Acinetobacter baylyi

A. baylyi under 10x ocular lens and 100x objective lens with crystal violet stain.

Acinetobacter baylyi
Scientific classification Edit this classification
Domain: Bacteria
Phylum: Pseudomonadota
Class: Gammaproteobacteria
Order: Pseudomonadales
Family: Moraxellaceae
Genus: Acinetobacter
Species:
A. baylyi
Binomial name
Acinetobacter baylyi
Carr et al. 2003

Acinetobacter baylyi is a bacterial species of the genus Acinetobacter, which was discovered in activated sludge in Victoria, Australia, alongside six other new Acinetobacter species in 2003. A. baylyi is named after the late Dr. Ronald Bayly, an Australian microbiologist who contributed significantly to the research of the Acinetobacter genus.[1][2][3] As with other species of Acinetobacter, it is a nonmotile, Gram-negative coccobacillus. It grows under strictly aerobic conditions, is catalase-positive, nitrate-negative, oxidase-negative, and non-fermentative.[4] The species is naturally competent, meaning that it can take up free exogenous DNA from its surroundings and incorporate the DNA into its own chromosomal DNA by transformation.[5]

A. baylyi has been observed as a novel, low-virulence nosocomial pathogen, despite lacking typical virulence-related genes, that is resistant to many common antibiotics. It is primarily acquired by patients who have immunosuppressing conditions like diabetes or malignancy. However, it is still treatable with alternative antimicrobial medications.[6][7] A. baylyi's survival depends on a bacterial "contact-dependent growth inhibition" (CDI) system that transports toxic self-produced proteins to the microbe's outer membrane in order to hinder the growth of neighboring cells. Despite aiding in A. baylyi biofilm formation, CdiA proteins do not contribute to enhancing the biofilm's attachment to host epithelial cells. A. baylyi has shown promise as a method for alternative fuel sources.[8]

Metabolism

A. baylyi metabolic pathways have been used for many studies in microbial metabolism, known for its fast growth rate and ability to be easily cultured.[5] A. baylyi can be cultured in mediums using organic carbon sources to survive such as glucose, pyruvate, acetate, and ethanol.[9] A. baylyi is also known as an omnipresent bacterium, meaning it can be found many places in nature.[2] This level of utility is due to the microbe's use of different compounds for metabolism as well as lack of a gene coding for pyruvate kinase, a vital enzyme in glycolysis for transforming phosphoenolpyruvate into pyruvate.[10][11] A. baylyi prefers to utilize organic carbon sources that can enter directly into the citric acid cycle, such as aromatic compounds. A. baylyi can also metabolize glucose as its sole carbon and energy source as it is oxidized into gluconate which feeds into the Entner-Doudoroff pathway, albeit slower in comparison.[12][11][9] This is because without pyruvate kinase, A. baylyi has to use a work-around of transforming the phosphoenolpyruvate into oxaloacetate, then malate, which can then become pyruvate and enter the pyruvate dehydrogenase complex and later the citric acid cycle.[11] A. baylyi is able to utilize aromatic compounds as organic carbon and energy sources through the unique B-ketoadipate pathway. Aromatic compounds are first transformed into catechol and protocatechuate, which then are transformed into the citric acid cycle substrates succinyl-CoA and acetyl-CoA, respectively.[12] Unlike other bacteria that can predominantly use L-amino acids, A. baylyi is an example of this metabolic tolerance for different compounds with its ability to utilize D-Asp and L-Asp amino acids as both a primary carbon and nitrogen source, thus opening the door to see how D-enantiomers can be used for bacterial growth.[13]

Previous experiments have shown that A. baylyi uses intracellular arginine to produce a biodegradable alternative to petroleum-based plastics known as polyaspartic acid. A. baylyi uses arginine to first produce cyanophycin polymers, which can then be converted to polyaspartic acid.[14]

ADP1 Strain

A. baylyi, specifically the strain ADP1, has been used for over a quarter of a century in several molecular biology studies due to its strong ability to easily undergo genetic transformation.[5][11] For these reasons, A. baylyi is used in multiple laboratory techniques. These include genetics, specifically gene duplication and amplification as well as bacterial metabolism.[5][10] The microbe has also been studied for its potential use an alternative triacylglycerol (TAG) source, as under nitrogen limiting conditions it is able to transform excess organic matter into wax esters and triacylglycerols (TAGs) as a lipid storage form through the isoenzymes wax ester synthase/diacylglycerol acyltransferase.[15][9] The concentration of wax esters and triacylglycerols that the ADP1 strain produces depends on the organic matter present in medium of which the A. baylyi is grown on.[9]

Genetic mechanisms

One major characteristic of A. baylyi is its ability to take in free DNA from the environment. It does so by importing the DNA by natural transformation, a highly efficient, unique mechanism that incorporates exogenous DNA into its chromosome.[5] This mechanism strongly depends on A. baylyi's DNA strand break repair system in order to ensure success of recombination with exogenous DNA.[16] Most bacteria struggle to achieve this exchange of adaptive traits from outside DNA via simple point mutations, so the ease at which A. baylyi can take in foreign DNA is beneficial to its survival.[17] Natural transformation contributes greatly to this bacteria's ability in antibiotic resistance and vaccination escape.[5][17] This also makes A. baylyi an ideal microbe for laboratory experiments.[5]

The facilitation of A. baylyi's ability in natural transformation, or horizontal gene transfer (HGT) processes, may be aided by the mechanisms of outer membrane vesicles (OMVs). OMVs are produced via vesiculation, the bulging of the outer membrane followed by the constriction and release of small, spherical structures from the bacterium, and are composed of various periplasmic components, including proteins and lipids, as well as some genetic material. OMVs play significant roles in intracellular communication, virulence/bacterial defenses, and adaptation to environmental stress. OMVs released by A. baylyi offer a mode of gene transfer that is not susceptible to degradation by nucleases, contributing to the microbe's high survival rate and antibiotic resistance; however, environmental stress factors can impact the efficiency of these OMVs, ranging from levels of vesicle release to genetic content and HGT abilities.[18]

Due to its increasing involvement in hospital infections and multi-drug resistance nature back in 2014, A. baylyi strains have also been closely associated with pathogenicity, bacterial adhesion, and biofilm formation. Biofilms arise from the aggregation of surface microbial cells enveloped within a matrix of extracellular polymeric substances. The biofilms of Acinetobacter strains have been associated with various infectious diseases, including cystic fibrosis or urinary tract infections, due to their ability to adhere to medical devices composed of plastic or glass.[4] A. baylyi's genetic transformation mechanisms may be attributed to its success as an infectious agent due to the optimal environment it creates with its biofilms.[19] It has been found that two possible genes may be significant to biofilm formation: Fimbrial-biogenesis protein (3317) and Putative Surface protein. These genes are very similar to the genetic mechanisms behind the antibiotic resistance characteristics of a similar strand, Acinetobacter baumannii.[4]

Pathophysiology

Virulence factors

A. baylyi has been on the rise as a pathogen, however, this is unusual due to lacking pathogenic traits and virulence-related genes like toxins, invasins, and secretory systems.[20] This results in A. baylyi being considered a low-virulence pathogen. A. baylyi is an opportunistic pathogen, typically acquired within a hospital setting by patients who already have coexisting diagnosis like diabetes mellitus or malignancy. It is important to note that A. baylyi's strong ability to perform natural transformation is a potential reason for its increasing antibiotic resistance, particularly to the beta-lactam type of drugs.[6] Beta-lactam drugs include a beta-lactam ring in their chemical structure, and examples include penicillins and cephalosporins.[21] These are ineffective against A. baylyi as the microbe has β-lactamase enzymes which can metabolize said medications.[6]

Survival

A. baylyi codes for a bacterial "contact-dependent growth inhibition" (CDI) system, a common feature of Gram-negative bacteria. Cognate transport CdiB proteins on A. baylyi's membrane export toxic CdiA proteins to the outer membrane, which undergo a reaction by outer membrane receptors in order to release the C-terminal toxic (CT) domain responsible for inhibiting the growth of neighboring cells. CdiA proteins also facilitate the growth of an A. baylyi biofilm, however, the proteins do not assist in strengthening the biofilm's attachment to host epithelial cells.[22]

Additionally, A. baylyi has a variety of other structural features that contributes to its survival, ranging from lipopolysaccharide (LPS) chains in their outer membranes to having thick capsules. The LPS chains serve to provide structural integrity, and the capsule protects the microbe from complement-mediated killing by complement proteins, a part of the innate immune system.[23][24]

Prophylaxis and Treatment

Prophylaxis

The most recommended method in preventing acquiring or spreading A. baylyi pathogens is to thoroughly wash hands and under fingernails with water and soap, especially before and after interacting with medical devices, "high-touch" surfaces, and wound care.[25]

Treatment

Studies have shown that by targeting the LysR-type transcriptional regulators present in the A. baylyi's with antibiotics has shown to show significant decline in the growth of the bacteria.[7] Antibiotics like imipenem, aminoglycosides, levofloxacin, trimethoprim/sulfamethoxazole, and chloramphenicol have demonstrated the greatest effectiveness in treating A. baylyi bacterial infection. Antimicrobial therapy has also been shown to help infected patients recover from infections caused by A. baylyi.[6]

Imipenem is part of the carbapenem drug class, which are resistant to degradation from β-lactamase enzymes. The drug affects A. baylyi by saturating and out-competing nutrients from entering the cell through specific nutrient uptake channels on its outer membrane, therefore blocking the channels. It especially impacts A. baylyi's ability to intake basic amino acids L-Arginine, lysine, and ornithine.[26]

Aminoglycosides are a category of drugs that inhibit the growth of Gram-negative bacteria, specifically by preventing protein synthesis.[27] Examples include streptomycin, neomycin, and tobramycin. They arose in response to multi-drug-resistance bacterial strains like A. baylyi, as well as Pseudomonas aeruginosa, Klebsiella pneumoniae, and Escherichia coli. Aminoglycosides are effective medications as they are first able to displace magnesium and calcium ions from bridging liposaccharides on the outer cell membrane, causing the bacteria's cell membrane to shift around and create pores due to the lack of cell membrane support. However, these pores also let in the aminoglycosides, which can then bind to the A-site of the 16S ribosomal RNA component of the 30S ribosomal subunit, inhibiting protein synthesis.[28]

Levofloxacin is a third-generation fluoroquinolone, acting upon nucleic acid transcription and replication as a non-intercalating topoisomerase poison, inhibiting the formation of new cells as a concentration-dependent bactericidal agent.[29][30] As A. baylyi is a Gram-negative bacteria, levofloxacin will both stabilize the temporary complex formed between the DNA helix and GyrB of DNA gyrase (a topoisomerase II enzyme), not allowing the enzyme to dissociate from the DNA, as well as block GyrA to pull the DNA strand through the C-gate, preventing the newly transcribed and replicated DNA from super-coiling, thereby not allowing the DNA to be stored away in the newly formed cell and killing it.[31][32][33]

Trimethoprim/sulfamethoxazole, typically given as a combination therapy known as co-trimoxazole, are two drugs that work together to inhibit A. baylyi's and other bacteria's folate synthesis metabolic pathway. Sulfamethoxazole, a sulphonamide, works first in the pathway by acting as a competitive inhibitor of dihydropteroate synthetase--an enzyme that only bacteria have and not humans--and preventing the synthesis of dihydropteroate from dihydropteroate diphosphate and p-aminobenzoic acid (PABA) by mimicking PABA.[34] The dihydropteroate is then condensed with L-Glutamic acid into dihydrofolate via an ATP-requiring reaction catalyzed by dihydrofolate synthetase.[35][34] Trimethoprim works next by inhibiting dihydrofolate reductase, an enzyme found in both humans and bacteria from reducing dihydrofolate into tetrahydrofolate. While dihydrofolate reductase is an enzyme found in both humans and bacteria, trimethoprim is "100,000 times" more selective towards the bacterial enzyme. Sulfamethoxazole and trimethoprim do not directly kill the bacteria, instead preventing the cell from growing and dividing in order to allow the body's immune system to fight it off.[34]

Chloramphenicol inhibits A. baylyi's peptidyl transferase enzyme, which is responsible for extending the protein chain by moving it from the tRNA on the bacterial ribosomal 50S subunit P-site to the next tRNA-bonded amino acid at the ribosomal 50S subunit A-site. As the transfer can't occur, the ribosome is no longer able to "read" the mRNA chain and protein synthesis is halted.[36]

Applications

Outside of a lab setting, A. baylyi's ability to create TAGs has been utilized as a potential alternative method of producing TAG-based products like cosmetics, oleochemicals, and biofuels. They are currently made with the TAG sources of vegetable oils, animal fats, and recycled greases.[8]

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