Filamentous Appendages
Many bacterial cells have protein appendages embedded within their cell envelopes that extend outward, allowing
interaction withtheenvironment. Theseappendages canattach toothersurfaces, transferDNA,orprovidemovement.
Filamentous appendages include fimbriae, pili, and flagella.
Fimbriae and Pili
Fimbriae and pili are structurally similar and, because differentiation between the two is problematic, these terms are
often used interchangeably.[22] [23]The termfimbriaecommonly refers to short bristle-like proteins projecting from
the cell surface by the hundreds (Figure 3.30). Fimbriae enable a cell to attach to surfaces and to other cells. For
pathogenic bacteria, adherence to host cells is important for colonization, infectivity, and virulence. Adherence to
surfaces is also important in biofilm formation.
The termpili(singular: pilus) commonly refers to longer, less numerous protein appendages that aid in attachment to
surfaces (Figure 3.30). A specific type of pilus, called theF pilusorsex pilus, is important in the transfer of DNA
between bacterial cells, which occurs between members of the same generation when two cells physically transfer
or exchange parts of their respective genomes (seeHow Asexual Prokaryotes Achieve Genetic Diversity
(http://cnx.org/content/m58845/latest/)).
Figure 3.30 Bacteria may produce two different types of protein appendages that aid in surface attachment.
Fimbriae typically are more numerous and shorter, whereas pili (shown here) are longer and less numerous per cell.
Group A Strep
Before the structure and function of the various components of the bacterial cell envelope were well
understood, scientists were already using cell envelope characteristics to classify bacteria. In 1933, Rebecca
Lancefield proposed a method for serotyping various β-hemolytic strains ofStreptococcusspecies using an
agglutination assay, a technique using the clumping of bacteria to detect specific cell-surface antigens (Figure
3.31). In doing so, Lancefield discovered that one group ofS. pyogenes, found in Group A, was associated
with a variety of human diseases. She determined that various strains of Group A strep could be distinguished
from each other based on variations in specific cell surface proteins that she named M proteins.
Today, more than 80 different strains of Group A strep have been identified based on M proteins. Various
strains of Group A strep are associated with a wide variety of human infections, including streptococcalMicro Connections
- J.A. Garnetta et al. “Structural Insights Into the Biogenesis and Biofilm Formation by theEscherichia coliCommon Pilus.”Proceedings
of the National Academy of Sciences of the United States of America109 no. 10 (2012):3950–3955. - T. Proft, E.N. Baker. “Pili in Gram-Negative and Gram-Positive Bacteria—Structure, Assembly and Their Role in Disease.”Cellular
and Molecular Life Sciences66 (2009):613.
Chapter 3 | The Cell 109