How prevalent are lactoferrin receptors in Gram-negative bacteria?

Surface receptors in Gram-negative bacteria that bind and extract iron from the host glycoproteins transferrin (Tf) or lactoferrin (Lf) was discovered 35 years ago in pathogenic species and subsequently was discovered in other pathogens of humans and food production animals. These bacterial species reside exclusively on the mucosal surfaces of the respiratory or genitourinary tract of their mammalian host and rely on their host specific Tf and Lf receptors to acquire iron for survival. Since the specificity of the bacterial Tf receptors was shown to be due to selective pressures on the host Tf, their presence in bacteria that reside in both mammals and birds indicates that they arose over 320 million years ago. Once Lf arose in mammals due to a gene duplication event, Lf receptors subsequently arose from Tf receptors. The focus on pathogens for discovery of these receptors has led to a limited understanding of how prevalent the Tf and Lf receptors are in commensal species and raises the question whether they are present in additional bacterial lineages. Since the Lf receptor provides a secondary iron acquisition system plus can provide protection from cationic peptides its presence varies in bacterial lineages.