A. baumannii is known as the most frequently isolated organism in intensive care units (ICUs), causing a variety of nosocomial infections, including pneumonia, urinary tract infections (UTIs), bacteremia as well as skin and soft tissue infections. These infections are usually associated with high mortality rates ranging between 26% among hospitalized patients and 43% among ICU patients.

Acinetobacter baumannii (A. baumannii) is Gram- negative, aerobic, glucose non-fermentative, non-motile coccobacillus, ubiquitous in nature, and persistent in healthcare settings. It is regarded as a significant opportunistic human pathogen.

This bacterium become a growing problem in hospitals as a predominant multidrug-resistant (MDR) bacterium. Horizontal acquisition of resistance genes is the main factor involved in the emergence of MDR .

There are many mechanisms to confer resistance to different classes of antibiotics in A. baumannii, one of them is multidrug efflux pumps. These efflux pumps are important source of MDR, which export antibiotics from the cell, increasing their antibiotic resistance.

AdeABC is one of the most important efflux systems, belonging to the RND family in Acinetobacter, which plays an important role in the resistance to a broad group of antibiotics; its genes are chromosomal and encode three genes, i.e., AdeB, AdeA, and AdeC, forming an operon in the vicinity. In addition, the expression of AdeABC is done by a two-component system, which includes a response regulator (AdeR) and a sensor kinase (AdeS) .

The ability of Acinetobacter spp. to form biofilm that enables bacterial survival in hospital settings, especially in ICUs, is the most significant contributing factor to their virulence, and this trait is also responsible for their notable antibiotic resistance. Several biofilm-related genes influence antimicrobial susceptibility, suggesting an association between the biofilm-forming ability of Acinetobacter spp. and their antibiotic resistance patterns (MDR/XDR)

Zinc oxide nanoparticles (ZnO NPs) are one of the most important nanoparticles of metal oxides; it is a unique and inorganic materials that can be used in several biological applications (anti-bacterial, anti-inflammatory). ZnO NPs exhibit distinctive properties other than other nanoparticles such as higher solubility, better biofilm penetration and effective drug delivery . ZnO NPs have been reported to have antimicrobial properties such as disrupting the cell membrane of pathogens, accumulating in the cell and producing toxic H2O2 (hydrogen peroxide)

Inclusion Criteria:

• All patients suffering from infections that can be caused by A. baumannii.

Exclusion Criteria:

• All patients suffering from infections that aren't caused by A. baumannii.

This study will be carried out at Department of Medical Microbiology and Immunology, Sohag Faculty of Medicine and Sohag University Hospitals.