Acinetobacter baumannii
Overview
 
Acinetobacter baumannii is a species of pathogen
Pathogen
A 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...

ic bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, 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. hospital patients each year, and specialists say "they could emerge as a bigger threat." The illness can cause severe pneumonia
Pneumonia
Pneumonia is an inflammatory condition of the lung—especially affecting the microscopic air sacs —associated with fever, chest symptoms, and a lack of air space on a chest X-ray. Pneumonia is typically caused by an infection but there are a number of other causes...

 and infections of the urinary tract, bloodstream and other parts of the body.

A.
Discussions
Encyclopedia
Acinetobacter baumannii is a species of pathogen
Pathogen
A 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...

ic bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, 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. hospital patients each year, and specialists say "they could emerge as a bigger threat." The illness can cause severe pneumonia
Pneumonia
Pneumonia is an inflammatory condition of the lung—especially affecting the microscopic air sacs —associated with fever, chest symptoms, and a lack of air space on a chest X-ray. Pneumonia is typically caused by an infection but there are a number of other causes...

 and infections of the urinary tract, bloodstream and other parts of the body.

A. baumannii forms opportunistic infection
Opportunistic infection
An opportunistic infection is an infection caused by pathogens, particularly opportunistic pathogens—those that take advantage of certain situations—such as bacterial, viral, fungal or protozoan infections that usually do not cause disease in a healthy host, one with a healthy immune system...

s.
There have been many reports of A. baumannii infections among American soldiers wounded in Iraq, earning it the nickname "Iraqibacter". Multidrug-resistant Acinetobacter baumannii is abbreviated as MDRAB. Multidrug-resistant Acinetobacter is not a new phenomenon; it has always been inherently resistant to multiple antibiotics.

Biology

See also the main article
Acinetobacter
Acinetobacter [asz−in−ée−toe–back−ter] is a genus of Gram-negative bacteria belonging to the Gammaproteobacteria. Acinetobacter species are non-motile and oxidase-negative, and occur in pairs under magnification....

 on the Acinetobacter genus

Acinetobacter baumannii is the most relevant human pathogen
Pathogen
A 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...

 within the Acinetobacter genus.
Most A. baumannii isolates are multiresistant
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...

, containing in their genome small, isolated islands of alien (meaning transmitted genetically from other organisms) DNA and other cytological and genetic material; this has led to more virulence. Acinetobacter have no flagellum
Flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...

; the name is Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 for "motionless".
During growth, they are rod-shaped, and up to 2.5 microns. During the stationary phase
Bacterial growth
250px|right|thumb|Growth is shown as L = log where numbers is the number of colony forming units per ml, versus T Bacterial growth is the division of one bacterium into two daughter cells in a process called binary fission. Providing no mutational event occurs the resulting daughter cells are...

, they become shorter and rounder, resembling small cocci as they shrink to roughly half their size. They can be found in pairs or groups. One of the tests that distinguish them from other pathogens is that they are oxidase-negative.

Transmission and prevalence

Acinetobacter enters into the body through open wounds, catheter
Catheter
In medicine, a catheter is a tube that can be inserted into a body cavity, duct, or vessel. Catheters thereby allow drainage, administration of fluids or gases, or access by surgical instruments. The process of inserting a catheter is catheterization...

s, and breathing tubes
Tracheal tube
A tracheal tube is a catheter that is inserted into the trachea in order for the primary purpose of establishing and maintaining a patent airway and to ensure the adequate exchange of oxygen and carbon dioxide. Many different types of tracheal tubes are available, suited for different specific...

. It usually infects those with compromised immune systems
Immunodeficiency
Immunodeficiency is a state in which the immune system's ability to fight infectious disease is compromised or entirely absent. Immunodeficiency may also decrease cancer immunosurveillance. Most cases of immunodeficiency are acquired but some people are born with defects in their immune system,...

, such as the wounded, the elderly, children, or those with immune diseases. Colonization poses no threat to people not already ill, but colonized healthcare workers
Iatrogenesis
Iatrogenesis, or an iatrogenic artifact is an inadvertent adverse effect or complication resulting from medical treatment or advice, including that of psychologists, therapists, pharmacists, nurses, physicians and dentists...

 and hospital visitors can carry the bacteria into neighboring wards and other medical facilities.
The number of nosocomial infections (hospital-acquired infections) caused by A. baumannii has increased in recent years, as have most other nosocomial pathogens (MRSA, VRSA, VRE, etc.) One of the reasons is that the bacteria can live up to 5 months on undisturbed surfaces, depending on humidity levels.

The first military outbreaks of severe A. baumannii infections occurred in April, 2003 in American soldiers returning from Iraq. Early reports attributed the infections to the Iraqi soil. Later testing demonstrated widespread contamination of field hospital
Field hospital
A field hospital is a large mobile medical unit that temporarily takes care of casualties on-site before they can be safely transported to more permanent hospital facilities...

s, via transportation of personnel and equipment from previously contaminated European hospitals, as the most plausible vector.

Virulence and pathogenicity

This opportunistic pathogen causes a wide variety of serious infections in humans, mostly in compromised patients. Recently, A. baumannii has emerged as an important pathogen among wounded soldiers, threatening civilian and military patients. A baumannii has also been implicated in severe life threatening infections such as necrotizing fasciitis.

Nosocomial
Nosocomial infection
A 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...

 A. baumannii 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....

 may cause severe clinical disease that is associated with an elevated mortality rate.
This opportunistic pathogen expresses myriad factors that could play a role in human pathogenesis
Pathogenesis
The pathogenesis of a disease is the mechanism by which the disease is caused. The term can also be used to describe the origin and development of the disease and whether it is acute, chronic or recurrent...

. Among these factors are the attachment to and persistence on solid surfaces, the acquisition of essential nutrients such as iron, the adhesion to epithelial cells and their subsequent killing by apoptosis
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...

, and the production and/or secretion of enzymes and toxic products that damage host tissues. However, very little is known about the molecular nature of most of these processes and factors, and almost nothing has been shown with regard to their role in bacterial virulence and the pathogenesis of serious infectious diseases. However, some of these gaps can now be filled by testing appropriate isogenic derivatives in relevant animal models that mimic the infections in humans, in particular the outcome of deadly pneumonia
Pneumonia
Pneumonia is an inflammatory condition of the lung—especially affecting the microscopic air sacs —associated with fever, chest symptoms, and a lack of air space on a chest X-ray. Pneumonia is typically caused by an infection but there are a number of other causes...

. Such an approach should provide new and relevant information on the virulence traits of this normally underestimated bacterial human pathogen.

A. baumannii strains have been found to exhibit reduced motility
Motility
Motility 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...

 in response to blue light. This specific photosensitivity
Photosensitivity
Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light.- Human medicine :Sensitivity of the skin to a light source can take various forms. People with particular skin types are more sensitive to sunburn...

 may have a role in biofilm
Biofilm
A biofilm is an aggregate of microorganisms in which cells adhere to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance...

 formation and other virulence factors.

Control and treatment of infections

Multidrug-resistant A. baumannii is a common problem in many hospitals in the US and Europe. First-line treatment is with a carbapenem
Carbapenem
Carbapenems are a class of β-lactam antibiotics with a broad spectrum of antibacterial activity. They have a structure that renders them highly resistant to most β-lactamases...

 antibiotic such as imipenem
Imipenem
Imipenem 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...

, but carbapenem resistance is increasingly common. Other treatment options include polymyxin
Polymyxin
Polymyxins are antibiotics, with a general structure consisting of a cyclicpeptide with a long hydrophobic tail. They disrupt the structure of thebacterial cell membrane by interacting with its phospholipids...

s, tigecycline
Tigecycline
Tigecycline is a glycylcycline antibiotic developed by Francis Tally and marketed by Wyeth under the brand name Tygacil. It was given a U.S. Food and Drug Administration fast-track approval and was approved on June 17, 2005...

, and 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.

The institution of strict infection-control measures, such as monitored hand washing
Hand washing
Hand washing for hand hygiene is the act of cleaning the hands with or without the use of water or another liquid, or with the use of soap, for the purpose of removing soil, dirt, and/or microorganisms....

, can lower hospital infection rates.

MDRAB infections are difficult and costly to treat. A study at a public teaching hospital found that the mean total hospital cost of patients having acquired MDRAB was $98,575 higher than that of control patients having had identical burn severity of illness indices.

There are some specific treatments in developing stage in order to overcome the resistance of this bacterium, using a specific bacteriophage
Bacteriophage
A bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...

against this type of bacteria.
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