The Impact of Lactoferrin on the Growth of Intestinal Inhabitant Bacteria

Abstract

Lactoferrin (Lf) is an iron-binding milk glycoprotein that promotes the growth of selected probiotic strains. The effect of Lf on the growth and diversification of intestinal microbiota may have an impact on several issues, including (i) strengthening the permeability of the epithelial cell monolayer, (ii) favoring the microbial antagonism that discourages the colonization and proliferation of enteric pathogens, (iii) enhancing the growth and maturation of cell-monolayer components and gut nerve fibers, and (iv) providing signals to balance the anti- and pro-inflammatory responses resulting in gut homeostasis. Given the beneficial role of probiotics, this contribution aims to review the current properties of bovine and human Lf and their derivatives in in vitro probiotic growth and Lf interplay with microbiota described in the piglet model. By using Lf as a component in pharmacological products, we may enable novel strategies that promote probiotic growth while conferring antimicrobial activity against multidrug-resistant microorganisms that cause life-threatening diseases, especially in neonates.

Keywords: bifidobacteria; immunomodulation; intestinal homeostasis; lactobacilli; lactoferricin; lactoferrin; milk.

Figure 1

The canonical structure of lactoferrin (Lf) (bovine lactoferrin Protein Data Bank (PDB) Identification (ID):1BLF; human lactoferrin PDB ID: LFG), consisting of a single polypeptide chain organized in N- and C-lobes that in turn contain N1/N2 and C1/C2 subdomains. Each subdomain forms a deep cleft in which one ferric ion (Fe³⁺) and one bicarbonate ion (HCO₃⁻¹) are bound. The N1-terminal subdomain contains the lactoferricin (Lfcin) domain, which is released in free form following the proteolysis of the full-length Lf with pepsin. The primary sequences of bovine (PDB ID: 1LFC) and human (PDB ID: 1Z6V) Lfcin are depicted below the full-length Lf. The molecular modeling and coloring of the tertiary structures were accomplished with the free-download software RasMol™ 1994, designed and launched by Roger Sayle©.

Figure 2

Simplified hypothetical mechanisms that may account for the in vitro growth-promoting effects of lactoferrin (Lf) on probiotics (bifidobacteria and lactobacilli). (a) Lf glycans undergo enzymatic hydrolysis to produce both intact oligosaccharides and the products of cleavage (disaccharides and monosaccharides). Intact and hydrolyzed glycans are transported via cell membrane permeases to the cytosol. In the cytosol, glycans are terminally hydrolyzed to yield monosaccharides used by cytosolic enzymes as substrates of fermentation via the Embden–Meyerhof–Parnas or bifid shunt pathways to produce energy for cell growth. (b) The cationic surface of Lf interacts electrostatically with acidic components of the cell wall that are negatively charged, such as teichoic acids. This interaction may favor the internalization of Lf in the periplasmic space and its eventual binding to cell membrane proteins for translocation to the cytosol. In the presence of ATP, Lf dissociates to the N-lobe and C-lobe. The latter is internalized in the nucleus and interacts with DNA to modulate genes involved in the mechanisms of DNA replication for cell growth. (c) Diferric Lf reaches the periplasmic space, where iron in ferric form is released and then translocated via a permease to cytosolic compartments. Ferric ions are considered an essential cofactor of enzymes involved in crucial processes of metabolism and DNA replication for growth. RasMol™ 1994 by Roger Sayle© was used as the software for modeling and coloring the tertiary protein structures.

Figure 3

The impact of Lf on the promotion of the growth and diversity of intestinal microbiota may entail (i) strengthening of the permeability of the epithelial cell monolayer; (ii) favoring of the microbial antagonism that discourages the colonization and proliferation of enteric pathogens, enhancing the growth and maturation of (iii) cell-monolayer components and (iv) gut nerve fibers; and (v) providing signals to balance the anti- and proinflammatory responses resulting in homeostasis.