonly infect burns and may result in complications owing to difficulty in treatment and eradication. Though less common, some evidence also links this bacterium to meningitis, most often following invasive surgery, and, in very rare cases, to community-acquired primary meningitis wherein the majority of the victims were children. Case reports also link ''A. baumannii'' to endocarditis, keratitis, peritonitis, and very rarely fatal neonatal sepsis.
The clinical significance of ''A. baumannii'' is partially due to its capacity to develop resistance against many available antibiotics. Reports indicate that it possesses resistance against broad-spectrum cephalosporins, β-lactam antibiotic
β-lactam antibiotics (beta-lactam antibiotics) are antibiotics that contain a beta-lactam ring in their chemical
structure. This includes penicillin derivatives (penams), cephalosporins and cephamycins (cephems), monobactams, carbapenems and ...
s, aminoglycosides
Aminoglycoside is a medicinal and bacteriologic category of traditional Gram-negative antibacterial medications that inhibit protein synthesis and contain as a portion of the molecule an amino-modified glycoside ( sugar). The term can also refer ...
, and quinolones
Quinolone may refer to:
* 2-Quinolone
* 4-Quinolone
4-Quinolone is an organic compound derived from quinoline. It and 2-quinolone are the two most important parent (meaning simplified) quinolones. 4-Quinolone exists in equilibrium with a mino ...
. Resistance to carbapenem
Carbapenems are a class of very effective antibiotic agents most commonly used for the treatment of severe bacterial infections. This class of antibiotics is usually reserved for known or suspected multidrug-resistant (MDR) bacterial infections. ...
s is also being increasingly reported. ''A. baumannii'' can survive on the human skin or dry surfaces for weeks and is resistant to a variety of disinfectants, making it particularly easy to spread in a hospital setting. Antibiotic resistance genes are often plasmid-borne, and plasmids present in ''Acinetobacter'' strains can be transferred to other pathogenic bacteria by horizontal gene transfer
Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). H ...
.
In healthy individuals, ''Acinetobacter'' colonies on the skin correlate with low incidence of allergies
Allergies, also known as allergic diseases, refer a number of conditions caused by the hypersensitivity of the immune system to typically harmless substances in the environment. These diseases include hay fever, food allergies, atopic dermat ...
; ''Acinetobacter'' is thought to be allergy-protective.
Treatment
''Acinetobacter'' species are innately resistant to many classes of antibiotics, including penicillin, chloramphenicol, and often aminoglycoside
Aminoglycoside is a medicinal and bacteriologic category of traditional Gram-negative antibacterial medications that inhibit protein synthesis and contain as a portion of the molecule an amino-modified glycoside (sugar). The term can also refer ...
s. Resistance to fluoroquinolones
A quinolone antibiotic is a member of a large group of broad-spectrum bacteriocidals that share a bicyclic core structure related to the substance 4-quinolone. They are used in human and veterinary medicine to treat bacterial infections, as we ...
has been reported during therapy, which has also resulted in increased resistance to other drug classes mediated through active drug efflux. A dramatic increase in antibiotic resistance in ''Acinetobacter'' strains has been reported by the Centers for Disease Control and Prevention
The Centers for Disease Control and Prevention (CDC) is the national public health agency of the United States. It is a United States federal agency, under the Department of Health and Human Services, and is headquartered in Atlanta, Georgi ...
(CDC), and the carbapenems are recognised as the gold-standard and treatment of last resort. ''Acinetobacter'' species are unusual in that they are sensitive to sulbactam
Sulbactam is a β-lactamase inhibitor. This drug is given in combination with β-lactam antibiotics to inhibit β-lactamase, an enzyme produced by bacteria that destroys the antibiotics.
It was patented in 1977 and approved for medical use in ...
, which is commonly used to inhibit bacterial beta-lactamase, but this is an example of the antibacterial property of sulbactam itself. Recently sulbactam-durlobactam, a new antibacterial combination undergoing phase 3 trial, has demonstrated good ''in vitro'' activity also against carbapenem-resistant ''A. baumannii'' isolates (92% susceptibility).
In November 2004, the CDC reported an increasing number of ''A. baumannii'' bloodstream infections in patients at military medical facilities in which service members injured in the Iraq
Iraq,; ku, عێراق, translit=Êraq officially the Republic of Iraq, '; ku, کۆماری عێراق, translit=Komarî Êraq is a country in Western Asia. It is bordered by Turkey to the north, Iran to the east, the Persian Gulf and K ...
/Kuwait
Kuwait (; ar, الكويت ', or ), officially the State of Kuwait ( ar, دولة الكويت '), is a country in Western Asia. It is situated in the northern edge of Eastern Arabia at the tip of the Persian Gulf, bordering Iraq to the nort ...
region during Operation Iraqi Freedom and in Afghanistan
Afghanistan, officially the Islamic Emirate of Afghanistan,; prs, امارت اسلامی افغانستان is a landlocked country located at the crossroads of Central Asia and South Asia. Referred to as the Heart of Asia, it is bordere ...
during Operation Enduring Freedom were treated. Most of these were multidrug-resistant. Among one set of isolates from Walter Reed Army Medical Center
The Walter Reed Army Medical Center (WRAMC)known as Walter Reed General Hospital (WRGH) until 1951was the United States Army, U.S. Army's flagship medical center from 1909 to 2011. Located on in the Washington, D.C., District of Columbia, it se ...
, 13 (35%) were susceptible to imipenem
Imipenem (trade name Primaxin among others) is an intravenous β-lactam antibiotic discovered by Merck scientists Burton Christensen, William Leanza, and Kenneth Wildonger in the mid-1970s. Carbapenems are highly resistant to the β-lactamase enz ...
only, and two (4%) were resistant to all drugs tested. One antimicrobial agent, colistin
Colistin, also known as polymyxin E, is an antibiotic medication used as a last-resort treatment for multidrug-resistant Gram-negative infections including pneumonia. These may involve bacteria such as ''Pseudomonas aeruginosa'', '' Klebsiella ...
(polymyxin E), has been used to treat infections with multidrug-resistant ''A. baumannii''; however, antimicrobial susceptibility testing for colistin was not performed on isolates described in this report. Because ''A. baumannii'' can survive on dry surfaces up to 20 days, they pose a high risk of spread and contamination in hospitals, potentially putting immunocompromised and other patients at risk for drug-resistant infections that are often fatal and, in general, expensive to treat. Trials to implement vaccines to prevent Acinetobacter infections were documented.
Reports suggest this bacterium is susceptible to phage therapy
Phage therapy, viral phage therapy, or phagotherapy is the therapeutic use of bacteriophages for the treatment of pathogenic bacterial infections. This therapeutic approach emerged at the beginning of the 20th century but was progressively re ...
.
Gene-silencing antisense oligomers in a form called peptide-conjugated phosphorodiamidate morpholino
A Morpholino, also known as a Morpholino oligomer and as a phosphorodiamidate Morpholino oligomer (PMO), is a type of oligomer molecule (colloquially, an oligo) used in molecular biology to modify gene expression. Its molecular structure contain ...
oligomers have also been reported to inhibit growth in tests carried out in animals infected with antibiotic-resistant ''A. baumannii''.
Aseptic technique
The frequency of nosocomial infection
A hospital-acquired infection, also known as a nosocomial infection (from the Greek , meaning "hospital"), is an infection that is acquired in a hospital or other health care facility. To emphasize both hospital and nonhospital settings, it is so ...
s in British hospitals prompted the National Health Service
The National Health Service (NHS) is the umbrella term for the publicly funded healthcare systems of the United Kingdom (UK). Since 1948, they have been funded out of general taxation. There are three systems which are referred to using the " ...
to research the effectiveness of anions for air purification, finding that repeated airborne ''Acinetobacter'' infections in a ward were eliminated by the installation of a negative air ioniser
An air ioniser (or negative ion generator or Chizhevsky's chandelier) is a device that uses high voltage to ionise (electrically charge) air molecules. Negative ions, or anions, are particles with one or more extra electrons, conferring a net n ...
—the infection rate fell to zero.[
]
Natural transformation
Bacterial transformation involves the transfer of DNA from a donor to a recipient bacterium through the intervening liquid medium. Recipient bacteria must first enter a special physiological state termed competence to receive donor DNA. ''A. calcoaceticus'' is induced to become competent for natural transformation by dilution of a stationary culture into fresh nutrient medium. Competence is gradually lost during prolonged exponential growth and for a period after entrance into the stationary state. The DNA taken up may be used to repair DNA damage or as a means to exchange genetic information by horizontal gene transfer. Natural transformation in ''A. calcoaceticus'' may protect against exposure to DNA-damaging conditions in the natural environment of these bacteria, as appears to be the case for other bacterial species capable of transformation.[http://www.hummingbirds.arizona.edu/Faculty/Michod/Downloads/IGE%20review%20sex.pdf]
References
Further reading
*
*
*
*
External links
Alliance for the Prudent Use of Antibiotics
''Acinetobacter'' sp. ADP1 Genome Page
CycSim: metabolic model of ''Acinetobacter baylyi adp1''
{{Taxonbar, from=Q310457
Bacteria genera
Healthcare-associated infections
Moraxellaceae