Acinetobacter [asz−in−ée−toe–back−ter] is a
genusIn 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
Gram-negativeGram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
bacteria belonging to the
GammaproteobacteriaGammaproteobacteria is a class of several medically, ecologically and scientifically important groups of bacteria, such as the Enterobacteriaceae , Vibrionaceae and Pseudomonadaceae. An exceeding number of important pathogens belongs to this class, e.g...
.
Acinetobacter species are
non-motileMotility is a biological term which refers to the ability to move spontaneously and actively, consuming energy in the process. Most animals are motile but the term applies to single-celled and simple multicellular organisms, as well as to some mechanisms of fluid flow in multicellular organs, in...
and
oxidase-negativeThe oxidase test is a test used in microbiology to determine if a bacterium produces certain cytochrome c oxidases. It uses disks impregnated with a reagent such as N,N,N′,N′-tetramethyl-p-phenylenediamine or N,N-Dimethyl-p-phenylenediamine , which is also a redox indicator...
, and occur in pairs under magnification.
They are important
soil organismsSoil life or soil biota is a collective term for all the organisms living within the soil.-Overview:In balanced soil, plants grow in an active and steady environment. The mineral content of the soil and its heartiful structure are important for their well-being, but it is the life in the earth that...
, where they contribute to the
mineralizationMineralization in soil science, is when the chemical compounds in organic matter decompose or are oxidized into plant-accessible forms,. Mineralization is the opposite of immobilization....
of, for example,
aromatic compoundsIn organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...
.
Acinetobacter are a key source of infection in debilitated patients in the hospital, in particular the species
Acinetobacter baumanniiAcinetobacter baumannii is a species of pathogenic bacteria, referred to as an aerobic gram-negative bacterium, that is resistant to most antibiotics. As a result of its resistance to drug treatment, some estimates state the disease is killing tens of thousands of U.S...
.
Etymology
Acinetobacter is a compound word from scientific Greek, meaning 'nonmotile rod'. The first element acineto- is an unusual transliteration of the Greek ακινητο-; the usual romanization in English is akineto-, as in akinetic.
Description
Species of the genus
Acinetobacter are
strictly aerobicAn aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment.Faculitative anaerobes grow and survive in an oxygenated environment and so do aerotolerant anaerobes.-Glucose:...
nonfermentativeFermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...
Gram-negativeGram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
bacilli. They show preponderantly a
coccobacillaryA coccobacillus is a type of rod-shaped bacteria. The word coccobacillus reflects an intermediate shape between coccus and bacillus . Coccobacilli rods are so short and wide that they resemble cocci. Haemophilus influenzae and Chlamydia trachomatis are coccobacilli...
morphology on nonselective agar. Rods predominate in fluid media, especially during early growth.
The morphology of
Acinetobacter sp. 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 agarMacConkey agar is a culture medium designed to grow Gram-negative bacteria and stain them for lactose fermentation.-Contents:It contains bile salts MacConkey agar is a culture medium designed to grow Gram-negative bacteria and stain them for lactose fermentation.-Contents:It contains bile salts...
(without salt). Although officially classified as nonlactose-fermenting, they are often partially lactose-fermenting when grown on MacConkey agar. They are
oxidaseAn oxidase is any enzyme that catalyzes an oxidation-reduction reaction involving molecular oxygen as the electron acceptor. In these reactions, oxygen is reduced to water or hydrogen peroxide ....
-negative,
nonmotileMotility is a biological term which refers to the ability to move spontaneously and actively, consuming energy in the process. Most animals are motile but the term applies to single-celled and simple multicellular organisms, as well as to some mechanisms of fluid flow in multicellular organs, in...
, and usually
nitrateThe nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
negative.
Taxonomy
The genus
Acinetobacter comprises 17 validly named and 14 unnamed (genomic) species. Some unrelated (genomic) species have common designations, while some other species seem to be congruent but have different names. The knowledge of the biology or ecology of acinetobacters at species level is limited. This is because identification of acinetobacters at species level is difficult. A phenotypic species identification system has been described, and a variety of genotypic methods has been explored and applied to investigate the diversity or phylogeny in the genus. These methods include high-resolution fingerprinting with AFLP, PCR-RFLP with digestion of PCR amplified sequences, and analysis of various DNA sequences. Of these, AFLP analysis and amplified 16SrRNA ribosomal DNA restriction analysis have been validated with large numbers of strains of all described species. Nucleotide sequence-based methods are expected to be the standard for identification in the near future.
However, because routine identification in the clinical microbiology laboratory is not (yet) possible, they are divided and grouped into three main complexes:
- Acinetobacter calcoaceticus-baumanii complex: glucose-oxidising nonhemolytic, (A.baumannii can be identified by OXA-51 typing)
- Acinetobacter lwoffii: glucose-negative nonhemolytic
- Acinetobacter haemolyticus: hemolytic
Hemolysis is the breakdown of red blood cells. The ability of bacterial colonies to induce hemolysis when grown on blood agar is used to classify certain microorganisms. This is particularly useful in classifying streptococcal species...
Identification
Different species of bacteria in this genus can be identified using Fluorescence-Lactose-Denitrification medium (FLN) to find the amount of acid produced by
metabolismMetabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
of
glucoseGlucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
.
The other reliable identification test at genus level is chromosomal DNA transformation assay (CTA):
In this aasy, 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 plated on a brain heart infusion agar. The growth is then harvested after incubation for 24 h at 30oC, plating on an Acinetobacter minimal agar (AMM), and incubating at 30oC for 108 h. Growth on the minimal agar medium indicates a positive transformation assay and confirmes 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.
Natural habitat
Acinetobacter spp are widely distributed in nature. They are able to survive on various surfaces (both moist and dry) in the hospital environment, thereby being a major source of infection in debilitated patients. Occasional strains are isolated from foodstuffs, and some are able to survive on various medical equipment and even on healthy human skin. On
Discovery ChannelDiscovery Channel is an American satellite and cable specialty channel , founded by John Hendricks and distributed by Discovery Communications. It is a publicly traded company run by CEO David Zaslav...
's
MythBustersMythBusters is a science entertainment TV program created and produced by Beyond Television Productions for the Discovery Channel. The series is screened by numerous international broadcasters, including Discovery Channel Australia, Discovery Channel Latin America, Discovery Channel Canada, Quest...
, hundreds of
Acinetobacter colonies were discovered on an everyday kitchen sponge.
In drinking water,
Acinetobacter have been shown to aggregate bacteria that otherwise do not form aggregates.
Clinical significance
In general,
Acinetobacter species are considered
nonpathogenicA pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
to healthy individuals. However, several species persist in hospital environments and cause severe, life-threatening infections in compromised patients. The spectrum of antibiotic resistances of these organisms together with their survival capabilities make them a threat to hospitals, as documented by recurring outbreaks both in highly developed countries and elsewhere. An important factor for their pathogenic potential is, it is presumed, an efficient means of
horizontal gene transferHorizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
.
Most infections occur in immunocompromised individuals, and the strain
A. baumanniiAcinetobacter baumannii is a species of pathogenic bacteria, referred to as an aerobic gram-negative bacterium, that is resistant to most antibiotics. As a result of its resistance to drug treatment, some estimates state the disease is killing tens of thousands of U.S...
is the second-most-commonly-isolated
nonfermentingFermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...
bacteria in human specimens.
Acinetobacter is frequently isolated in
nosocomial infectionA nosocomial infection , also known as a hospital-acquired infection or HAI, is an infection whose development is favoured by a hospital environment, such as one acquired by a patient during a hospital visit or one developing among hospital staff...
s and is especially prevalent in
intensive care unitthumb|220px|ICU roomAn intensive-care unit , critical-care unit , intensive-therapy unit/intensive-treatment unit is a specialized department in a hospital that provides intensive-care medicine...
s, where both sporadic cases as well as
epidemicIn epidemiology, an epidemic , occurs when new cases of a certain disease, in a given human population, and during a given period, substantially exceed what is expected based on recent experience...
and
endemicIn epidemiology, an infection is said to be endemic in a population when that infection is maintained in the population without the need for external inputs. For example, chickenpox is endemic in the UK, but malaria is not...
occurrence is common.
A. baumannii is a frequent cause of nosocomial pneumonia, especially of
late-onset ventilator associated pneumoniaVentilator-associated pneumonia is a sub-type of hospital-acquired pneumonia which occurs in people who are receiving mechanical ventilation. VAP is not characterized by the causative agents; rather, as its name implies, definition of VAP is restricted to patients undergoing mechanical...
. It can cause various other infections including skin and wound infections,
bacteremiaBacteremia 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....
, and
meningitisMeningitis 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...
, but
A. lwoffi is mostly responsible for the latter.
A. baumannii can survive on the human skin or dry surfaces for weeks.
Biofilm formation is an important feature of most clinical isolates of Acinetobacter spp. Biofilms are assemblages of surface microbial cells that are enclosed in an extracellular polymeric matrix.
It is clear from the epidemiologic evidence that Acinetobacter biofilms play a role in infectious diseases such as cystic fibrosis and periodontitis, and in bloodstream and UTI because of their ability to
indwell medical devices. Antibiotic resistance markers are often plasmid-borne, and plasmids present in Acinetobacter strains can be transferred to other pathogenic bacteria. The ability of Acinetobacter species to adhere to the surfaces, form biofilms, and display antibiotic resistance and gene transfer means that there is an urgent need to study the factors responsible for their spread.
Since the start of the
Iraq War, over 700 U.S. soldiers have been infected or colonized by
A. baumannii. Four civilians undergoing treatment for serious illnesses at
Walter Reed Army Medical CenterThe Walter Reed Army Medical Center was the United States Army's flagship medical center until 2011. Located on 113 acres in Washington, D.C., it served more than 150,000 active and retired personnel from all branches of the military...
in Washington, D.C. contracted
A. baumannii infections and died. At
Landstuhl Regional Medical CenterThe Landstuhl Regional Medical Center is an overseas military hospital operated by the United States Army and the Department of Defense. LRMC is the largest military hospital outside of the continental United States. It is located near Landstuhl, Germany, and serves as the nearest treatment center...
, a U.S. military hospital in Germany, another civilian under treatment, a 63-year-old German woman, contracted the same strain of
A. baumannii infecting troops in the facility and also died. These infections appear to have been hospital-acquired. Based on genotyping of
A. baumannii cultured from patients prior to the start of the Iraq War, one can presume that it is likely the soldiers contracted the infections while hospitalized for treatment in Europe.
Treatment
Acinetobacter species are innately resistant to many classes of antibiotics, including
penicillinPenicillin is a group of antibiotics derived from Penicillium fungi. They include penicillin G, procaine penicillin, benzathine penicillin, and penicillin V....
,
chloramphenicolChloramphenicol is a bacteriostatic antimicrobial that became available in 1949. It is considered a prototypical broad-spectrum antibiotic, alongside the tetracyclines, and as it is both cheap and easy to manufacture it is frequently found as a drug of choice in the third world.Chloramphenicol is...
, and often
aminoglycosideAn 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. Resistance to fluoroquinolones 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 resistanceAntibiotic 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...
in
Acinetobacter strains has been reported by the
CDCThe Centers for Disease Control and Prevention are a United States federal agency under the Department of Health and Human Services headquartered in Druid Hills, unincorporated DeKalb County, Georgia, in Greater Atlanta...
and the carbapenems are recognised as the gold-standard and treatment of last resort. Acinetobacter species are unusual in that they are sensitive to
sulbactamSulbactam is a molecule that is given in combination with beta-lactam antibiotics to inhibit beta-lactamase, an enzyme produced by bacteria that destroys the antibiotics...
; sulbactam is most commonly used to inhibit bacterial beta-lactamase, but this is an example of the antibacterial property of sulbactam itself.
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
IraqIraq ; officially the Republic of Iraq is a country in Western Asia spanning most of the northwestern end of the Zagros mountain range, the eastern part of the Syrian Desert and the northern part of the Arabian Desert....
/
KuwaitThe State of Kuwait is a sovereign Arab state situated in the north-east of the Arabian Peninsula in Western Asia. It is bordered by Saudi Arabia to the south at Khafji, and Iraq to the north at Basra. It lies on the north-western shore of the Persian Gulf. The name Kuwait is derived from the...
region during Operation Iraqi Freedom (OIF) and in
AfghanistanAfghanistan , officially the Islamic Republic of Afghanistan, is a landlocked country located in the centre of Asia, forming South Asia, Central Asia and the Middle East. With a population of about 29 million, it has an area of , making it the 42nd most populous and 41st largest nation in the world...
during
Operation Enduring Freedom (OEF) were treated. Most of these were multidrug-resistant. Among one set of isolates from
Walter Reed Army Medical CenterThe Walter Reed Army Medical Center was the United States Army's flagship medical center until 2011. Located on 113 acres in Washington, D.C., it served more than 150,000 active and retired personnel from all branches of the military...
, 13 (35%) were susceptible to
imipenemImipenem is an intravenous β-lactam antibiotic developed in 1980. It has an extremely broad spectrum of activity.Imipenem belongs to the subgroup of carbapenems. It is derived from a compound called thienamycin, which is produced by the bacterium Streptomyces cattleya...
only, and two (4%) were resistant to all drugs tested. One antimicrobial agent,
colistinColistin is a polymyxin antibiotic produced by certain strains of Bacillus polymyxa var. colistinus. Colistin is a mixture of cyclic polypeptides colistin A and B. Colistin is effective against most Gram-negative bacilli and is used as a lingerdoodle. It is one of the last-resort antibiotics for...
(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 for up to 20 days, they pose a high risk of spread and contamination in hospitals, potentially putting immune-compromised and other patients at risk for drug-resistant infections that are often fatal and, in general, expensive to treat.
Reports suggest that this bacteria is susceptible to
phage therapyPhage therapy is the therapeutic use of bacteriophages to treat pathogenic bacterial infections. Although extensively used and developed mainly in former Soviet Union countries circa 1920, this method of therapy is still being tested for treatment of a variety of bacterial and poly-microbial...
. A phage directed against
Acinetobacter showed a remarkable lytic activity both
in vitro and
in vivo: as few as 100 pfu of phage protected mice against
Acinetobacter.
Biotechnology
Many of the characteristics of
Acinetobacter ecology, taxonomy, physiology, and genetics point to the possibility of exploiting its unique features for future applications.
Acinetobacter strains are often ubiquitous, exhibit metabolic versatility, and are robust. And some provide convenient systems for modern molecular genetic manipulation and subsequent product engineering. These characteristics are being exploited in various biotechnological applications including
biodegradationBiodegradation or biotic degradation or biotic decomposition is the chemical dissolution of materials by bacteria or other biological means...
and
bioremediationBioremediation is the use of microorganism metabolism to remove pollutants. Technologies can be generally classified as in situ or ex situ. In situ bioremediation involves treating the contaminated material at the site, while ex situ involves the removal of the contaminated material to be treated...
, novel lipid and peptide production,
enzyme engineeringEnzyme engineering is the application of modifying an enzyme's structure or modifying the catalytic activity of isolated enzymes to produce new metabolites, to allow new pathways for reactions to occur, or to convert from some certain compounds into others...
, biosurfactant, and biopolymer production and engineering of novel derivatives of these products. It is anticipated that progress in these fields will broaden the range of applications of
Acinetobacter for modern biotechnology.
External links