II. THE PROKARYOTIC CELL: BACTERIA
B. PROKARYOTIC CELL STRUCTURE
4. STRUCTURES LOCATED OUTSIDE THE CELL WALL
a. Glycocalyx (capsule)
The overall purpose of this Learning Object is:
1) to learn the chemical makeup and the functions associated with the bacterial glycocalyx;
2) to introduce the process of phagocytosis by eukaryotic cells; and
3) learn the significance of bacteria producing biofilms.
LEARNING OBJECTIVES FOR THIS SECTION
In this section on Prokaryotic Cell
Structure we are looking at the various organelles or structures that make up
a bacterium. As mentioned in the introduction to this section, a typical bacterium
usually consists of:
Structures located outside the cell wall of bacteria include the glycocalyx (capsule), flagella, and pili. We will now look at the glycocalyx.
1. The Glycocalyx (Capsules and Slime Layers )
All bacteria secrete some sort of glycocalyx (def), an outer viscous covering of fibers extending from the bacterium (see Fig. 1, Fig. 2, and Fig. 3). If it appears as an extensive, tightly bound accumulation of gelatinous material adhering to the cell wall, it is called a capsule (def) as shown in the photomicrograph in Fig. 2. If the glycocalyx appears unorganized and more loosely attached, it is referred to as a slime layer (def).
A. Structure and Composition
The glycocalyx is usually a viscous polysaccharide or polypeptide slime. Actual production of a glycocalyx often depends on environmental conditions.
- A capsule stain of Streptococcus Lactis.
B. Functions and Significance to Bacterial Pathogenicity
Although a number of functions have been associated with the glycocalyx, such as protecting bacteria against drying, trap nutrients, etc., for our purposes there are two very important functions. The glycocalyx enables certain bacteria to resist phagocytic engulfment by white blood cells in the body or protozoans in soil and water. The glycocalyx also enables some bacteria to adhere to environmental surfaces (rocks, root hairs, teeth, etc.), colonize, and resist flushing.
a. Role of glycocalyx in resisting phagocytosis
As will be seen in Unit 4, there are several steps involved in phagocytosis. First the surface of the microbe must be attached to the cytoplasmic membrane of the phagocyte. Attachment of microorganisms is necessary for ingestion and may be unenhanced or enhanced.
- Unenhanced attachment is a general recognition of what are called pathogen-associated molecular patterns or PAMPs (def) - components of common molecules such as peptidoglycan, teichoic acids, lipopolysaccharide, mannans, and glucans common in microbial cell walls but not found on human cells - by means of glycoprotein known as endocytic pattern-recognition receptors (def) on the surface of the phagocytes (see Fig. 4).
- Enhanced attachment is the attachment of microbes to phagocytes by way of an antibody (def) molecule called IgG or proteins produced during the complement pathways (def) called C3b and C4b (see Fig. 5). Molecules such as IgG and C3b that promote enhanced attachment are called opsonins (def) and the process is called opsonization (def). Enhanced attachment is much more specific and efficient than unenhanced.
Following attachment, polymerization and then depolymerization of actin filaments (def) send pseudopods out to engulf the microbe (see Fig. 6) and place it in a vesicle called a phagosome (def) (see Fig. 7).
Movie of a bacterium being engulfed by a neutrophil.
© James Sullivan, author. Licensed for use, ASM MicrobeLibrary.
Finally, lysosomes (def), containing digestive enzymes and microbicidal chemicals, fuse with the phagosome containing the ingested microbe and the microbe is destroyed (see Fig. 8).
Capsules enable bacteria to resist phagocytosis by evading the complement and antibody body defense responses. For example, capsules can resist unenhanced attachment by preventing the glycoprotein receptors on phagocytes from recognizing the bacterial cell wall components and mannose-containing carbohydrates (see Fig. 10). Also, some capsules simply cover the C3b that does bind to the bacterial surface and prevent the C3b receptor on phagocytes from making contact with the C3b (see Fig. 9). This will be discussed in greater detail later in Unit 1 under Bacterial Pathogenesis.
Examples of bacteria that use their capsule to resist phagocytic engulfment include Streptococcus pneumoniae (inf), Haemophilus influenzae type b (inf), Neisseria meningitidis (inf), Bacillus anthracis (inf) , and Bordetella pertussis (inf).
The body's immune defenses, however, can eventually get around the capsule by producing opsonizing antibodies (IgG) against the capsule. The antibody then sticks the capsule to the phagocyte. In vaccines against pneumococccal pneumonia (inf) and Haemophilus influenzae type b (inf), it is capsular polysaccharide that is given as the antigen (def) in order to stimulate the body to make opsonizing antibodies against the encapsulated bacterium.
Highlighted Bacterium: Streptococcus pneumoniae
Click on this link, read the description of Streptococcus pneumoniae, and be able to match the bacterium with its description on an exam.
- Movie of an encapsulated bacterium resising engulfment by a neutrophil. Phagocytosis. © James Sullivan, author. Licensed for use, ASM MicrobeLibrary.
b. Role of glycocalyx in adhering to and colonizing environmental surfaces
The glycocalyx also enables some bacteria to adhere to environmental surfaces (rocks, root hairs, teeth, etc.), colonize, and resist flushing. For example, many normal flora bacteria produce a capsular polysaccharide matrix or glycocalyx to form a biofilm (def) on host tissue (see Fig. 3) as discussed below.
Many pathogenic bacteria, as well as normal flora and many environmental bacteria, form complex bacterial communities as biofilms. Biofilms are groups of bacteria attached to a surface and enclosed in a common secreted adhesive matrix, typically polysaccharide in nature.
Bacteria in biofilms are often able to communicate with one another by a process called quorum sensing (discussed later in Unit 2) and are able to interact with and adapt to their environment as a population of bacteria rather than as individual bacteria. By living as a community of bacteria as a biofilm, these bacteria are better able to:
- resist attack by antibiotics;
- trap nutrients for bacterial growth and remain in a favorable niche;
- adhere to environmental surfaces and resist flushing;
- live in close association and communicate with other bacteria in the biofilm; and
- resist phagocytosis and attack by the body's complement pathways.
Biofilms are, therefore, functional, interacting, and growing bacterial communities. Biofilms even contain their own water channels for delivering water and nutrients throughout the biofilm community.
- Electron micrograph of a biofilm of Haemophilus influenzae from Biomedcentral.com
- Photomicrograph of a biofilm with water channelsfrom Centers for Disease Control and Prevention Rodney M. Donlan: "Biofilms: Microbial Life on Surfaces"
- Biofilm of Pseudomonas aeruginosa from the Ausubel Lab, Department of Molecular Biology, Massachusetts General Hospital
To initiate biofilm formation, planktonic bacteria (free individual bacteria not in a biofilm) contact an environmental surface through their motility or by random collision. These planktonic bacteria then attach to that surface using pili or cell wall adhesins. This attachment then signals the expression of genes involved in quorum sensing and, ultimately, biofilm formation. As the biofilm matrix is secreted, motile bacteria lose their flagella and become nonmotile.
Planktonic Pseudomonas aeruginosa, for example, uses its polar flagellum to move through water or mucus and make contact with a solid surface such as the body's mucous membranes. It then can use pili and cell wall adhesins to attach to the epithelial cells of the mucous membrane. Attachment activates signaling and quorum sensing genes to eventually enable the population of P. aeruginosa to start synthesizing a polysaccharide biofilm composed of alginate. As the biofilm grows, the bacteria lose their flagella to become nonmotile and secrete a variety of enzymes that enable the population to obtain nutrients from the host cells. Eventually the biofilm mushrooms up and develops water channels to deliver water and nutrients to all the bacteria within the biofilm. As the biofilm begins to get too crowded with bacteria, quorum sensing enables some of the Pseudomonas to again produce flagella, escape the biofilm, and colonize a new location (See Figs. 6A-6G).
Streptococcus mutans, and Streptococcus sobrinus , two bacteria implicated in initiating dental caries, break down sucrose into glucose and fructose. Streptococcus mutans can uses an enzyme called dextransucrase to convert sucrose into a sticky polysaccharide called dextran that forms a biofilm enabling the bacteria to adhere to the enamel of the tooth and form plaque. This will be discussed in greater detail later in Unit 2 under Bacterial Pathogenicity. S. mutans and S. sobrinus also ferment glucose in order to produce energy. The fermentation of glucose results in the production of lactic acid that is released onto the surface of the tooth and initiates decay.
- Scanning electron micrograph of Streptococcus growing in the enamel of a tooth.© Lloyd Simonson, author. Licensed for use, ASM MicrobeLibrary.
- Scanning electron micrograph of dental plaque.© H. Busscher, H. van der Mei, W. Jongebloed, R Bos, authors. Licensed for use, ASM MicrobeLibrary.
- Scanning electron micrograph of Staphylococcus aureus forming a biofilm in an indwelling catheter courtesy of CDC.
- Biofilm of Staphylococcus aureus from Montana State University
Many chronic and difficult-to-treat infections are caused by bacteria in biofilms. Within biofilms, bacteria grow more slowly, exhibit different gene expression than free planktonic bacteria, and are more resistant to antimicrobial agents such as antibiotics because of the reduced ability of these chemicals to penetrate the dense biofilms matrix. Biofilms have been implicated in tuberculosis, kidney stones, Staphylococcus infections, Legionnaires' disease, and peridontal disease. It is further estimated that as many as 10 million people a year in the US may develop biofilm-associated infections as a result of invasive medical procedures and surgical implants.
E-Medicine article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free.
Kaiser's Microbiology Home Page
Copyright © Gary E. Kaiser
All Rights Reserved
Updated: Aug., 2012
Please send comments and inquiries to Dr. Gary Kaiser