## 1 Introduction  
The emergence of drug-resistant bacteria and the difficulty in killing them are mainly associated with the aggregation of microorganisms within a self-produced matrix called a biofilm [1]. In their natural habitat, microorganisms exist prevalently in the form of biofilms, which are considered an adaptive response to environmental stress and confer bacteria with increased resistance compared to that of their planktonic counterparts. According to the National Institute of Health, it is estimated that 65-80% of all bacterial infections are due to biofilm-associated microorganisms [2]. The biofilm-related infections are highly resistant not only to common disinfectants, but also to conventional antibiotics that are generally used to kill planktonic bacteria, rendering biofilm infections extremely difficult to eradicate [3, 4]. Enterohemorrhagic _Escherichia coli_ O157:H7 (EHEC) is a human pathogen belonging to the attaching and effacing (A/E) _E. coli_ group. It possesses virulence factors that are essential for adhesion to intestinal epithelial cells (attachment), and it is responsible for the destruction of the brush border of microvilli (effacement) [5]. The occurrence of these pathogens in the food chain is responsible for food-borne diseases. This pathogen can cause bloody diarrhea and hemorrhagic colitis, and approximately 4% of the cases develop into hemolytic uremic syndrome. The ability of _E. coli_ O157:H7 to adhere to different biological surfaces to form sessile communities, the presence of persisting forms that cause severe food safety issues, and the absence of an effective therapy against EHEC infections have led to the need for new antimicrobial agents. Thus, the search for alternative strategies for treating and preventing bacterial infections is highly demanded. In recent years, antimicrobial peptides (AMPs) have attracted a growing interest as a therapeutic alternative to conventional anti-infective agents. Currently, there are more than 3100 experimentally reported AMPs [6]. AMPs are naturally occurring polypeptides containing cationic and hydrophobic amino acids (~12-50 residues) with direct antibacterial activity. AMPs are widely distributed in the animal and plant kingdoms, and they act to neutralize a broad range of microbes. Recently, many peptides have been identified with a prevalent biofilm inhibitory action [7; 8; 9]. More than a thousand AMPs arising from the skin secretions of amphibian species have been reported in the last few years. These peptides rapidly kill a broad range of microbes by perturbing their anionic plasma membrane and/or inactivating intracellular targets [10; 11; 12]. Esculentins-1 are a class of frog skin peptides with a highly conserved amino acid sequence consisting of 46 residues and characterized by a C-terminal loop [13; 14]. Esculentin-1a and -1b differ by only one amino acid in position 11 of their sequence [15]. During the last few years, two peptides named Esc(1-21) and Esc(1-18), corresponding to the first 20 and 18 amino acids of the native esculentin-1a and -1b, respectively (Figure 1),were synthesized and found to display a large spectrum of bactericidal activity [14], encompassing the Gram-positive _Streptococcus agalacliae_ and _Corynebacterium jiekeium_ bacterial strains, the Gram-negative _E. coli_ and _Pseudomonas aeruginosa_ strains, and the yeast _Candida albicans_[16; 17; 18; 19; 20; 21]. Considering the good activity of Esc(1-18) and Esc(1-21) on non-pathogenic reference _E. coli_ strains [16; 22], we assessed their antibacterial and antibiofilm activity against the bacterial pathogen _E.coli_ O157:H7 EDL933 reference strain. We examined the effects of these two peptides on the expression of a set of genes implicated in the: (i) regulation of biofilm formation and dispersal; (ii) response to oxidative and osmotic stresses; (iii) stringent response, which is induced by nutritional stress conditions.