Sialic acid utilization by Cronobacter sakazakii

BACKGROUND: The Cronobacter genus is composed of seven species, and can cause infections in all age groups. Of particular concern is C. sakazakii, as this species is strongly associated with severe and often fatal cases of necrotizing enterocolitis and meningitis in neonates and infants. Whole genome sequencing has revealed that the nanAKT gene cluster required for the utilisation of exogenous sialic acid is unique to the C. sakazakii species (ESA_03609–13). Sialic acid is found in breast milk, infant formula, intestinal mucin, and gangliosides in the brain, hence its metabolism by C. sakazakii is of particular interest. Therefore its metabolism could be an important virulence factor. To date, no laboratory studies demonstrating the growth of C. sakazakii on sialic acid have been published nor have there been reports of sialidase activity. The phylogenetic analysis of the nan genes is of interest to determine whether the genes have been acquired by horizontal gene transfer. RESULTS: Phylogenetic analysis of 19 Cronobacter strains from 7 recognised species revealed the nanAKTR genes formed a unique cluster, separate from other Enterobacteriaceae such as E. coli K1 and Citrobacter koseri, which are also associated with neonatal meningitis. The gene organisation was similar to Edwardsiella tarda in that nanE gene (N-acetylmannosamine-6-phosphate-2epimerase) was not located within the nanATK cluster. Laboratory studies confirmed that only C. sakazakii, and not the other six Cronobacter species, was able to use sialic acid as a carbon source for growth. Although the ganglioside GM1 was also used as carbon source, no candidate sialidase genes were found in the genome, instead the substrate degradation is probably due to β–galactosidase activity. CONCLUSIONS: Given the relatively recent evolution of both C. sakazakii (15–23 million years ago) and sialic acid synthesis in vertebrates, sialic acid utilization may be an example of co-evolution by one species of the Cronobacter genus with the mammalian host. This has possibly resulted in additional virulence factors contributing to severe life-threatening infections in neonates due to the utilization of sialic acid from breast milk, infant formula, milk (oligosaccharides), mucins lining the intestinal wall, and even gangliosides in the brain after passing through the blood–brain barrier.