**1. Introduction**

Leafy greens are mostly eaten raw, based on the increased consumers' preferences for natural, nutritious diets. The consumption of leafy greens is recommended to reduce the risk of malnutrition, diet-related chronic diseases such as cardiovascular diseases, cancer, metabolic disorders, and may help to slow down the cognitive decline with aging [1–4]. For preserving their bioactive compounds, leafy greens are commonly consumed raw, and the lack of a kill-step to inactivate the potentially present pathogens leads to greater risk to the health of consumers. Among other fresh produce, leafy greens are more exposed to pathogen contamination because they grow low to the ground and can be easily contaminated in open fields. The increased consumption of fresh, ready-to-eat leafy greens has been repeatedly, reported worldwide and linked to pathogenic *Escherichia coli* (herein *E. coli or E. coli* O157:H7) associated foodborne illnesses outbreaks.

Pathogenic *Escherichia coli* group is comprised of six pathotypes out of which Shiga toxin-producing *E. coli* (STEC)—STEC (also be referred to as Verocytotoxinproducing *E. coli* (VTEC) or enterohemorrhagic *E. coli* (EHEC)) is the one most

associated with foodborne outbreaks [5]. Some other STEC *E. coli* strains, namely *E. coli* O145, *E. coli* O26, and *E. coli* O104:H4 were involved in rare foodborne outbreaks due to consumption of shredded lettuce, raw clover sprouts, and raw sprouted seeds [6–8].

According to World Health Organization (WHO) pre-harvest food safety is an important element in creating sustainable food safety policies and must be considered in the context of *farm-to-fork* for the protection of human health [9]. Despite the existing food safety regulations and the undertaken on-farm food safety measures, according to recent studies performed by the United States Center of Diseases Control (CDC), European Centre for Disease Prevention and Control (ECDC) in collaboration with the European Food Safety Authority (EFSA) the incidence of infections caused by pathogenic *Escherichia coli*, between 2016 and 2019, remained almost unchanged or even increased (**Table 1**) [10–14]. In this context, in the past years, these fresh produces emerged as a food safety concern that, ultimately, increased the awareness about their primary contamination stage, namely pre-harvest. It has been established that once contaminated leafy greens leave the farm's site it will be difficult to prevent further transmission of *E. coli* to consumers. Usually, large quantities of contaminated leafy greens are recalled from the markets, a fact which pose a great economic burden on leafy greens growers but also on public health [15]. The on-farm contamination with pathogenic *Escherichia coli* largely depends on agricultural and environmental factors, unsafe on-site agronomic practices including the harvesting stage, and ineffective or missing post-harvest decontamination steps. However, eliminating completely the presence of the *E. coli* from the on-farm, the natural growing environment of leafy greens, during pre-harvest stage, proves to be impossible due to the high number of factors which are involved in the harboring and transmission of this pathogen. Based on the vast number of on-field and experimental results it was unanimously agreed that it is more feasible to first understand the main agricultural factors affecting the


#### **Table 1.**

*Selected pathogenic E. coli outbreaks associated with fresh leafy greensa,b .* *Pathogenic* Escherichia coli*: An Overview on Pre-Harvest Factors That Impact the Microbial… DOI: http://dx.doi.org/10.5772/intechopen.101552*

prevalence, incidence, and survival of this pathogen, as well as pathogen contamination of leafy greens, which ultimately negatively impacts the microbial safety of the produce. In turn, this will assist leafy greens producers to improve their on-farm pre-harvest agronomic practices for reducing the pathogen contamination to levels that will be a lesser hazard to public health.

Subsequent to the reported pathogenic *Escherichia coli*-related foodborne outbreaks, epidemiological trace-back studies identified the following as main contamination factors: (a) the use of manure, as a soil organic fertilizer; (b) irrigation water; (c) the domestic and wild animals which either can be found in the proximity of the growing sites or as free-roaming animals; and (d) on-farm human activity [15–17].

Similarly, in European countries, over the years, the consumption of fresh leafy greens led to multiple foodborne outbreaks. For example, in Germany (2011) the consumption of sprouts led to 3816 total illnesses (810 hospitalizations and 54 deaths) due to *E.coli* O104:H4. Between 2010 and 2011, in England, Wales and Scotland, 252 fell ill and one died following the consumption of raw leeks; the identified pathogen being *E.coli* O157 PT8. In Denmark (2010), the consumption of lettuce resulted in 264 illnesses due to *E. coli* ETEC O6:K15:H16. The consumption of fresh basil, provoked in Denmark (2006) about 200 illnesses due to *E. coli* ETEC O92:H- and O153:H2 [18].
