The intricacies of Acinetobacter baumannii: a multifaceted comprehensive review of a multidrug-resistant pathogen and its clinical significance and implications

Abstract

Acinetobacter baumannii, a highly adaptive and formidable nosocomial pathogen, has emerged as a symbol of modern medicine's struggle against multidrug resistance (MDR). As a Gram-negative dweller in moist hospital environments, A. baumannii has proven its ability to colonize the most vulnerable-critically ill patients-leaving behind a trail of infections highlighted by high morbidity and mortality and rendering nearly all antibiotics ineffective. This literature review aims to provide an in-depth, comprehensive overview of microbiological features, virulence factors, clinical manifestations, epidemiology, and antibiotic resistance mechanisms of A. baumannii. It also highlights the different diagnostic approaches, possible treatment strategies, and infection control, as well as the profound public health burden this pathogen imposes. The genus Acinetobacter has undergone a pivotal taxonomic journey and categorization. In addition, the intricate virulence mechanisms and factors of A. baumannii, including but not limited to outer membrane components and nutrient acquisition systems, have contributed to its pathogenicity and severe clinical manifestations ranging from respiratory tract infections and meningitis to urinary tract infections, skin infections, and bloodstream infections. This review also describes the epidemiological trend of A. baumannii established by its global prevalence and distribution, risk factors, hospital-acquired vs. community-acquired infections, and its geographical variations. In terms of antibiotic resistance, this pathogen has demonstrated resilience to a wide range of first-line and last-resort antibiotics due to its different evasion mechanisms. The current diagnostic approaches, treatment strategies, and infection control measures are further analyzed in detail, underscoring the need for prompt and precise identification of A. baumannii to guide appropriate therapy and reinforce the optimal approaches to limit its transmission and control outbreaks. Finally, the review addresses the substantial public health implications, reflecting on the hindrance that A. baumannii brings to healthcare systems, and the urgent need for global surveillance, effective infection control protocols, innovative research, and therapeutic approaches to mitigate its global threat.

Keywords: Acinetobacter baumannii; multidrug-resistant; nosocomial; pathogen; review.

Figure 1

A circular map of the Acinetobacter baumannii chromosome depicting the locations of genes used in the Oxford and Institut Pasteur MLST methods. Stars (Turton PCRs) denote genes found by trilocus typing by PCR. Created in BioRender. ezz, z. (2025) https://BioRender.com/a0dpfdb.

Figure 2

Schematic representation of the major virulence factors of Acinetobacter baumannii. Created in BioRender. b, a. (2025) https://BioRender.com/nflrr8d. OMPs, outer membrane vesicles.

Figure 3

Acinetobacter baumannii virulence associated with secretion systems and released effector proteins. The names of the main protein components for each secretion system are indicated, and outer membrane vesicles (OMVs) are also shown. OM, outer membrane; IM, inner membrane. Created in BioRender. ezz, z. (2025) https://BioRender.com/c5l6dl2.

Figure 4

Mechanisms of iron acquisition and utilization by Acinetobacter baumannii. A. baumannii employs various strategies to obtain and utilize iron from the host. HphA is transported across the inner membrane into the periplasm via the Sec translocon and is then secreted into the extracellular medium by Slam (highlighted in blue). The bacterium's phospholipase C lyses host erythrocytes to extract heme, which is transported into the cell through the TonB-dependent HphR outer membrane receptor. The enzyme HemO liberates iron from heme for cellular use. Created with BioRender. Created in BioRender. ezz, z. (2025) https://BioRender.com/ies2m3c.

Figure 5

Acinetobacter baumannii siderophores: acinetobactin, fimsbactin, and baumannoferrin.

Figure 6

Iron transport mechanisms of Acinetobacter baumannii siderophores. The siderophore Acinetobactin has a high affinity for iron and utilizes an inner membrane ATP-binding cassette (ABC) transporter, consisting of BauB, BauC, BauD, and BauE, along with the BauA TonB-dependent outer membrane receptor, driven by proton-motive force. Baumannoferrin also exhibits a strong affinity for iron, transporting it through an inner membrane ABC transporter and the BfnH TonB-dependent outer membrane receptor, both using proton-motive force. Similarly, Fimsbactin transports iron using the FeoA transporter and the FbsN TonB-dependent outer membrane receptor, with both mechanisms relying on proton-motive force. The cytoplasmic membrane's FeoAB is essential for importing ferrous iron (Sheldon and Skaar, 2020). Created in BioRender. ezz, z. (2025) https://BioRender.com/pels3ql.