Lipopolysaccharides (LPSs) from different Gram-negative bacteria are structurally distinct. Even closely related serovars of single bacterial species may possess different LPS molecules. Further variability may then be superimposed on this ground-state structural diversity as a result of variable expression of other LPS structures. This variable expression is due in some cases to high-frequency, reversible, random "on-off" switching of genes required for biosynthesis of particular LPS structures. In other cases differential expression of LPS biosynthesis genes may be part of a programmed response to environmental stimuli, which may occur as adaptations by pathogenic bacteria to changing environments within the host during the course of infection.