by Kevin D. Young
I'm finally waking up, a twist in my story...
My grandmother took a lot of pills. Each week she arranged them in little piles that she dropped into the compartments of a multi-well device, a way to be sure she took each group on the right day and at the right time. The neat thing (to a kid) was their range of colors and shapes–round ones, ovals, squares, triangles and even hot dog shapes, of every primary color and several pastels. Simply magical. But, of course, appearances were irrelevant. Pill size, shape, and color were dictated by medical tradition and marketing; none of these overt characteristics had any impact on their medicinal value. Efficacy was dictated solely by the nature of the compounds embedded in each lozenge.
A handsome helical Helicobacter
A similar situation exists for bacteria. They, too, come in a range of sizes and shapes (though not colors when viewed individually, more’s the pity), and for decades these overt characteristics have been used to differentiate these organisms. However, with only slight hints to the contrary, morphology has not been thought to be medically important. Instead, as with my grandmother’s pills, a bacterium’s pathogenicity has been attributed solely to specific virulence factors embedded in these odd packages. This view is beginning to change.
That earlier hint that cell shape might aid pathogenesis? Uropathogenic E. coli grow as filamentous cells during one stage of bladder infection, and these forms were proposed (click here and here) to promote dissemination as the bacterium moved from one host cell to another. However, this was not an airtight conclusion because E. coli also filaments in response to DNA damage, and bladder cells may inflict just such damage to impede bacterial growth. Thus, the observed morphological change might be only a secondary phenotype rather than a primary virulence factor. Along this same line, but in a different domain entirely, cell size of the fungal pathogen Cryptococcus neoformans has also been implicated as a virulence factor. Here, though, the increase in diameter of individual cells from 5-10 µm to 100 µm keeps them from being phagocytized. Thus, the shear bulk of this particular change may be more important than morphology per se.