Figure 2.

Factors Driving and Maintaining Diversity of Polysaccharide Antigens. (A) Major diversifying forces in the world of bacterial polysaccharide antigens: host immunity, bacteriophages, and cell–cell interactions (including host glycan diversity, eukaryotic predators and other host commensals). These forces are likely to drive and maintain the polysaccharide antigen diversity we observe today. (B) These factors should not be viewed as mutually exclusive, but rather as different forces operating at different scales of time and space. Coevolution with bacteriophages could select for novel polysaccharide diversity on short timescales from just a few bacterial generations, could occur within virtually any ecological niche including host associated or non-host associated, and could have transient or long-lasting impacts on both bacterial and phage population dynamics. The impact of genetic variation in host glycan diversity is expected to take much longer to affect bacterial population structures, depending on host diversity and generation times. Host immunity, the diversity of other host commensals, and the impact of predators are likely to operate somewhere between the two. Phages may promote within-host diversity of antigens, but a serotype which provides resistance against a given phage population may not spread in the population due to its low between-host fitness. Likewise, glycan diversity in different host populations may promote diversity over space: different populations found in different locations may promote different bacterial antigens, but a single type within each population.