Microbiology, 2021

100 3 • The Cell
• How can the PA definitively identify the cause of Barbara’s pneumonia?
• What form of treatment should the PA prescribe, given that the amoxicillin was ineffective?
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Filamentous Appendages
Many bacterial cells have protein appendages embedded within their cell envelopes that extend outward,
allowing interaction with the environment. These appendages can attach to other surfaces, transfer DNA, or
provide movement. 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 term fimbriae commonly refers to short bristle-like proteins
projecting from the cell surface by the hundreds. 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 term pili (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 the F pilus or sex 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 (see How Asexual Prokaryotes Achieve
Genetic Diversity).
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. (credit: modification of work by American Society
for Microbiology)
MICRO CONNECTIONS
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 of Streptococcus species using an
agglutination assay, a technique using the clumping of bacteria to detect specific cell-surface antigens. In
doing so, Lancefield discovered that one group of S. 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.
22 J.A. Garnetta et al. “Structural Insights Into the Biogenesis and Biofilm Formation by the Escherichia coli Common Pilus.”
Proceedings of the National Academy of Sciences of the United States of America 109 no. 10 (2012):3950–3955.
23 T. Proft, E.N. Baker. “Pili in Gram-Negative and Gram-Positive Bacteria—Structure, Assembly and Their Role in Disease.” Cellular
and Molecular Life Sciences 66 (2009):613.
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