## 1 Introduction  
Post-weaning diarrhoea (PWD) is one of the main economic losses in the pig breeding industry because of its large impact on the most pivotal phase of the pig production cycle. Even if several predisposing and contributing factors have been identified, the main etiological agent is representedby enterotoxicigenic _Escherichia coli_ (ETEC) [1]. These bacterial strains are considered to be the most significant global cause of severe watery diarrhoea in piglets, with side effects varying from growth retardation to sudden death [2].  
The onset of PWD symptoms in pigs is primarily associated with _Escherichia coli_ K88 (_E. coli_ K88), the main representative amongst ETEC strains [3]. The bacterium can enter the digestive tract of weaning piglets via the oral route. From the upper jejunum to the ileum, _E. coli_ K88 colonizes the intestinal mucosal surface employing its fimbrial adhesins, hair-like polymeric structures essential for ETEC pathogenicity and capable of binding to receptors on enterocytes [4].  
However, the cornerstone of _E. coli_ K88 pathogenicity lies in its ancillary plasmids, which harbour genes encoding for heat-labile (LT) and heat-stable (ST) toxins, which target intestinal cells. Although by different means, toxins activate signal transduction pathways that finally bring the severe intestinal secretion of electrolytes, disruptions of cellular tight-junctions, loss of water, and, consequently, diarrhoea [5; 6].  
Antibiotics (AB) represent one of the main approaches against PWD. However, scientists and political institutions are promoting action towards a reduction of their use, because of worries related to the continuous spread of antimicrobial resistance [7].  
This issue, together with the consumer demand for novel molecules with antibacterial properties, have encouraged research towards alternatives, such as organic acids (OA) and nature identical compounds (NIC). These classes of compounds all have a recognized bactericidal effect at different concentrations against several pathogens [8; 9].  
Organic acids embody a wide group of organic compounds considered one of the most-used feed additives in pig nutrition [10]. OA have been proved to have antimicrobial effects because of their action on cellular metabolism and because of their ability to deplete cellular energy while triggering ATPase pumps to let H\({}^{+}\) anions out of cells [11; 12]. Interestingly, this bactericidal activity only occurs against pH-sensitive bacteria, such as bacteria belonging to _Escherichia_, _Clostridium_, _Salmonella_, and _Listeria_ genus. Non-pH-sensitive bacteria, such as _Bifidobacteria_ and _Lactobacillus_, can innately face and manage lower intracytoplasmic pH levels, without being negatively affected by OA anions [13].  
Nature-identical compounds are pure chemicals which reflect molecules naturally occurring in essential oils, thus overcoming the intrinsic variability in their composition [14]. Their mode of action is related to their capacity to form pores on the bacterial membrane, induce cell lysis, and inhibit enzyme activity, in addition to anti-inflammatory and antioxidant activities [15; 16; 17].  
This study aimed to evaluate the in vitro susceptibility of a field strain of _E. coli_ K88 to selected antibiotics and antimicrobial compounds. More precisely, we investigated how AB, OA, and NIC could affect _E. coli_ K88 growth, survival, and expression of various virulence genes involved in motility, quorum sensing, biofilm formation, adherence to cells, and LT and ST toxin production.