Acinetobacter
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 Acinetobacter baumannii
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'A. baumannii |
Acinetobacter is a Gram-negative genus of bacteria belonging to the phylum Proteobacteria. Non-motile, Acinetobacter species are oxidase-negative, and occurs in pairs under magnification.
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[edit] Identification
Using FLN, or Fluorescence-Lactose-Denitrification medium, to find the amount of acid produced by metabolism of glucose, different species of bacteria under this genus can be identified.
[edit] Description
[edit] Microbiology
Species of the genus Acinetobacter are stricly aerobic nonfermentative gram-negative bacilli. They show preponderently a coccobacillary morphology on nonselective agar. Rods predominate in fluid media, especially during early growth.
The morphology of Acinetobacter spp. 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 (without salt). Although officially classifed as non-lactose fermenting, they are often partially lactose fermenting when grown on MacConkey agar. They are oxidase negative, nonmotile and usually nitrate negative.
[edit] Taxonomy
There are over 25 species in the Acinetobacter group. 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
- Acinetobacter lwoffii: glucose-negative nonhemolytic
- Acinetobacter haemolyticus: hemolytic
[edit] 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 an important 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.
[edit] Clinical Significance
Species of the genus Acinetobacter are generally considered nonpathogenic to healthy individuals. Most infections occur in debilitated individuals, and the strain A. baumannii is the second most commonly isolated nonfermenter in human specimens.
Acinetobacter is frequently isolated in nosocomial infections and is especially prevalent in intensive care units, where both sporadic cases as well as epidemic and endemic occurance is common. A. baumannii is a frequent cause of nosocomial pneumonia, especially of late-onset ventilator associated pneumonia. It can cause various other infections including skin and wound infections, bacteremia, and meningitis, but A. lwoffi is mostly responsible for the latter. A. baumannii can survive on the human skin or dry surfaces for weeks.
NEEDS MAJOR REWRITE
Since the start of the Iraq War, over 300 cases of A. baumannii had infected U.S. soldiers in the Middle East. At least five have died.
Ethanol has been found to stimulate the virulence of A. baumannii.[1] Tests on infected nematode worms dosed with ethanol found that the worms laid fewer eggs and their life spans were only 80% of worms infected with a non-ethanol responsive strain of A. baumannii, suggesting the common misconception that drinking alcohol kills infections is false and drinking alcohol may actually help the infection's spread.
[edit] Treatment
Acinetobacter species are innately resistant to many classes of antibiotics, including penicillin, chloramphenicol, and often aminoglycosides. Resistance to fluoroquinolones has been reported during therapy and this 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 CDC and the carbapenems are recognised as the gold-standard and/or treatment of last resort. Rather worringly is an increase in resistance to the carbapenems which leaves very little treatment option although there some success reported with polymyxin B as well as the use of novel combinations of antibiotics.[2] Acinetobacter species are unusual in that they are sensitive to sulbactam; sulbactam is most commonly used to inhibit bacterial β-lactamase, but this is an example of the antibacterial property of sulbactam itself.[3]
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/Kuwait region during Operation Iraqi Freedom (OIF) and in Afghanistan during Operation Enduring Freedom (OEF) were treated.[4] Most of these were multidrug-resistant. Among one set of isolates from Walter Reed Army Medical Center, 13 (35%) were susceptible to imipenem only, and two (4%) were resistant to all drugs tested. One antimicrobial agent, colistin (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 generally expensive to treat.
[edit] References
- ^ Smith, M.G., and M. Snyder (2005). "Ethanol-induced virulence of Acinetobacter baumannii". American Society for Microbiology meeting, Atlanta.
- ^ Rahal J (2006). "Novel antibiotic combinations against infections with almost completely resistant Pseudomonas aeruginosa and Acinetobacter species.". Clin Infect Dis 43 Suppl 2: S95-9. PMID 16894522.
- ^ Wood GC, Hanes SD, Croce MA, Fabian TC, Bougher BA. (2002). "Comparison of ampicillin-sulbactam and imipenem-cilastin for the treatment of Acinetobacter ventilator-associated pneumonia". Clin Infect Dis 34: 1425–30.
- ^ (2004) "Acinetobacter baumannii infections among patients at military medical facilities treating injured U.S. service members, 2002-2004.". MMWR Morb Mortal Wkly Rep 53 (45): 1063-6. PMID 15549020.
Jones ME, Draghi DC, Thornsberry C, Karlowsky JA, Sahm DF, Wenzel RP. Emerging resistance among bacterial pathogens in the intensive care unit-a European and North American Surveillance study (2000-2002). Ann Clin Microbiol Antimicrob. 2004 Jul 29;3:14.
Manual of Clinical Microbiology. ASM Press.
