## 1 Introduction
There are several reviews available on the topic [29; 30; 31]. Another parameter that results from the composition of PIFs is their buffering capacity (BC), i.e., their ability to resist changes in pH. Studies have shown that the BC of PIFs mainly depends on the type of proteins involved in its composition, as well as on minerals [32]. Regarding proteins, PIFs containing mainly caseins have a higher buffering capacity than whey-based or plant-based formulae. Furthermore, the composition of PIFs indirectly affects their reconstitution temperature, especially those containing thermally sensitive vitamins (C, B\({}_{6}\)), nucleotides, or probiotics that are sensitive to temperatures above 55-60 \({}^{\circ}\)C. As regards the composition of the infant formulae, both the EU and the USA have specific legislation in place that governs the production of PIFs in terms of their safety and nutritional adequacy, as well as their production and distribution. In the EU, Regulation (EU) No 609/2013 is in force, which has been complemented by Regulation (EU) 2016/127 that provides detailed requirements on the composition of infant formulae [33; 34], while, in the US, the Federal Food, Drug, and Cosmetic Act (FD&C Act), Section 412 applies [15]. Due to the particular age group that PIFs are meant for, it is not possible to introduce new ingredients or change radically their composition without authorization. Nevertheless, it should be noted that, although the legislation establishes the framework for PIFs, it is still rather flexible in terms of differentiating the ingredients used in their formulation. One of the most common interventions in PIFs is their enrichment with probiotic bacteria [26; 27]. The use of probiotics in PIFs is not explicitly regulated by EU regulation; yet, it is not forbidden. The manufacturers of PIFs have to prove that any supplementary substances, such as probiotics, are safe and suitable for infant intake. Probiotics are defined as live microorganisms which, when administered in adequate amounts, confer a benefit to the host [35]. Species like _Lactobacillus_, _Lacticasiebacillus_, _Bifidobacterium_, _Enterococcus_, _Bacillus_, and _Streptococcus_ have been shown to be beneficial in treating a variety of clinical disorders, including necrotizing enterocolitis [36]. Currently, there are PIFs available on the market that are enriched with probiotic bacteria, such as Enfamil Nutramigen\({}^{\circledR}\), with Enflora LGG, which contains _Lacticasiebacillus rhamnos_ GG (LGG), formerly named _Lactobacillus rhamnos_; Gerber Good Start\({}^{\circledR}\) Baby Formula Powder and Lactogen\({}^{\circledR}\) Pro 1, both enriched with the probiotic _Limosilactobacillus reuteri_, formerly named _Lactobacillus reuteri_; as well as Good Start\({}^{\circledR}\) PLUS 1 Baby Formula, that contains _Bifidobacterium lactis_. Strains of the genus _Bifidobacterium_ are well-known for their probiotic properties, such as the protection against pathogens, production of short-chain fatty acids (SCFA), modulation of the immune system, and amelioration of digestion, along with the resistance to bile salts in the human's small intestine. One of the world's most well-documented probiotic species _Bifidobacterium animals_ subsp. _lactis_ BB-12\({}^{\circledR}\) (BB-12\({}^{\circledR}\)) has been studied for its beneficial health effect within gastrointestinal health and immune function, in many clinical studies [37; 38]. BB-12\({}^{\circledR}\) was also reported to have positive effects on the re-establishment of a balanced composition of the gut microbiota of newborns and children [39]. The inhibition of _C. sakazakii_ growth in the co-culture with probiotic bacteria is well-known and has been attributed to several mechanisms, often complementary, with the main one being the decrease in the pH of the growth medium due to the production of weak organic acids by the bacteria [26; 27]. However, there is limited knowledge about the potential involvement of the growth medium in the bacterial inhibition phenomena.