In 2017, the World Health Organization released a global list of resistant bacteria, identifying multidrug-resistant (MDR) Gram-negative bacteria (GNB) such as carbapenem-resistant Acinetobacter baumannii, carbapenem-resistant Pseudomonas aeruginosa, third generation cephalosporin-resistant Enterobacteriaceae, and carbapenem-resistant Enterobacteriaceae as critical priorities. Indeed, the diffusion of these organisms is an unprecedented threat, for several reasons. Two of them are extremely important: (i) the first is the magnitude of the diffusion, with prevalences as high as 10-50% in many country, although with wide local variability; (ii) the second is the important shortage of therapeutic options, which in the last decades has frequently forced physicians to use potentially toxic drugs, or with somewhat suboptimal pharmacokinetics.

The epidemiology and the clinical impact of MDR-GNB have been already described in patients with different local and national backgrounds. However, what is less clear is the impact of MDR-GNB in immunocompromised populations. These encompass very different groups of patients, which share an increased risk of acquiring or dying from infections due to MDR-GNB. Such a risk might be increased because of severe immunosuppression, as in neutropenic hematology patients, but possibly also because of less severe impairments of immune functions, as in patients with nonhematologic cancers or autoimmune diseases, as well as recipients of biological therapies.

The aim of this special issue is to focus on the epidemiology and clinical impact (e.g., mortality, length of hospital stay, and quality of life) of MDR-GNB infections in the abovementioned special patient populations. In particular, we encourage submission of original articles, although review articles describing the current state of art with regard to the management of MDR-GNB infections in immunocompromised patients are also welcome. In addition, case reports highlighting difficulties and questions arising from the management of MDR-GNB bacteria in patients with peculiar impairments in immune defenses will also be considered.

Potential topics include but are not limited to the following:

• Elucidation of the epidemiology of MDR-GNB infections in immunocompromised populations, for example, hematology patients, but also patients with nonhematological cancers, patients with autoimmune diseases, and/or patients receiving biological therapies
• Therapeutic considerations regarding the management of MDR-GNB infections in immunocompromised populations
• PK/PD and tolerability of antimicrobials in immunocompromised patients
• Dedicated infection-control and antimicrobial stewardship initiatives in immunocompromised patients at risk of MDR-GNB infections
• Peculiar microbiological aspects to be considered in immunocompromised patients at risk of MDR-GNB infections
• Description of specific immune profiles predisposing to infections due to MDR-GNB, or impairing their outcome
• Adjunctive non-antibiotic therapy in immunocompromised patients with infections due to MDR-GNB