ACINETOBACTER BAUMANNII: CHARACTERISTICS, MORPHOLOGY, SYMPTOMS - BIOLOGY - 2024

Acinetobacter baumannii is a gram-negative bacterium, belonging to the order of Pseudomonas. Throughout its taxonomic history it has been located in the genera Micrococcus, Moraxella, Alcaligenes, Mirococcuscalco-aceticus, Herellea and Achromobacter, until it was located in its current genus in 1968.

A. baumannii is a pathogenic bacterium, considered the species most frequently implicated in infections within its genus. It has been reported as being involved in various types of infections such as septicemia, pneumonia, and meningitis.

By Vader1941 - Visualized on SEM-EDS machine at CeNSE, IISc, BangalorePreviously published: Unpublished, CC BY-SA 4.0, It is an opportunistic parasite that has a significant incidence in hospital or nosocomial diseases. Transmission by mechanical ventilation has been identified as an important cause of contagion, especially in Intensive Care Units.

Despite being considered a low-grade pathogen, it has the ability to increase its virulence in strains involved in frequent infections. It has a great capacity to develop resistance and multi-resistance to antibiotics.

In hospitals it is transmitted mainly through the contact of personnel, through the use of contaminated hospital materials and also through the air over short distances.

The World Health Organization includes A. baumannii on a list of resistant pathogens for which new antibiotics are urgently required, assigning it category 1, with critical priority.

Biological characteristics

All species of the genus Acinetobacter have a wide distribution in different natural niches. A. baumannii can naturally inhabit the skin of healthy people, being able to colonize mucosal surfaces, which constitutes an important epidemiological reservoir. However, the habitat of A. baumannii is almost exclusive to hospital environments.

These bacteria lack flagella or structures used for locomotion. However, they achieve movement through structures that allow them to extend and retract, and through chemical mechanisms such as the excretion of a film of a high molecular weight exopolysaccharide behind the bacteria.

A. baumannii can colonize a large number of living or inert environments and has a great ability to survive on artificial surfaces for an extended period of time.

This ability is possibly due to its ability to resist dehydration, to use various carbon sources through various metabolic pathways, and because of the possibility of forming biofilms. For this reason it is common to find it in hospital supplies, such as catheters and mechanical ventilation devices.

Morphology

A. baumannii is a coccobacillus, intermediate in shape between cocci and rods. They measure 1.5 to 2.5 by 1 to 1.5 microns when populations are in a logarithmic phase of growth. They are more spherical when they reach the stationary phase.

Metabolism

The bacterium A. baumannii is not a glucose fermenter; It is strict aerobic, that is, it requires oxygen for its metabolism.

The species of the genus Acinetobacter are the only ones in the Moraxellaceae family that lack cytochrome c oxidases, which is why they show negative results in oxidase tests.

A. baumannii grows at temperatures that vary between 20 and 44ºC, its optimum temperature being between 30 and 35ºC.

Antibiotic resistance

The constant generation of antibiotic resistance not only hinders the treatment and control of infections caused by A. baumannii, but also promotes the selection of endemic and epidemic multi-resistant strains.

Some intrinsic mechanisms of A. baumannii that promote resistance to antibiotics are known:

• The presence of β-lactamases confers resistance to b-lactams.
• The production of specific enzymes such as ammoniglucoside-3'-phosphotransferase VI, inactivates amikacin.
• The presence of oxacillinase OXA-51 hydrolyzes penicillins and carbapenems.
• The presence and overexpression of reflux pumps, which are pumps that expel small molecules out of the cell that manage to penetrate the cytoplasm, thus reducing their susceptibility to antibiotics.

Biofilms generated by A. baumannii alter the metabolism of microorganisms, reducing their sensitivity to antibiotics, in addition to providing a physical barrier against large molecules and preventing dehydration of bacteria.

Pathologies

A. baumannii colonizes a new host by contact with infected people, or with contaminated medical equipment. First of all, this bacterium attaches itself to the skin and mucosal surfaces. To reproduce it must survive antibiotics and inhibitors and the conditions of these surfaces.

The increase in the number of bacteria on mucosal surfaces, particularly in conditions of hospitalization in contact with intravascular catheters or endotracheal tubes, may increase the risk of infection of the respiratory tract and bloodstream.

Hospital-acquired pneumonia is the most common infection caused by A. baumanii. Commonly contracted in Intensive Care Units, by patients receiving mechanically assisted respiration.

A. baumanii has also caused major infection problems in military personnel with postwar trauma, specifically in Iraq and Afghanistan. Specifically, due to osteomyelitis and soft tissue infections, which can cause necrosis and cellulitis.

There are also risks of A. baumanii meningitis in patients recovering from neurosurgery.

Individuals susceptible to A. baumanii infections include those who have previously been subjected to the use of antibiotics, major surgeries, burns, trauma, immunosuppression, or the use of invasive medical devices, mainly mechanical ventilation, in Intensive Care Units.

Symptoms

There is no specific symptomatology of A. baumanii infections. Each of the different infections produced by this bacterium has its own characteristic symptoms.

In general, the symptoms of infections that may involve A. baumanii or other opportunistic bacteria such as Klebsiella pneumoniae and Streptococcus pneumoniae, can include fever, chills, rash, painful urination, urgent need to urinate frequently, confusion or altered mental states. nausea, muscle pain, chest pain, and cough.

Treatment

Antibiotics to treat A. baumannii infections are extremely limited, due to their great capacity to acquire resistance and multi-resistance. For this reason, it is important to determine the susceptibility of each strain to different antibiotics to guarantee the effectiveness of each treatment.

Given the resistance to carbapenems, the use of polymyxins, specifically colistin, has been resorted to, despite having a relatively low resistance index and its side effects on the kidneys.

However, strains resistant to colistin have already been detected. As an alternative to resistance to these antibiotics, combination therapy has been used.

References

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3. Hernández Torres, A., García Vázquez, E., Yagüe, G. &, Gómez Gómez, J. (2010) Multiresistant Acinetobacter baumanii: current clinical situation and new perspectives Revista Española de Quimioterapia, 23 (1): 12-19.
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7. Vanegas-Múnera, JM, Roncancio-Villamil, G. & Jiménez-Quiceno, JN (2014). Acinetobacter baumannii: clinical importance, resistance mechanisms and diagnosis. CES Medicine Magazine, 28 (2): 233-246.