Bacteriol . Henrik Hasman , Mark A . Schembri and Per Klemm K - 12 Escherichia coli Colony Morphology of Antigen 43 and Type 1 Fimbriae Determine

Colony morphology has been used as an important identification and characterization criterion in bacteriology for many decades. However, the molecular mechanisms underlying the appearance of different colony types have been given little attention. The synthesis of O antigen is defunct in Escherichia coli K-12, and colonies should accordingly only appear to be rough. However, previous reports have noted the presence of different interchangeable colony morphology types. In this study we have addressed the influence of two phase-variable surface structures, antigen 43 and type 1 fimbriae, on colony morphology. Due to differential expression of these structures, four different colony phenotypes could be distinguished. By creating and studying defined mutants of the respective loci, i.e., flu and fim, we conclude that the presence or absence of the corresponding gene products on the cells correlates with the observed colony morphology forms. Interestingly, the habitat specificity of bacteria under static liquid conditions seems to correlate with the colony phenotypes. In nature bacteria often grow as colonies on surfaces. Observation of colony development and morphology has for many years been an important tool for identification and characterization of bacteria because individual species often form colonies of characteristic size and appearance. However, even when a pure bacterial culture is plated out, different morphological colony types often arise. Traditionally, the colony morphology of Escherichia coli is identified as either a rough or a smooth form. The two forms are readily distinguished, as the colonies of the former are rough, flat, and irregular and colonies of the latter are smooth, high, and circular. E. coli K-12 strains are rough, their lipopolysaccharide having a complete core but no O antigen due to the insertion of an IS5 element in the rfb gene cluster controlling O antigen biosynthesis (23). Only extremely rarely are K-12 strains able to revert to the smooth form, as this requires the integrity of the rfb gene cluster to be restored. Nonetheless, K-12 strains have been observed to exhibit different interchangeable colony types. Diderichsen (7) partially identified a mechanism controlling the colony morphology of K-12 strains. The observed phenotypes were somewhat similar to the ones seen for rough and smooth strains, but the strains frequently switched between the two. To distinguish these from the rough and smooth phenotypes related to lipopolysaccharide status, the rough, flat, and irregular variant was designated frizzy, or form 1, and the smooth, convex, circular variant was designated glossy, or form 2 …