Outer membrane proteins (OMPs) are a class of integral membrane proteins anchored in the outer membrane with a β-barrel structure. OmpA is one of the most abundant porins in the outer membrane of A. baumannii (Park et al., 2011; Uppalapati et al., 2020). OmpA is connected to the diaminopimelic acid of the peptidoglycan by two conserved residues (Asp271 and Arg286) in its periplasmic C-terminal domain (Park et al., 2012.). These characteristics give OmpA high stability in the membrane and the capability to fight against harsh environments (Moon et al., 2012). Indeed, being exposed to the outside of the bacterial cell OmpA provides the first line of contact between the bacterium and its surroundings. Given its central position, OmpA acts as an adhesion factor in virulence, channels for the uptake of nutrients, siderophore receptors, and enzymes such as proteases and lipases. Three OMPs were identified as fibronectin-binding proteins, such as OmpA, TonB-dependent copper receptor, and 34 kDa Omp (Smani et al., 2012). OmpA forms a non-selective channel in bacterial outer membranes that permits the passage of ions and other solutes (Sugawara and Nikaido, 2012; Confer and Ayalew, 2013). Furthermore, OmpA contributes to the antimicrobial resistance of A. baumannii (Sugawara and Nikaido, 2012; Smani et al., 2014). Indeed, disrupting the OmpA gene decreases the minimal inhibitory concentrations (MICs) of aztreonam, chloramphenicol, and nalidixic acid by 8, 8, and 2.7-fold, respectively. This data suggests that OmpA participates in the extrusion of antibiotics from the periplasmic space through the outer membrane and couples with inner membrane efflux systems (Smani et al., 2014). In A. baumannii, OmpA serves multiple functions, both in vitro and in vivo, including adherence to epithelia, induction of epithelial cell death, drug resistance, channels for the uptake of nutrients, siderophore receptors, binding to factor H (Choi et al., 2005, 2008; Gaddy et al., 2009; Kim et al., 2009). OmpA enhances the survival and persistence of A. baumannii by facilitating biofilm formation (Gaddy et al., 2009; Shin et al., 2009). In particular, outer membrane receptor proteins are significantly upregulated in biofilm than in planktonic cultures (Shin et al., 2009). Moreover, it has been reported that overexpression of OmpA represents a significant risk factor for pneumonia, bacteremia, and enhanced mortality in patients infected with A. baumannii (Sánchez-Encinales et al., 2017). In A. baumannii, virulence factors, including OmpA and certain tissue-degrading enzymes, are delivered to host cells via OMVs (Jin et al., 2011). OMVs are spherical elements with a 20–200 nm diameter, secreted by various Gram-negative pathogenic bacteria (Kulp and Kuehn, 2010). They mainly comprise lipopolysaccharide (LPS), outer membrane and periplasmic proteins, phospholipids, and nucleic acids, representing delivery vehicles for bacterial effectors to host cells (Ellis and Kuehn, 2010). OMVs are central in delivering A. baumannii virulence factors, including OmpA, and certain tissue-degrading enzymes, such as proteases and phospholipases (Lee et al., 2017). Furthermore, OmpA has the highest content in OMVs, which is involved in the mitochondrial decomposition of the host’s cell apoptosis (Choi et al., 2005; Tiku et al., 2021).

Phospholipases are lipolytic enzyme essential for phospholipid metabolism and a major virulence factor in many Gram-negative bacteria. Phospholipids are the primary building blocks of biological membranes and a carbon and energy source in the human host. In A. baumannii, have been identified two phospholipases C (A1S_0043 and A1S_2055) and three phospholipases D (PLD1, PLD2, PLD3), all with substrate specificity toward the eukaryotic membrane component phosphatidylcholine (PC; Flores-Díaz et al., 2016). PC is abundant in eukaryotic membranes representing 50% of all phospholipids and increasing up to 80% in the lung and tracheobronchial secretions (Girod et al., 1992; Bernhard et al., 2001; Tomaras et al., 2003) Experimental evidence suggests that it may serve as a nutrient source during lung infections by pathogens like Pseudomonas aeruginosa and A. baumannii (McConnell et al., 2013; Sun et al., 2014; Özarslan et al., 2023). Phospholipids’ degradation compromises the stability of host cell membranes, interfering with cellular signaling, thus resulting in changes in the host immune response (Flores-Díaz et al., 2016). In particular, the 1,2-diacylglycerol released by cellular phospholipases C plays roles in modifying biophysical membrane properties, including charge, fluidity, and permeability, and can recruit cytosolic proteins that induce spatial reorganization of signaling complexes, which in turn affect diverse cellular processes (Toker, 2005; Flores-Díaz et al., 2016). Consequently, products generated by bacterial phospholipases could affect the immune response and promote the infection’s establishment or progression (van der Meer-Janssen et al., 2010). In A. baumannii, phospholipases D concertedly promote serum resistance, epithelial cell invasion, and in vivo pathogenesis (Jacobs et al., 2010; Stahl et al., 2015). Interestingly, PLD1 and PLD2 appear to result from a gene duplication characterized by the HxKx4Dx6GSxN (HKD) pattern similar to eukaryotic cells and required for catalytic activity (Stahl et al., 2015). Despite their similarity, PLD2 is more important for invasion and virulence than the other two PLDs (Jacobs et al., 2010; Stahl et al., 2015). Since phospholipases are conserved across numerous strains of A. baumannii and are essential for host invasion, they may represent promising targets for developing enzyme inhibitors and potential vaccine candidates to limit the impacts on human diseases (Flores-Díaz et al., 2016).

The Type II secretion system (T2SS) is a two-step process, dependent on the general secretory pathway (Sec) or the Twin-arginine (Tat) system for substrate translocation to the periplasm before secretion in the extracellular environment (Weber et al., 2017). The T2SS was first described in A. baumannii ATCC17978, with the specific apparatus encoded by genes designated, general secretory pathway (GspA-O), located in six separate operons (Eijkelkamp, 2014). Secretion of type II effector proteins includes enzymes such as lipase, elastase, alkaline phosphatase, and phospholipases, which are essential for A. baumannii virulence (Elhosseiny and Attia, 2018). In A. baumannii, major T2SS effectors include the metalloendopeptidase, CpaA, and the lipases, LipA and LipH (Johnson et al., 2016; Weber et al., 2017). Secretion of CpaA and LipA requires specific membrane-associated chaperones CpaB and LipB (Zheng et al., 2013; Harding, 2016). In particular, LipA contributes to extracellular lipolytic activity by using long-chain fatty acids as carbon sources for growth and may use fatty acids derived through lipid hydrolysis as signaling molecules allowing bacterial escape from innate immunity (Johnson et al., 2016; Lee et al., 2017). In addition, CpaA is a zinc-dependent metalloendopeptidase forming an active complex with its chaperone (CpaAB), essential for secretion. It targets the common coagulation pathway by interfering with fibrinogen, factor XII and factor V, disrupting blood clotting and allowing the dissemination and colonization of A. baumannii (Waack et al., 2018; Urusova et al., 2019). Moreover, mutations in gspD and lipA showed a significant virulence reduction in both G. mellonella and murine models (Harding, 2016; Johnson et al., 2016).

Previous studies showed that A. baumannii strains produce a type VI secretion system (T6SS) involved in interbacterial competition (Fitzsimons et al., 2018). The T6SS is a complex nanomachine structurally and mechanistically analogous to an intracellular membrane-attached contractile phage tail (Cianfanelli et al., 2016). T6SS is an efficient weapon that can inject toxic effectors into the extracellular environment or directly into eukaryotic or prokaryotic cells (Cianfanelli et al., 2016). In addition, this system is implicated in bacterial competition and DNA uptake released by the prey cells, which promotes horizontal gene transfer (HGT; Weber et al., 2017). Indeed, HGT plays a significant role in the spread of antibiotic resistance cassettes and pathogenicity islands. Therefore, the potential involvement of T6SS in acquiring antibiotic resistance in A. baumannii has attracted considerable attention (Weber et al., 2017). It remains to be determined what, if any, benefit the T6SS may provide to A. baumannii during infection. In particular, G. mellonella infected with A. baumannii defective for the T6SS did not succumb to infection as quickly as did worms infected with the wild-type but were killed to the same extent at later time points (Repizo et al., 2015).

The quinones/fluoroquinolones are antibiotics that inhibit two enzymes involved in DNA synthesis: DNA gyrases and Topoisomerase IV. A. baumannii has genetic mutations providing resistance. Mutations in the gyrA and parC genes of the DNA gyrase subunit and Topoisomerase IV subunit C play a major role in conferring direct antibiotic resistance (Roy et al., 2021). Other important antibiotic resistance mechanisms of A. baumannii involve efflux pumps, permeability defects, and alteration of the target site (Figure 1). More generally, three resistance-nodulation cell division (RND)-family efflux pump systems, such as AdeABC, AdeFGH, and AdeIJK, and the multi-antimicrobial extrusion protein family (MATE) efflux pump in A. baumannii are overexpressed due to amino acid substitutions in their regulatory genes (Fernandez and Hancock, 2013; Sun et al., 2014; Darby et al., 2023). These two systems allow a broad spectrum of antibiotic resistance to aminoglycoside, chloramphenicol, erythromycin, tetracycline, and tigecycline (Magnet et al., 2001; Fournier et al., 2006; Vila et al., 2007). The plasmid-encoded qepA gene is an efflux pump belonging to the major facilitator superfamily that decreases susceptibility to hydrophilic fluoroquinolones, especially ciprofloxacin (Jacoby et al., 2014). With less antibiotic resistance efficiency, mutations in the aminoglycoside transferase AAC(6′)-Ib-Cr by Tryp102Arg and Asp179Typ substitution permit N-acetylation modification of two fluoroquinolones (ciprofloxacin and norfloxacin; Roy et al., 2021; Venkataramana et al., 2022).

The aminoglycosides antibiotic family inhibits protein synthesis by binding to the 16S ribosomal RNA of the 30S ribosome, with high affinity. Two main mechanisms, involving aminoglycoside modifying enzymes and RNA 16S methylase modification, are associated with increased resistance. Several reports reviewing clinical A. baumannii isolates find a match in genes coding for aminoglycosides enzymes modification ant(3″)-I, aac(3)-I, aph(3′)-I, aac(6′)-Ib and aph(3′)-IIb; and a gene coding for an rRNA 16S methylase armA allowing a high antibiotic resistance (Nie et al., 2014; Hasani et al., 2016).

Carbapenems are the most important class of antibiotics against A. baumannii and, generally, for Gram-positive and negative isolates (Meletis, 2016). Indeed, carbapenems are considered the drugs of choice to treat A. baumannii infections and the first-line agents for empirical therapy in areas with low rates of resistant strains (Pandey and Cascella, 2022). However, different mechanisms of β-lactam resistance have been described resulting in overexpression of OXA β-lactamases and chromosomal cephalosporinases, which have been classified as Acinetobacter-derived cephalosporinases (ADCs; Paton et al., 1993). The ADCs overexpression is caused by an insertion sequence (ISAba1) close to these resistance genes (Heritier et al., 2006). The first ADC gene was reported in Spain in 2000 (Bou and Martínez-Beltrán, 2000). Currently, several variants have been described worldwide conferring resistance against penicillins, extended-spectrum cephalosporins, monobactam (aztreonam), and β-lactamase inhibitors (sulbactam; Rodríguez-Martínez et al., 2010; Tian et al., 2011; Kuo et al., 2015; Ingti et al., 2020). The extensive use of carbapenems has been regarded as one of the main risk factors promoting the emergence and spread of MDRAB (Garnacho-Montero et al., 2015). The most effective resistance mechanism is the acquisition of carbapenem-hydrolyzing enzymes. In CRABs, the most common are class D oxacillinases (OXA type) β-lactamases classified in subgroups, with more than 400 OXA-type enzymes identified. Specifically, OXA-23, OXA-24, OXA-51, and OXA-58 subgroups are widespread in A. baumannii (Evans et al., 2013). Nevertheless, other β-lactamases classes are involved in carbapenem resistance, such as class A β-lactamases and class B metallo-β-lactamases (MBLs; Smet et al., 2008). OXA-type β-lactamases (especially OXA-23) have also been commonly detected in cefiderocol-resistant A. baumannii clinical isolates (Iregui et al., 2020; Kohira et al., 2020; Abdul-Mutakabbir et al., 2021; Yamano et al., 2021). Moreover, PER-like β-lactamases and, to a lesser extent, NDM β-lactamases have been shown to contribute to a decreased susceptibility to cefiderocol (Poirel et al., 2021). Therefore, combined factors, including the presence of β-lactamases such as NDM-like enzymes, modification of the penicillin-binding proteins (target gene PBP-3), permeability defects associated with efflux overexpression and reduced expression or mutation of genes involved in the ion transport, might contribute to resistance to cefiderocol in A. baumannii (Malik et al., 2020; Wang et al., 2022). More seldom is the presence of mutations affecting iron transport genes (pirA and piuA) in cefiderocol-resistant A. baumannii isolates (Malik et al., 2020). The pirA and piuA genes encode components of the pyoverdine and ferric iron uptake systems, respectively. Cefiderocol is transported across the outer cell membrane via iron transporters; thus, mutations in these genes may reduce antibiotic susceptibility. Nevertheless, the finding that mutations in these iron transport genes are relatively rare in A. baumannii isolates may suggest that iron acquisition is central to A. baumannii survival and, at the same time, genes involved in drug efflux, cell envelope modification, and cell wall biosynthesis may be more efficient in providing resistance to cefiderocol (Moynié et al., 2017).

The increase in colistin treatments after the rise of CRAB has led to a critical emergence of resistant strains, particularly in the hospital environment (Katip et al., 2021a,b). The first recorded case of a colistin-resistant Acinetobacter sp. was in 1949 in the Czech Republic (Sun et al., 2020). Currently, the high-resistant clonal lineage of A. baumannii has been described across 12 hospitals in Italy, Greece, and Spain, with resistance rates for colistin of 50% (Nowak et al., 2017). Moreover, 42% of A. baumannii isolates causing bloodstream infections in intensive care unit (ICU) patients from a Greek hospital have been found resistant to colistin and directly linked to fulminant septic shock and high mortality (Papathanakos et al., 2020). Despite that discovery, the resistance mechanisms to colistin in A. baumannii are only partially understood. Colistin is positively charged and interacts electrostatically with the negatively charged phosphate groups of lipid A, the LPS component of Gram-negative bacilli outer membrane. Colistin’s binding causes displacement of calcium (Ca²⁺) and magnesium (Mg²⁺) ions, associated with lipid A phosphoresters, thus affecting the stability of the LPS molecules. Subsequently, colistin inserts its hydrophobic terminal acyl fatty chain, causing disruption and permeabilization of the outer membrane. When permeabilization occurs, colistin penetrates the outer membrane, affecting the integrity of the inner membrane’s phospholipid bilayer, leading to membrane destabilization and cell death (Rhouma et al., 2016; Novović and Jovčić, 2023). Unlike Gram-negative bacteria such as Salmonella spp., Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa, A. baumannii does not possess a PhoP/PhoQ two-component system. The primary polymyxin resistance mechanisms in A. baumannii relies on the PmrA/PmrB two-component system. The PmrA/PmrB is a major regulatory system implied in the lipid A modification (Hua et al., 2020) and is well-characterized in E. coli, P. aeruginosa, or K. pneumoniae (Chen and Groisman, 2013). The histidine-kinase PmrB sensor reacts to various stress conditions, such as low Mg²⁺ and Ca²⁺ concentrations, acid pH, and high Fe³⁺ concentrations (Figure 2).

In Gram-negative bacteria, resistance to polymyxins results mostly from LPS modifications, which is the drug target. These modifications originate from the addition of cationic groups such as 4-amino-L-arabinose (L-Ara4N) and/or phosphoethanolamine (PEtN) on the lipid A (Ezadi et al., 2019). Unlike Enterobacterales, A. baumannii lacks all the genes of the arn operon required for L-Ara4N biosynthesis. Consequently, colistin resistance is caused by the addition of PEtN to the lipid A on position 1 or 4′ by the chromosomally-encoded EptA-like phosphoethanolamine transferase by the pmrC gene (El-Sayed Ahmed et al., 2020).

Mutations in the PmrAB system have been found in a large number of colistin-resistant A. baumannii isolates. These mutations constitutively activate the PmrAB regulatory system, which in turn, upregulates the expression of the operon pmrCAB (Adams et al., 2009). The self-regulation of the pmrCAB transcription enables the modification of lipid A (Olaitan et al., 2014). The colistin resistance-related mutations in the coding sequence for the amino acids Pro102 and Ile13 of PmrA and Pro233, Thr235, and Gln270 of PmrB caused an overactivity of PmrA. These mutations in pmrA were located in the sulfatase domain, while in pmrB were in the histidine kinase domain. Mutations in pmrA-pmrB promote phosphorylation of the PmrB receptor kinase, activating PmrA. The activated PmrA modulates the expression of the pmrC gene that encodes the phosphoethanolamine transferase that catalyzes the addition of PEtN to the 1′- or 4′-phosphate group of lipid A (Sun et al., 2020). Another study, analyzing the genetic determinants associated with colistin resistance in A. baumannii isolates collected from various regions of Greece, identified additional mutations in PmrB (Glu140 or Leu178) and PmrA (Lys172 or Asp10) genes (Palmieri et al., 2020). Besides, PmrA also regulates the naxD transcription coding for an N-acetylhexosamine deacetylase which is involved in the deacetylation of the β-galactosamine modifying the Lipid A (Adams et al., 2009; Moffatt et al., 2010; Llewellyn et al., 2012; Deveson Lucas et al., 2018; Sun et al., 2020; Ilsan et al., 2021).

Recently, a plasmid-mediated resistance to polymyxin has been described in Enterobacterales. The mcr (mobile colistin resistance) genes also encode a phosphoethanolamine transferase that adds PEtN to lipid A (Partridge et al., 2018). The mcr-1 remains the predominant plasmid-mediated colistin resistance gene, while mcr-2, -3, -4, -5, -6, -7, and -8 have been detected in isolates from animals, humans, and different environments worldwide. Currently, 56 mcr variant sequences are available in GenBank (Partridge et al., 2018). The mcr genes initially found in Enterobacterales, have been only recently described on A. baumannii (Ma et al., 2019; Al-Kadmy et al., 2020), for which resistance to polymyxin was previously restricted to chromosome-encoded elements (Jeannot et al., 2017; Poirel et al., 2017; Partridge et al., 2018).

More recently, a colistin-resistant mutation has also been shown in A. baumannii by insertion into the hns gene, an H-NS family transcriptional regulator. That mutation alters the expression of more than 150 genes, including the eptA gene. Overexpression of this LPS modifying enzyme codes for EptA, a PEtN transferase homolog to PmrC, which confer colistin resistance (Deveson Lucas et al., 2018; Trebosc et al., 2019; Palmieri et al., 2020; Ilsan et al., 2021).

Instead of lipid A modification, A. baumannii can acquire resistance to colistin due to the complete loss of LPS by inactivating the lipid A biosynthesis genes (lpxA, lpxC, and lpxD; Moffatt et al., 2010; Cafiso et al., 2019). LpxA, LpxC, and LpxD are three enzymes involved in the first main steps of LPS biosynthesis of A. baumannii occurring in the cytoplasm compartment (Powers and Trent, 2018). Specifically, mutations in LpxA and LpxC can lead to modifications in the fatty acid chains of lipid A, while mutations in LpxD can affect the addition of PEtN groups to lipid A. These changes can reduce the outer membrane’s net negative charge and permeability, decreasing colistin susceptibility (Palmieri et al., 2020).

Generally, biofilm production relies on the initial reversible bacterial attachment to a surface in response to environmental stimuli (Toyofuku et al., 2016). Thus, early adhesion is essential in the colonization process and in establishing an A. baumannii infection. The CsuA/BABCDE chaperon-usher assembly system encodes for the bacterial pili that mediate the attachment of A. baumannii to various abiotic surfaces (Longo et al., 2014). The Csu pili comprise four protein subunits, CsuA/B, CsuA, CsuB, and CsuE, assembled via the chaperone–usher pathway (Tomaras et al., 2008). The CsuC chaperone assists the CsuA/B polymerization in forming the major pilus subunit (Pakharukova et al., 2015). In addition, CsuD functions as the usher, CsuE forms a tip adhesin, while CsuA and CsuB constitute minor pilin subunits (Tomaras et al., 2003, 2008; Pakharukova et al., 2015). Previous studies showed that the inactivation of the csuE gene abolishes pilus production and biofilm (Amala Reena et al., 2017; Ghasemi et al., 2018). However, a study conducted with 52 different clinical strains revealed that biofilm formation and the ability to attach host cells are independent abilities and not necessarily associated (Eijkelkamp et al., 2011). Indeed, most A. baumannii carry the csuA/BABCDE locus; nevertheless, a subset of clinical isolates is csu deficient, indicating that these pili may be dispensable for biofilm formation and maintenance and that other pili systems may functionally replace them (Wright et al., 2016). In A. baumannii, the expression of the csu operon is mainly regulated by the two-component system BfmRS where BfmS acts as a sensor kinase and BfmR functions as a response regulator (Tomaras et al., 2008; Gaddy and Actis, 2009). Indeed, the two-component system BfmRS is considered the master regulator of resistance to stress in A. baumannii (Law and Tan, 2022). BfmrR-P can act directly or indirectly on regulating genes for osmotic and oxidative stress, heat shock, the biosynthesis of siderophores, and the production of capsular polysaccharides, in addition to pili production. Furthermore, BfmR is also important for pellicle formation in A. baumannii (Krasauskas et al., 2019). A pellicle is an alternative biofilm growing at the air-liquid interface that may favor the colonization and persistence of A. baumannii in respiratory tracts, humidifiers, and moist surfaces (Martí et al., 2011; Nait Chabane et al., 2014).

The two-component system BfmRS is also responsible for the subsequent irreversible adhesion starting with the production of factors under the control and early extracellular DNA (eDNA) release. An early eDNA release was demonstrated to be responsible for the first tridimensional biofilm formation. Notably, eDNA release is independent from the cell lysis in the early stage of biofilm formation and is mediated by membrane vesicles (Sahu et al., 2012).

In A. baumannii, the AdeABC, AdeIJK, and AdeFGH RND-type efflux systems are critical in biofilm formation (Coyne et al., 2011). Mutant strains of AdeABC, AdeIJK, and AdeFGH efflux pumps produce a significantly lower level of biofilm than the wild-type strain (Yoon et al., 2015). Moreover, mutation of AdeABC and AdeIJK efflux pumps showed lower expression of several pilus system-encoding proteins, including CsuA/B, CsuC, and FimA. These proteins play a central role in the initial stages of adhesion, surface colonization, and biofilm maturation in A. baumannii (He et al., 2015; Shadan et al., 2023).

The AdeRS two-component system regulates the AdeABC efflux pump’s expression (Richmond et al., 2016; Xu et al., 2019). In particular, the deletions of adeRS and adeB reduced the biofilm growth of A. baumannii without affecting the number of adherent cells. This observation suggests that cells might be unable to produce a mature biofilm without this efflux pump (Richmond et al., 2016). After the initial surface attachment, biofilm maturation occurs. During this process, individual cells produce the biofilm matrix entering the irreversible attachment stage. In A. baumannii, biofilm maturation is modulated by the Biofilm-associated proteins (Bap) and their interaction with the extracellular polymeric substances (EPS; Soroosh et al., 2020; Upmanyu et al., 2022). The main elements of the A. baumannii EPS are alginates and poly-β-(1-6)-N-acetylglucosamine (PNAG) compounds that interact with each other, with ions or heterologous molecules to form an elastic structure (Marvasi et al., 2010). The pgaABCD locus is involved in the synthesis of PNAG, facilitating cell adhesion, promoting biofilm integrity, and limiting desiccation (Choi et al., 2009; Morris et al., 2019; Flannery et al., 2020). Accordingly, deleting the pgaABC genes in A. baumannii impairs biofilm formation (Choi et al., 2009). The Bap are large surface proteins orthologous to the Staphylococcus aureus Bap protein (Cucarella et al., 2001; Loehfelm et al., 2008). A type I secretion system secretes Bap. It is required in cell-to-cell adhesion and for developing higher-order structures on polystyrene and titanium (Loehfelm et al., 2008; Harding et al., 2017). Moreover, the Bap protein increases host colonization by facilitating A. baumannii adherence to human neonatal keratinocytes and bronchial epithelial cells (Brossard and Campagnari, 2012). In addition to the Bap protein, the AdeABC efflux pump, normally related to antibiotic resistance, may also contribute to biofilm maturation (Richmond et al., 2016). Notably, in mature A. baumannii biofilms, can be observed two types of colonies: the avirulent translucent (AV-T) colonies that produce dense biofilms and virulent opaque (VIR-O) colonies that exhibit low biomass but enhanced virulence in G. mellonella, increased surface motility an antibiotic resistance phenotype (Tipton et al., 2015). Several genes are linked to different genomic expression profiles. Among them, ABUW_1132, a highly conserved gene that encodes a LysR-type transcriptional regulator (LTTR) that contributes to the passage of AV-T to VIR-O; its overexpression up-regulates abaI and activates the abaI/abaR quorum sensing (QS) signal (Tierney et al., 2021). In A. baumannii, the QS system is regulated by the two-component system, AbaI/AbaR, which is homologous to the typical LuxI/LuxR system found in other Gram-negative bacteria. abaI encodes the autoinducer synthase, which catalyzes the synthesis of N-(3-hydroxy dodecanol)-L-HSL (AHL), which at high density interacts with the cognate receptor AbaR leading to downstream cellular responses (Oh and Han, 2020). Previous studies have found that abaI and abaR disruption reduces biofilm formation (Niu et al., 2008; Anbazhagan et al., 2012). Moreover, A. baumannii cultured in the presence of AHL showed increased expression of Csu pili and biofilm formation (Luo et al., 2015).

In the final stage, the cells within the biofilm disperse and colonize new surfaces. Biofilm dispersal is induced prevalently under environmental stress, including the A. baumannii SOS response, and the activation of the UmuDAb RecA-dependent repressor inactivated by RecA cleavage when DNA damage occurs. As a result, the UmuDAb mutant cannot activate the transcription of bmfR. Thus, no Csu pili or biofilm is formed (Ching et al., 2019). Notably, dispersed cells exhibit variable phenotypes, antibiotic susceptibility, transcriptomic patterns, and metabolic activities (Rumbaugh and Sauer, 2020). For example, dispersed clinical isolates of A. baumannii are more hydrophobic and adhere more efficiently to the surface than the planktonic cells (Berlanga et al., 2017). Moreover, the dispersed cells were more susceptible to ciprofloxacin and tetracycline than the same cells in the planktonic state (Berlanga et al., 2017). In contrast, another A. baumannii clinical strain disseminating from ciprofloxacin-exposed biofilms is highly resistant to ciprofloxacin, erythromycin, and tetracycline (Penesyan et al., 2019). These studies suggest that the ability of the dispersed cells to evolve, acquiring higher antibiotic resistance, could complicate the management and treatment of the infection (Law and Tan, 2022).

The ability of A. baumannii to form biofilms has been reported as an essential factor contributing to its persistence and tolerance to antimicrobial agents (Roy et al., 2022). The proportion of A. baumannii clinical isolates that produce biofilms can vary significantly depending on the study and sample population. In a collection of 20 clinical isolates of A. baumannii, emerged that 80% of the strains formed biofilm, perhaps because of a dominant clone (Sechi et al., 2004). Bardbari et al. compared biofilm-production ability between clinical and environmental A. baumannii. In this study emerged that the majority of both clinical and environmental isolates could form varying degrees of biofilm. Specifically, the prevalence of strong biofilm producers in clinical and environmental strains was 58.7 and 31.2%, respectively (Bardbari et al., 2017). Others reported that among 154 A. baumannii isolated in Taiwan, 45.4% possessed strong biofilm formation ability (Yang et al., 2019). Moreover, among 100 A. baumannii clinical isolates from three hospitals in Iran, 58% were strong biofilm producers (Zeighami et al., 2019). Another study investigating 92 unrelated strains of A. baumannii isolated from two Spanish hospitals found that 63% of isolates formed biofilm, mainly from device-associated infections. Notably, these isolates were less frequently resistant to imipenem or ciprofloxacin than non-biofilm-forming isolates (Rodrı́guez-Bano et al., 2008).

A study from 4 Chinese hospitals analyzed the contribution of biofilm formation in the epidemic spread of A. baumannii by comparing biofilm-forming abilities and genetic characteristics of international clonal lineage II (ICL II) and non-ICL II isolates. From a total of 114 clinical A. baumannii isolates, collected from various specimens, including blood, sputum, urine, and wound, emerged that 36% of the clinical isolates were able to form biofilm, but only 19.5% were strong biofilm producers. Of the A. baumannii isolates, the biofilm formation capacity of ICL II was significantly lower than that of non-ICL II isolates. The authors concluded that biofilm formation might not be a critical factor for the epidemic spread of A. baumannii, particularly for the ICL II lineage. They suggested that other factors, such as antimicrobial resistance and virulence, could play a more critical role in the epidemic potential of A. baumannii (Hu et al., 2016). Despite the propensity to produce biofilm, the clinical impact of biofilm in A. baumannii isolates is still debated. Indeed, a recent multicenter study in Taiwan including 711 patients showed that higher APACHE II score, shock status, lack of appropriate antimicrobial therapy, and carbapenem resistance were independent risk factors of 28-day mortality in the patients with A. baumannii bacteremia but not the level of biofilm formation. In addition, biofilm formation was most commonly observed in survivors than in non-survivors (38.4% vs. 31.9%; Chiang et al., 2022). Similar results have been previously observed in a cohort of 273 patients with A. baumannii bacteremic pneumonia (Wang et al., 2018). Accordingly, other studies have shown that infections caused by biofilm-producing A. baumannii are not necessarily associated with worse clinical outcomes (Rodrı́guez-Bano et al., 2008; Wang et al., 2018). Therefore, the impact and pathogenesis of biofilm production remain elusive and, in many cases, related to the patient’s underlying condition or to the strain that causes the infection (Rodrı́guez-Bano et al., 2008; Barsoumian et al., 2015; Wang et al., 2018; Di Domenico et al., 2020, 2021).

A. baumannii is known for its ability to develop resistance to multiple antibiotics, making treatment of infections particularly challenging. In addition, the formation of biofilms further exacerbates this issue, as the extracellular matrix can act as a physical barrier, limiting the penetration of antibiotics and protecting the bacteria from the host’s immune system (Perez et al., 2007; Antunes et al., 2011). Several antibiotics and antibiotic combinations have shown promise in combating A. baumannii biofilms. However, their effectiveness may vary depending on the strain and resistance profile. The use of two or more antibiotics with different mechanisms of action can enhance the therapeutic efficacy by affecting multiple bacterial targets. In particular, the combination of colistin and rifampicin was more effective at eradicating biofilms formed by multidrug-resistant A. baumannii isolates than either antibiotic alone (Batoni et al., 2016). The antimicrobial combinations of colistin-levofloxacin, colistin-tigecycline, and tigecycline-levofloxacin or these combinations with clarithromycin were effective as lock solutions in the treatment of A. baumannii catheter-related infections (Ozbek and Mataraci, 2013). Nevertheless, candidate antibiotics were active against biofilm-embedded A. baumannii cells at 400-fold the MIC. This concentration is unachievable in human serum, making those antimicrobials an undesirable option for systemic use in A. baumannii biofilm-associated infections (Ozbek and Mataraci, 2013). Synergistic effects were also observed on biofilm-embedded carbapenem-resistant and carbapenem-susceptible A. baumannii strains. In particular, meropenem was active against biofilm-embedded carbapenem-susceptible A. baumannii, whereas meropenem plus sulbactam exhibited synergism against biofilm CRAB and caused significantly more damage to the biofilm architecture than colistin or tigecycline used alone (Wang et al., 2016). Additionally, clinical isolates of MDRAB exhibited different degrees of biofilm formation in the presence of sub-minimum inhibitory concentrations of colistin and tigecycline (Sato et al., 2018). A recent study showed that biofilm-embedded MDRAB had been eradicated with colistin but not tigecycline. Notably, the eradication increased with a combination of colistin and high concentrations of tigecycline (Sato et al., 2021). Moreover, combining azithromycin and polymyxin B displayed synergistic activity against biofilm-producing A. baumannii clinical isolates, improving antimicrobial efficacy (Peng et al., 2020). These data suggest that the effects of different antibiotics may depend on bacterial strains and the response of A. baumannii may vary under specific environmental stress conditions, such as in the presence of multiple antimicrobial agents. Nevertheless, one of the main challenges in analyzing these studies is the considerable heterogeneity in the design, methodologies, and patient populations examined (Table 1). While reflecting the field’s richness, such diversity can make it difficult to draw firm conclusions or compare findings directly across studies. Additionally, there is not yet a universally accepted definition or a standardized method for determining biofilm formation by A. baumannii. The absence of such standards introduces variability between studies and complicates the comparison of results. Furthermore, many of our findings are based on in vitro studies. While these studies provide valuable insights, they cannot fully capture the complexity of clinical infections. The behavior of A. baumannii in a real-world clinical setting can be influenced by myriad factors not present under laboratory conditions.