Phosphoglucosamine mutase (EC 188.8.131.52) catalyzes the interconversion of glucosamine-6-phosphate into glucosamine-1-phosphate, an essential step in the biosynthetic pathway leading to the formation of peptidoglycan precursor uridine 5′-diphospho-N-acetylglucosamine. The gene (glmM) of Escherichia coli encoding the enzyme has been identified previously. We have now identified a glmM homolog in Streptococcus gordonii, an early colonizer on the human tooth and an important cause of infective endocarditis, and have confirmed that the gene encodes phosphoglucosamine mutase by assaying the enzymatic activity of the recombinant GlmM protein. Insertional glmM mutant of S. gordonii did not produce GlmM, and had a growth rate that was approximately half that of the wild type. Morphological analyses clearly indicated that the glmM mutation causes marked elongation of the streptococcal chains, enlargement of bacterial cells, and increased roughness of the bacterial cell surface. Furthermore, the glmM mutation reduces biofilm formation and increases sensitivity to penicillins relative to wild type. All of these phenotypic changes were also observed in a glmM deletion mutant, and were restored by the complementation with plasmid-borne glmM. These results suggest that, in S. gordonii, mutations in glmM appear to influence bacterial cell growth and morphology, biofilm formation, and sensitivity to penicillins.