**1. Introduction**

Since the discovery and application of penicillin in 1940, antibiotics have played an unprecedented role in the prevention, control, and treatment of infectious diseases in both humans and animals [1]. However, in animal production, they have also been used at subtherapeutic doses [2]. It is estimated that the global consumption of antibiotics in animal production could increase by 67% in the coming years [3] mainly because of the growing global demand for animal protein [2, 4]. Although it has been reported that in developed countries the total consumption of antibiotics has decreased by around 4%, consumption of antibiotics in the USA increased slightly [5]. Furthermore, it has been reported that the amount of antibiotics used in animal production in the USA is 100–1000 times higher than human medicine, being used ~80–90% at subtherapeutic doses, and for prophylactic purposes, while the remaining 10–20% at therapeutic doses [6, 7].

The inclusion of antibiotics at subtherapeutic doses into the feed was generalized in the early 1950s, both in the EU and the USA since they could be used to prevent diseases and positively influence the promotion of growth and feed efficiency of animals [3, 8, 9].

Nevertheless, in the last decades, these practices have changed considerably due to the concern of the increase of bacteria resistant to antibiotics, since they can be transmitted zoonotically from animals to humans, causing serious problems in public health and even death because of the failure of the antibiotic at therapeutic doses [10]. Furthermore, another problem for human health is the presence of antibiotic residues in animal-derived food, by the use of antibiotics for long periods of time, since it is associated in some cases with allergic reactions, imbalance of the intestinal microbiota, and especially, the development of antibacterial resistance [11].

Consequently, one of the measures taken in the face of the problems of bacterial resistance was the restriction of antibiotics at subtherapeutic doses in the EU in 2006 [12] and the USA in 2017 [13], and although in countries as Mexico they have not been officially banned, the Ministry of Agriculture and Rural Development (SADER), through its decentralized administrative body, the National Health Service, Food Safety and Food Quality (SENASICA), has promoted initiatives to prevent their use since 2012 [14–17]. However, as a consequence of this measure, the incidence of enteric diseases in animals has increased significantly [18], as well as the use of antibiotics, but at therapeutic doses for the purpose of controlling and preventing diseases, which could lead to a worse scenario of bacterial resistance [2, 19–21]. In this context, the European One Health Action Plan against antimicrobial resistance calls for the phasing out of routine prophylactic (Prevent) and metaphylactic (Control) antimicrobial use in animal production and investment in the research of new alternatives [22], since they could be regulated in the coming years.

Therefore, the poultry industry has been under pressure to seek and investigate new alternatives to reduce the problems of bacterial resistance, prevent and control diseases, reduce the mortality rate, and finally promote the growth of animals. Among these alternatives, the most popular are probiotics (yeasts or bacteria) since it has been reported that they can improve the performance [23, 24], as well as prevent and control enteric pathogens in poultry [25–27]. Furthermore, it has been reported that probiotics could be an interesting alternative to prevent and control the toxic effects of aflatoxins. For these reasons, the probiotic market has expanded rapidly and is expected to grow to around 7% in 2020. However, this market is led mainly by Asia and Europe given the growing demand for dietary supplements [18].
