ABSTRACTThe sialic acids (N-acylneuraminates) are a group of nine-carbon keto-sugars existing mainly as terminal residues on animal glycoprotein and glycolipid carbohydrate chains. Bacterial commensals and pathogens exploit host sialic acids for nutrition, adhesion, or antirecognition, whereN-acetyl- orN-glycolylneuraminic acids are the two predominant chemical forms of sialic acids. Each form may be modified by acetyl esters at carbon position 4, 7, 8, or 9 and by a variety of less-common modifications. Modified sialic acids produce challenges for colonizing bacteria, because the chemical alterations toN-acetylneuraminic acid (Neu5Ac) confer increased resistance to sialidase and aldolase activities essential for the catabolism of host sialic acids. Bacteria withO-acetyl sialate esterase(s) utilize acetylated sialic acids for growth, thereby gaining a presumed metabolic advantage over competitors lacking this activity. Here, we demonstrate the esterase activity ofEscherichia coliNanS after purifying it as a C-terminal HaloTag fusion. Using a similar approach, we show thatE. colistrain O157:H7 Stx prophage or prophage remnants invariably include paralogs ofnanSoften located downstream of the Shiga-like toxin genes. These paralogs may include sequences encoding N- or C-terminal domains of unknown function where the NanS domains can act as sialateO-acetyl esterases, as shown by complementation of anE. colistrain K-12nanSmutant and the unimpaired growth of anE. coliO157nanSmutant onO-acetylated sialic acid. We further demonstrate thatnanShomologs inStreptococcusspp. also encode active esterase, demonstrating an unexpected diversity of bacterial sialateO-acetyl esterase.IMPORTANCEThe sialic acids are a family of over 40 naturally occurring 9-carbon keto-sugars that function in a variety of host-bacterium interactions. These sugars occur primarily as terminal carbohydrate residues on host glycoproteins and glycolipids. Available evidence indicates that diverse bacterial species use host sialic acids for adhesion or as sources of carbon and nitrogen. Our results show that the catabolism of the diacetylated form of host sialic acid requires a specialized esterase, NanS. Our results further show thatnanShomologs exist in bacteria other thanEscherichia coli, as well as part of toxigenicE. coliprophage. The unexpected diversity of these enzymes suggests new avenues for investigating host-bacterium interactions. Therefore, these original results extend our previous studies ofnanSto include mucosal pathogens, prophage, and prophage remnants. This expansion of thenanSsuperfamily suggests important, although as-yet-unknown, functions in host-microbe interactions.
Unexpected Diversity of Escherichia coli SialateO-Acetyl Esterase NanS
