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Although pili have been observed in some species of Gram positive bacteria, the preliminary research that I have done indicates that pili are significantly more common in Gram negative bacteria.

Is this an accurate assessment of the current body of scientific knowledge? If so, do we know (or have any leading theories on) why this is the case?

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Pili were at least discovered more recently in gram-positive bacteria.

Pili are formed differently in gram-positive and gram-negative bacteria. Here's a pretty good review on the differences: http://www.ncbi.nlm.nih.gov/pubmed/18953686

Researchers seem to think that pili might be involved in adhesion/formation of biofilms (adhesion is important for pathogenicity). Certain types of pili are specifically involved in gene transfer. This is a good review on pili and pathogencity in gram-positive bacteria: http://www.nature.com/nrmicro/journal/v4/n7/pdf/nrmicro1443.pdf. If you can't access it, let me know, and I'll do my best to summarize.

EDIT:

The Wikipedia entry for pilin interprets the second paper I linked as saying that pili are more common in gram-negative bacteria and that pili are implicated in pathogenicity.

Straight from the paper:

Prevalence:

Over the past five decades, several distinct pilus types have been identified, most of which were described and characterized in Gram-negative bacteria.

Function:

A common feature of Gram-negative pili, however, is their role in adhesion to eukaryotic cells. It has been proposed that bacteria use these structures to form an initial association with host cells, which can then be followed by a more ‘intimate’ attachment that brings the bacterium into proximity to the host-cell surface.

and

Pilus-like structures on the surface of Gram-positive bacteria were first detected in Corynebacterium renale, by electron microscopy. More recently, surface appendages were reported to be present in Actinomyces naeslundii and were subsequently found in other species, including Corynebacterium diphtheriae, Streptococcus parasanguis (Streptococcus parasanguinis), Streptococcus salivarius and Streptococcus sanguis (Streptococcus sanguinis). Finally, in the past year, pili were also characterized in all three of the principal streptococcal pathogens that cause invasive disease in humans — group A Streptococcus (GAS; that is, Streptococcus pyogenes), group B Streptococcus (GBS; that is, Streptococcus agalactiae) and Streptococcus pneumoniae — in which they have been shown to have key roles in the adhesion and invasion process and in pathogenesis. [emphasis added by me]

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this doesn't precisely answer the question. are they more common in gram negative bacteria, and if so, why? –  shigeta Feb 14 '13 at 4:15
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@shigeta: see my edited answer. Given the diversity of pili structure and function (see the first article I posted), I don't know if you'll get a "why" that explains pili prevalence in gram-negative bacteria. –  dd3 Feb 15 '13 at 1:19
    
of course perfect answers aren't always going to happen with biology, but it helps the site to give the answer here. thanks! –  shigeta Feb 15 '13 at 3:32

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