Infectious diseases caused by bacteria, viruses or fungi are among the leading causes of death worldwide. The emergence of drug-resistance mechanisms, especially among bacteria, threatens the efficacy of all current antimicrobial agents, some of them already ineffective. As a result, there is an urgent need for new antimicrobial drugs. Host defense antimicrobial peptides (HDPs) are natural occurring and well-conserved peptides of innate immunity, broadly active against Gram-negative and Gram-positive bacteria, viruses and fungi. They also are able to exert immunomodulatory and adjuvant functions by acting as chemotactic for immune cells, and inducing cytokines and chemokines secretion. Moreover, they show low propensity to elicit microbial adaptation, probably because of their non-specific mechanism of action, and are able to neutralize exotoxins and endotoxins. HDPs have the potential to be a great source of novel antimicrobial agents. The goal of this review is to provide an overview of the advances made in the development of human defensins as well as the cathelicidin LL-37 and their derivatives as antimicrobial agents against bacteria, viruses and fungi for clinical use.

There are currently no defined optimal therapies available for multidrug-resistant (MDR) Acinetobacter baumannii infections. We evaluated the efficacy of rifampin, imipenem, sulbactam, colistin, and their combinations against MDR A. baumannii in experimental pneumonia and meningitis models. The bactericidal in vitro activities of rifampin, imipenem, sulbactam, colistin, and their combinations were tested using time-kill curves. Murine pneumonia and rabbit meningitis models were evaluated using the A. Acinetobacter baumannii is an important nosocomial pathogen worldwide (5, 35), with pneumonia, bacteremia, and surgical site and urinary tract infections being the most important infections caused by this organism (16). A Spanish study showed A. baumannii as the cause of nearly 9% of cases of ventilator-associated pneumonia (VAP) (2), with a crude mortality of 40% to 70% (14). A. baumannii may also cause meningitis and ventriculitis, especially in patients undergoing neurosurgical procedures or with head trauma (17), with mortality rates between 20% and 27% (5).The well-known ability of A. baumannii to acquire resistance to almost all groups of available antibiotics leads to serious problems in the management of infections caused by multidrug-resistant (MDR) A. baumannii infections (5, 16). In these cases, carbapenems have been considered the treatment of choice. However, increasing numbers of carbapenem-resistant A. baumanii isolates have been reported worldwide (1, 28), prompting the search for other therapeutic options.Sulbactam has been used successfully in cases of meningitis and pneumonia caused by A. baumannii (17,21,39). Colistin has good in vitro activity (37) but has shown contradictory results in clinical practice (12) and experimental models (23). Rifampin has demonstrated in vitro and in vivo bactericidal activities against MDR A. baumannii in an experimental pneumonia model (23), but rifampin-resistant mutants appear shortly after treatment initiation with rifampin alone (23, 27). The combination of rifampin plus imipenem has been evaluated in clinical infections caused by highly imipenem-resistant A. baumannii strains, with inconclusive results (33). Two clinical studies have shown efficacy rates of 76% to 100% for colistin plus rifampin in VAP, bacteremia, and meningitis (4, 25). The aims of this study were to evaluate the efficacies of rifampin and its combinations with imipenem, sulbactam, and colistin in experimental pneumonia and meningitis models caused by MDR A. baumannii strains.

Objectives Escherichia coli is characterized by three resistance patterns to β-lactams/β-lactamase inhibitors (BLs/BLIs): (i) resistance to ampicillin/sulbactam and susceptibility to amoxicillin/clavulanic acid and piperacillin/tazobactam (RSS); (ii) resistance to ampicillin/sulbactam and amoxicillin/clavulanic acid, and susceptibility to piperacillin/tazobactam (RRS); and (iii) resistance to ampicillin/sulbactam, amoxicillin/clavulanic acid and piperacillin/tazobactam (RRR). These resistance patterns are acquired consecutively, indicating a potential risk of developing resistance to piperacillin/tazobactam, but the precise mechanism of this process is not completely understood. Methods Clinical isolates incrementally pressured by piperacillin/tazobactam selection in vitro and in vivo were used. We determined the MIC of piperacillin/tazobactam in the presence and absence of piperacillin/tazobactam pressure. We deciphered the role of the blaTEM genes in the new concept of extended-spectrum resistance to BLs/BLIs (ESRI) using genomic analysis. The activity of β-lactamase was quantified in these isolates. Results We show that piperacillin/tazobactam resistance is induced in E. coli carrying blaTEM genes. This resistance is due to the increase in copy numbers and transcription levels of the blaTEM gene, thus increasing β-lactamase activity and consequently increasing piperacillin/tazobactam MICs. Genome sequencing of two blaTEM-carrying representative isolates showed that piperacillin/tazobactam treatment produced two types of duplications of blaTEM (8 and 60 copies, respectively). In the clinical setting, piperacillin/tazobactam treatment of patients infected by E. coli carrying blaTEM is associated with a risk of therapeutic failure. Conclusions This study describes for the first time the ESRI in E. coli. This new concept is very important in the understanding of the mechanism involved in the acquisition of resistance to BLs/BLIs.