Bacterial galactosemia is caused by cytoplasmic interference of an essential cell wall biosynthesis glycosyltransferase

Bacterial galactosemia or "galactose death," triggered by incomplete galactose metabolism, was first discovered in Escherichia coli and Salmonella six decades ago, and later in many other microorganisms, yet the mechanism for the toxicity and subsequent cell death remains unclear. In Bacillus subtilis, galactosemia is manifested by a buildup of uridine-diphosphate-galactose (UDP-Gal) and a strong toxicity phenotype characterized by cell shape abnormality and rapid cell lysis. Here we present evidence that in B. subtilis, the toxicity is due to inhibition of cell wall biosynthesis through interference of the essential glycosyltransferase MurG that carries out lipid II synthesis from lipid I and uridine-diphosphate-N-acetyl-glucosamine (UDP-GlcNAc). Single-molecule imaging reveals real-time inhibition of cell wall biosynthesis and MurG activities in cells exhibiting toxicity. We further show that in vitro, MurG is able to utilize UDP-Gal as a substrate generating a "toxic" lipid II, causing a potential poisoning effect on peptidoglycan crosslinking. Evidence also suggests a similar mechanism in Vibrio cholerae and Staphylococcus aureus. Lastly, a strong synergistic lethality was seen in S. aureus wild-type cells treated with both galactose and sub-lethal doses of cell-wall antibiotics. Our study provides mechanistic explanation of the toxicity associated with bacterial galactosemia and its potential application in antibacterial solutions. SignificanceGalactosemia is a potentially fatal genetic disorder due to incomplete galactose metabolism, found in both eukaryotic and prokaryotic organisms. The molecular mechanisms of galactosemia-associated toxicity remain unclear in all cases. Here we present evidence that in the bacterium Bacillus subtilis, the toxicity is due to interference of an essential glycosyltransferase, MurG, which concerts lipid I to lipid II during peptidoglycan biosynthesis, by a nucleotide sugar derived from galactose metabolism. This interference leads to a halt of cell wall biosynthesis and structural defects causing rapid cell lysis. Our evidence also suggests a similar mechanism in other bacteria such as Staphylococcus aureus and Vibrio cholerae. Our study may help solve the long-time puzzle of bacterial galactosemia first uncovered six decades ago.