Bacterial Resistance of Acinetobacter baumannii: A Global Concern

Acinetobacter baumannii ( A. baumannii ), one of the five most important bacteria with global threat to human health due to constantly increasing resistance (ESKAPE organisms), identified as a enormous threat in healthcare facilities, can create antibiotic resistance. The implementation of early detection and identification of multidrug-resistant A. baumannii is serious to control its spread. The this study presents the human infection of A. baumannii , pathological findings, virulence factors of A. baumannii , antibiotic resistance mechanisms, and the therapeutic options available for treating A. baumannii infections. The ability of A. baumannii to develop antibiotic resistance mechanisms allows the organism to prosper in hospital settings, facilitating the global spread of multidrug-resistant strains. To dominate this problem, knowledge of the pathogenesis and antibiotic resistance mechanisms of A. baumannii is important. As reported, A. baumannii resistance to aminoglycosides, fluoroquinolones, and carbapenems increased, and resistance to lipopeptides, such as polymyxin B and colistin, are lower compared to that of other antimicrobial drugs. Therefore, novel prevention and treatment strategies against A. baumannii infections are warranted.


Introduction
Antimicrobial or antibiotic resistance (AMR) is an important and triggering phenomenon with increasing costs for healthcare systems worldwide. In recent years, AMR has been related to significant morbidity, mortality, and increased costs resulting from prolonged hospitalization and treatment. Data from multicenter studies in the previous decades have demonstrated an increase in both community-acquired and nosocomial AMR as well as a rise in the number of older patients with primary or secondary immunodeficiencies 1,2 .
The World Health Organization (WHO) has long recognized the need for an improved and coordinated global effort to contain AMR 3,4 . The first WHO Global report on AMR surveillance was conducted on national and international surveillance networks for the first time, indicating the extent of AMR surveillance in different parts of the world and the presence of large gaps in the existing surveillance 4 .
Acinetobacter baumannii (A. baumannii) belongs to the Moraxellaceae family and is a Gram-negative bacterium that predominantly causes nosocomial infections. Acinetobacter taxonomy involves phenotypic traits and chemotaxonomic methods 5 . Acinetobacter baumannii is part of the A. baumannii -A. calcoaceticus complex (Acb), initially including four species, namely A. calcoaceticus, A. baumannii, A. nosocomialis, and A. pittii 4 . Subsequently, several other species, such as A. seifertii, have been proposed for inclusion in this complex 5 , A. lactucae 6 , and Acinetobacter species between 1 and 3 6 .
Acinetobacter infections include meningitis, urinary tract infections, hospital-acquired (HAP) and ventilator-associated pneumonia (VAP), bacteremia, and gastrointestinal and skin/wound infections 7 . Gupta et al. observed that the infection was common in patients aged group >50 years followed by those younger than 10 years old 8 , indicating a wide range of infections at different ages. Due to clusters of closely related species, distinguishing Acinetobacter species is difficult. Among Acinetobacter species, Acinetobacter baumannii is the most important member associated with hospital-acquired infections worldwide 9 .
Acinetobacter baumannii is one of the ESKAPE organisms (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, A. baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) that pose a global threat to human health and a therapeutic challenge due to constantly increasing resistance 10 . Resistance of A. baumannii isolates to broad-spectrum antibiotics, such as amikacin, expanded-spectrum cephalosporins, carbapenems, and tigecycline is on the rise, so physicians are increasingly left with very few effective therapeutic options 11 . Carbapenem-resistant A. baumannii (CRAB) was ranked in 2018 by WHO as the number one priority for antibiotic research and development. Carbapenem was chosen as a marker because carbapenem resistance is usually associated with a broad range of co-resistance to other antibiotic classes 12 . Imipenem-resistant A. baumannii constituted more than 50% of a worldwide collection of clinical trials between 2005 and 2009 13 .
The overall prevalence of multidrug-resistant strains in patients with A. baumannii HAP and VAP is estimated to be 79.9%, ranging from 56.5% in Argentina and 61.8% in Taiwan to 100% in Central America, Pakistan, Lebanon, Qatar, and Croatia, while its overall mortality can be as high as 56.2% 14 . The patterns of carbapenem resistance differ throughout Europe and also within the countries of the Arab League. Increased incidence of carbapenemresistant A. baumannii isolates has been observed in Northern and Eastern Europe as well as in the Levant countries of the Arab League (Iraq, Jordan, Lebanon, Palestinian territories, and Syria) 15 . This aerobic Gramnegative coccobacillus had been regarded as a low-grade pathogen, but is a successful pathogen responsible for opportunistic infections of the skin, bloodstream, urinary tract, and other soft tissues 16 . Since many A. baumannii infections have suddenly been reported among veterans and soldiers who served in Iraq and Afghanistan 17 , A. baumannii is referred to as Iraqibacter. The frequency of community-acquired A. baumannii infections has increased gradually 18 . Many reports have shown that isolates of A. baumannii rapidly develop resistance to antimicrobials and multidrug-resistant strains 19 .
A systematic review concluded that the acquisition and spread of A. baumannii appeared to be related to a large number of variables, the most important of which were deficiencies in the implementation of infection control guidelines and the use of broad-spectrum antibiotics 20 .
Moradi in Iran found that the antimicrobial resistance of A. baumannii has increased, which may affect the antimicrobial resistance of this organism worldwide 21 .
This review summarizes the role of A. baumannii in human infectious pathology, virulence factors of A. baumannii, antibiotic resistance mechanisms, and the therapeutic options available for treating A. baumannii infections.

Risk factors
Age, respiratory and cardiovascular system diseases, diabetes mellitus, high APACHE 2 score, immunesuppression, antibiotic use, hospitalization before infection, especially in intensive care units (ICUS), central venous and nasogastric catheter, mechanic ventilation are listed as risk factors of A. baumannii infections. Transfusion, hemodialysis, and trauma are not known as risk factors for this infection 22 .
Comorbidities refer to hypertension, coronary heart disease, diabetes, cancer, chronic renal insufficiency, and cerebral infarction; ICU days refer to patients with ICU admission until they test positive for the first time. The combination of antimicrobial agents is the use of two or more than two kinds of antibacterial drugs. Invasive operation refers to a tracheotomy, nasal feeding, indwelling catheter, arteriovenous catheter, abdominal puncture, and ventilator 23 .
Chiang and Silvia Munoz et al. also found that the tumor underlying disease is a risk factor causing the death of patients with A. baumannii bloodstream infection 24,25 .
In previous studies, risk factors of A. baumannii bloodstream infection included serious underlying illnesses, exposure to antibacterial drugs, colonization of bacteria, history of surgery, central venous catheter and indwelling catheter, parenteral alimentation, mechanical ventilation as well as time in ICU 26 .

Virulence factors of Acinetobacter baumannii
Several effective factors can play a role in the disease process caused by A. baumannii 27 . Table 1 demonstrates the important virulence factors and their role in A. baumannii pathogenesis.
Pieces of evidence elucidated these virulence factors, including adherence and invasion in host cells and host cell death, outer membrane protein A (OmpA), phospholipids, extracellular polysaccharides capsule, siderophore-mediated iron-acquisition system, phospholipases, and biofilm formation have an important role in bacterial pathogenicity 28 .
These virulence factors, along with MDR trait, make this pathogen create havoc, at least in hospitals, and act as the emerging cause of nosocomial respiratory and urinary tract infections. As this is a nosocomial pathogen, every individual admitted to hospitals or undergoing antibiotic treatment has the potential risk of acquiring infection 28,29 .

Isolation and identification methods
There is a variety of methods to assess the diversity of Acinetobacter spp. Several methods or combinations of methods have been found useful in delineating species 5 .
The phenotypic and genotypic identification methods (biochemical systems and 16S rRNA gene sequencing) utilized in the species determination of Acinetobacter spp.. The 16S rRNA gene sequencing is not too conserved to distinguish all Acinetobacter spp. 6 . A more effective identification method is required for clinical or laboratory applications. In this case, rpoB sequencing and Matrix-Assisted Laser Desorption/Ionization Time of Flight Mass Spectrometry (MALDI-TOF-MS) was evaluated as two alternative methods.
The rpoB gene has a housekeeping role, and its size differs between species 37 . The variability of the rpoB gene sequence ensures that it is impossible to design universal primers to amplify this gene for all bacteria. As a result, rpoB is more suitable for typing subspecies, and is frequently used as a multiple-locus sequence typing (MLST) locus for many bacterial species 37 .

Antimicrobial resistance of A. baumannii
The severity of A. baumannii infections is caused by the high ability of this bacterium to survive in extreme environmental conditions through the multitude of resistance mechanisms 6 . The A. baumannii revealed both intrinsic and acquired resistance mechanisms based on chromosomal mutations and the acquisition of ARGs (antibiotic resistance genes) through HGT (Horizontal gene transfer ) 35 . The A. baumannii antibiotic resistance is driven by multiple mechanisms 37 .
Acinetobacter has impressive genetic plasticity, facilitating rapid genetic mutations and rearrangements as well as the integration of foreign determinants carried by mobile genetic elements 37 . Of these, insertion sequences are considered one of the key forces shaping bacterial genomes and evolution 37 . Additionally, A. baumannii can form biofilms and thus prolong its survival on medical devices, such as ventilators in ICUs 38 . However, the relationship between biofilm formation and antibiotic resistance remains unclear 38 .
Biofilms are sessile microbial cells embedded in a selfproducing exopolysaccharides (EPS) matrix. In biofilms, bacterial cells possess increased resistance to antibiotics or biocides, the host's immune response, antimicrobial agents, and a high ability to survive in extreme environmental conditions 6 . The impaired diffusion of antimicrobial agents causes increased bacterial resistance in biofilms due to microbial aggregation, overexpression of matrix exopolymers, and alteration of phenotypic and genotypic microbial characteristics to stress response 6,38 . The phenotypic and genotypic characteristics of bacterial cells are triggered when producing quorum-sensing signals in a cell-dependent manner, signals that allow communication between cells when significant changes in environmental conditions occur 39 . Antimicrobial agents such as antibiotics or biocides can trigger the formation of biofilms if administered at concentrations lower than the minimum inhibitory concentration (MIC) 38 . Therefore, treating infections caused by biofilm-forming bacteria requires higher doses of antibiotics and antimicrobial agents 6 .
A systematic review concluded that the acquisition and spread of A. baumannii appeared to be related to a large number of variables, the most important of which were deficiencies in the implementation of infection control guidelines and the use of broad-spectrum antibiotics 41 .
Certain strains of A. baumannii are highly resistant to most antibiotics available in clinical practice. A number of resistance mechanisms to different classes of antibiotics are known to exist in A. baumannii, including β-lactamases, multidrug efflux pumps, aminoglycoside-modifying enzymes, permeability defects, and the alteration of target sites 42 . Most of these resistance mechanisms can target different classes of antibiotics. However, several other mechanisms can work together to contribute to the resistance to a single class of antibiotics. Carbapenems were the preferred treatment for MDR A. baumannii infections, but their prior use has led to an increased incidence of carbapenem resistance during the last years 43 . Polymyxins are now widely used as antibiotics for MDR A. baumannii infections. They were initially avoided due to their systemic toxicities (nephrotoxicity and neurotoxicity) 44 . Extensive drug-resistant (XDR) A. baumannii is called an isolate resistant to three or more classes of antimicrobials (penicillins and cephalosporins-including inhibitor combinations, fluoroquinolones, and aminoglycosides, resistant to carbapenems in most cases), while pan drugresistant (PDR) A. baumannii is an XDR isolate resistant to polymyxins and tigecycline. Lately, extensively drug-  Table  2 shows the different antimicrobial resistance mechanisms of A. baumannii.

Treatment aspects of infections caused by Acinetobacter baumannii
The most important aspect of the infection with A. baumannii strains is their resistance to entirely known antibiotics, suggesting the need for urgent action by the global healthcare community. Due to the high antibiotic resistance rate, this pathogen can survive for a long time in the hospital environment and spread nosocomial 1, 2 .
Acinetobacter baumannii may cause pneumonia, wound infections, bacteremia, urinary tract infections and meningitis 7 . Among the identified risk factors leading to colonization or infection with A. baumannii (sometimes difficult to distinguish), prolonged hospitalization, ICU admission, recent surgical procedures, antimicrobial agent exposure, central venous catheter use, prior hospitalization, nursing home residence and local colonization pressure on susceptible patients are well known 22,23 .
Those infections can be treated with a combination of β-lactam and aminoglycoside. The combination of a βlactam with an aminoglycoside appears at least synergistic in vitro and allows a rapid bactericidal effect 49 . Fluoroquinolones also exhibited a rapid bactericidal effect against susceptible A. baumannii and therefore can be used in combination with a β-lactam 46 . The increasing resistance trend observed for fluoroquinolones, aminoglycosides, and broad-spectrum β-lactams has consequently led to the use of carbapenems alone or in combination with nonclassical molecules, such as polymyxin, rifampin and sulbactam [44][45][46] . Tigecyline is often active against multidrug-resistant A. baumannii; however, recent reports described the emergence of tigecycline resistance 47 . The control of MDR in A. baumannii can be one of the significant challenges in clinical microbiology in the near future. Those infections can be treated with a combination of βlactam and aminoglycoside. The combination of a βlactam together with an aminoglycoside appears at least synergistic in vitro and allows a rapid bactericidal effect 48 .
Minocycline, has been proposed for treating drugresistant A. baumannii 49 . However, approximately 20% of A. baumannii isolates are not susceptible to minocycline since the introduction of minocycline, 49 . Minocycline therapy combined with colistin is effective for treating minocycline-resistant A. baumannii infections 48 , and minocycline therapy combined with rifampicin, colistin, or imipenem has a synergistic effect in most isolates without the tetB gene 49 . Lin et al. (2014) observed that only a few effective anti-Acinetobacter currently available drugs, such as polymyxins and tigecycline 50 .
The colistin resistance of baumannii isolates in MDRA (10.4%) was lower than that of rifampicin (47.8%) or tigecycline (45.5%) resistance 51 . Therefore, colistin seems to be the only effective antimicrobial agent against MDR A. baumannii infections. Unfortunately, the emergence of colistin-resistant A. baumannii strains has increased worldwide 52 .
Trimethoprim-sulfamethoxazole alone effectively kills all carbapenem-resistant A. baumannii strains, and trimethoprim-sulfamethoxazole combined with colistin also rapidly kills all strains for up to 24 h 52 .
Fluoroquinolones also exhibited a rapid bactericidal effect against susceptible A. baumannii; therefore, it can be used in combination with a β-lactam 46,47 . The increasing resistance trend observed for fluoroquinolones, aminoglycosides, and broad-spectrum β-lactams has consequently led to the use of carbapenems alone or in combination with nonclassical molecules, such as polymyxin, rifampin, and sulbactam 46,53 . Tigecyline is often active against MDR A. baumannii; however, recent reports described the emergence of tigecycline resistance 41,42 . It is likely that the now widely distributed blaNDM carbapenemase genes, increasingly reported in Enterobacteriaceae, first spread among Acinetobacter spp. before disseminating into Enterobacteriaceae 38 . Acinetobacter baumannii exhibits different factors potentially involved in the persistence of antimicrobial resistance in healthcare institutes (either antibiotics or antiseptics) and also shows a robust metabolism that is possibly responsible for higher survival on inorganic surfaces compared with most enterobacterial species 29,47 . Nevertheless, the current main problem with regard to the A. baumannii resistance is that carbapenems are often associated with multidrug or even pandrug resistance.
Since antibiotic-based therapies may become more and more limited when dealing with A. baumannii, alternative therapies are being explored. These experimental therapies include bacteriophage-based therapy or antibacterial peptides 44,46 . The main problem with these therapies is that their efficacy has been evaluated only in vitro. The pharmacokinetic/pharmacodynamic profiles of these compounds, including half-life, diffusion in the host organism, and potential degradation by human body fluids, limit their clinical efficacy. Accordingly, bacteriophage therapy is quite hazardous, and there is not enough data regarding the in vivo activity of such compounds. Moreover, control of the virus after treatment seems to be impossible. In addition, the emergence of bacteriophageresistant strains under therapy can rapidly occur (modification of their membrane target site). However, it is believed that antibiotic-use policies and control of antibiotic resistance are crucial for controlling the emergence and spread of antibiotic resistance in A. baumannii 48 .

Conclusion
Acinetobacter infections are becoming an increasingly common clinical entity which may very well affect the antimicrobial resistance of this organism worldwide. The enormous adaptability of A. baumannii, as well as the ability to survive in extreme environmental conditions, lead to a permanent need to unravel the diversity of mechanisms involved in the acquisition and transfer of resistance determinants.
Aminoglycosides, fluoroquinolones, and carbapenems are common antibiotics for treatment of A. baumannii infections. Results of this study shows that A. baumannii resistance to these agents increased. Also, resistance to lipopeptides such as polymyxin B and colistin are lower compared with that of other antimicrobial groups. Therefore, novel prevention and treatment strategies against A. baumannii infections are warranted.