what are the surface components of a lesson 1.3...

W o r k b o o kLesson 1.3 1

Bacterial structures

Before we can discuss processes used to iden-tify infectious diseases, details about specific diseases, and immune responses, we need to become familiar with the structures of bacteria, viruses, and immune barriers. In this lesson we will focus on the structures of bacteria that directly relate to infectious diseases. As we will see here and in future lessons, these structures are often precisely adapted in microbes that cause disease.

LESSON 1.3 WORKBOOK

The Bacterial EnvelopeWhen bacteria attempt to grow in, or colonize, an organism, the normal immune system responds to the foreign invader immediately. The immune system recognizes the surface of the bacteria called its envelope. The bacte-rial envelope is composed of a capsule, plasma membrane and cell wall. Because the immune system is so vigilant, pathogenic bacteria must try to stay one step ahead of the immune system by continually modifying their surface components.

The Capsule is camouflage and protection from the environment

The capsule is not essential to bacterial life and not all bacteria have one, but those that do have an advantage in infecting their hosts. The capsule is composed of a slimy layer of sugars and lipids. It plays two roles: First, it is durable and so can protect the bacterium from physical stresses such as osmotic challenges. Second, it camouflages the bacteria from the immune system. It can do this because the capsule consists of the same sugars that are found on the surface of the infected host’s cells. The immune system therefore doesn’t recognize the bacterium as a foreign interloper and doesn’t target it for death.. Examples of bacteria that use a capsule to evade immune system recognition are the Streptococci that cause both strep throat and ‘flesh eating’ disease, the Pneumococci that cause pneumonia and the Meningococci that cause meningitis.

DEFINITIONS OF TERMS

Lipids –Hydrophobic, or water-fearing molecules, including fats.

Osmosis-Movement of water molecules through a selectively permeable membrane that bal-

ances out an unequal concentra-tion across that membrane

Plasma membrane- The bio-logical membrane that separates

the interior of all cells from the outside environment.

For a complete list of defined terms, see the Glossary.

What are the surface components of a bacterium? ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Why is it hard for the immune system to recognize the capsule as foreign?_______________________________________________________________________________________________________________________________________________________________________________

What is the purpose of the cell wall?__________________________________________________________________________________________________________________________________________________________________________________________________________________

What is the purpose of the plasma mem-brane?___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________


The Cell Wall protects the bacteria from environmental stresses In contrast to the capsule, all bacteria have some kind of cell wall. It is located internal to the capsule (if there is one) and external to the plasma membrane. The cell wall reinforces the cell membrane and protects it from environmental stress. If we think about the stresses bacteria face in their natural environment, the reasons they need a cell wall become clear. For example, intestinal bacteria, such as E. coli, are constantly exposed to bile salts that have detergent-like properties able to dissolve an unprotected cell membrane. The cell wall plays key roles in bacterial infections, which will be addresses by groups 2 and 3.

As in eukaryotic cells, the Plasma Membrane separates the cell from the environmentThe bacterial plasma membrane lies internal to the cell wall. It is similar in many respects to eukaryotic plasma membranes, but it also plays specific roles in bacterial infection. For example, the bacterium Streptococcus exports toxins across its plasma membrane. These toxins give rise to sore throat.

The Acid-Fast solution to membrane protection: slow growing but very tough and well camouflaged

Acid-fast bacteria have cell walls that contain large amounts of waxes. This protective cover makes them im-pervious to many chemicals and able to avoid being killed by immune cells. The cost of this protection, is that they grow very slowly, probably because they cannot take up nutrients very rapidly. For example, the tubercle bacillus divides only once every 24 hours. Only a few pathogenic bacteria are acid fast. One notable example is the tubercle bacillus that causes tuberculosis. Its acid-fast coating means it can wall itself off from the immune system when it infects the lungs. Because of this, and because it divides very slowly, the infection can persist for a long time.

Figure 1.3.1: The bacterial envelope consists of the capsule (if there is one), the cell wall, and the plasma membrane.

LESSON MATERIALS What is the purpose of the cell wall?__________________________________________________________________________________________________________________________________________________________________________________________________________________

What is the purpose of the plasma mem-brane?_____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________Name a benefit to being acid-fast_______________________________________________________________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________

Name a drawback to being acid-fast_________________________________________________________________________________________________________ ___________________________________ ____________________________________________________________________________________________________________________________________________

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The Gram-positive solution to membrane protec-tionAll bacteria protect their cell membranes with a thick exterior cell wall that plays a critical role in maintaining shape and rigidity. In Gram-positive bacteria the wall is made of a polymer composed of sugars and amino acids - called murein. Bacteria are the only organisms to have murein. The murein on the surface of a Gram-positive bacterium can absorb a purple dye - the Gram stain – hence the term ‘Gram - positive’. This is used to identify Gram positive bacteria in an infection.

How does the murein work? Murein is composed of sugar chains that are cross-linked to one another in an organization resembling a chain link fence. Layers of murein are wrapped around the length and width of the bacterium to form a sac. Depending on the shape of the murein sac, Gram positive bacteria may resemble rods (bacilli) like Bacillus anthracis, the bacterium that causes anthrax. Other Gram-positive bacteria may resemble spheres (cocci) like the Staphylo-coccus aureus that causes MRSA.

Why is murein useful? The thick dense layer of murein allows bacteria to survive in environments where the osmotic pressure (pres-sure on the membrane) is high. This allows the bacteria to live in solutions that have a low or high salt concentra-tion. However if the murein coat is breached the bacteria will burst.

Gram positive cell wall components can cause illness. Gram-bacterial cell walls contain other unique polymers such as the lipid (fat) molecule LPA. Both murein and LPA are involved in how the immune system recognizes an infection has occurred. Taking the Gram-positive bacterium Staphlyococcus aureus as an example: the staphylococcus bacterium produces a slimy capsule, but in its case the cap-sule cannot camouflage it from the immune system. Instead, cells of the im-mune system can recog-nize both the murein and LPA on the S. Aureus cell wall. The immune system then initiates a response to the invading bacteria that gives rise to typical symptoms of bacterial in-fection such as fever.

LESSON MATERIALS


Membrane Proteins – proteins attached to or associated with the

membrane of a cell


What is murein? _____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

What is LPA?_______________________________________________________________________________________________________________________________________________________________________________

What is the main function of murein?__________________________________________________________________________________________________________________________________________________________________________________________________________________

In what kinds of environments can gram-positive bacteria survive and why?__________________________________________________________________________________________________________________________________________________________________________________________________________________What is the drawback to the gram positive strategy? ____________________________________________________________________________________________________________________________________________

■■ ■.

Figure■1.3.2: Gram positive bacteria have a rigid external cell wall made from murein (purple) and containing LPA. (green) The red shapes in the lipid bilayer are membrane proteins.

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The Gram-negative solution to membrane pro-tection Gram-negative bacteria have adopted a radically different solution to the problem of how to protect their plasma membranes. Their cell wall also has a murein component for rigidity, but it is far less prominent than in the Gram-positive cell wall. Instead, Gram-negative bacteria build a second membrane external to the murein wall. This outer membrane is just like other typical biological membranes, except it contains molecules called phos-pholipids that are unique to Gram-negative bacteria. Because of this cell membrane Gram negative bacteria fail to absorb the purple Gram dye, instead they remain pale pink.

The unique phospholipids of Gram-negative bacteria can cause symptoms of disease.

The major phosphoipid found in the Gram-negative outer membrane is LPS. Like the LPA found in Gram-positive bacteria, LPS is critically important for infection and stimulates a strong immune response. LPS is very potent - even small amounts of LPS in the bloodstream will cause the host to become severely ill. The LPS is different in each Gram-negative species. As a consequence each different species of Gram-negative bacteria stimulates the immune system to produce different specific antibodies against it. These antibodies can then be used to determine which Gram-negative bacterium is present in an infection

The Gram-negative cell wall can inhibit antibiotics.

The inner membrane of Gram-negative bacteria is surrounded by the murein layer for strength together with a gel-like solution of enzymes. These enzymes play important roles in disease. One enzyme in particular - β-lactamase - can inactivate certain types of antibiotics like penicillins and cephalosporins that interfere with the synthesis of the bacterial cell wall. For this reason Gram-negative bacteria are somewhat more resistant to antibiotics than Gram positives. Examples of the many disease-causing bacteria with Gram-negative cell walls are the Escherichia coli (E.coli) that cause intestinal diarrhea (hamburger disease) and Haemophilus influenzae (H. influenzae) that causes flu-like symptoms.

The complex architecture of the Gram-negative cell wall must work very well because in nature (but not necessarily in the human body) Gram-negatives outnum-ber Gram-positives!

LESSON MATERIALS


Phospholipids – a class of lipids that are a major component of cell

membranes.

Enzymes – proteins that catalyze, or increase the rate of, chemical

reactions.


Figure 1.3.3: Gram negatve bacteria have an additional outer membrane external to the murein cell wall. The outer membrane also contains LPS . The red shapes in the lipid bilayer are membrane proteins.

Why don’t gram negative bacteria stain purple with Gram Stain?

__________________________________________________________________________________________________________________________________________________________________________________________________________________

What are some differences between LPS and LPA?

________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

How does the immune system respond to gram negative bacteria versus gram-positive bacteria?

_________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________


Flagella - How bacteria get aroundMany successful pathogens are actively motile, which helps them spread in both the environment and the body. This motility is largely produced by long helical flagella. Depending on the species, bacteria may have one or several flagella.

Why bacteria need to move.

Flagella allow bacteria to move toward substances that attract them – such as nutrients and away from those that they want to avoid such as the cellular predators of the immune system. This process of movement is called chemotaxis. Swimming allows bacteria to chemotax toward or away from a stimulus.

How flagella help bacteria move.

Flagella are attached to the surface of one end of the bacterium. When they spin around counterclockwise, the movement will push the bacteria forward in a straight line like a propeller. If the bacterium has several flagella, they will form a bundle. When all the flagella in a bundle spin counterclockwise the bacteria will also be propelled in a straight line. However, if any of the flagella begin to rotate clockwise, their movement becomes erratic caus-ing the bacteria to tumble randomly. These two types of motion, swimming and tumbling, both occur during chemotaxis. In the absence of a stimulus bacteria alternate between swimming and tumbling and move ran-domly, but when an attractive stimulus, like nutrients, or a negative stimulus, like a white blood cell, appears, the flagella begin to rotate counterclockwise so that swimming predominates. Then the bacteria can move towards the stimulus or away from it.

How flagella impact disease.

Flagella play a direct role in disease: The protein that makes up flagella (named flagellin or the bac-terial H antigen) can be recognized by the immune system, so bacteria have to continually modify it in an attempt to camouflage themselves. For exam-ple, the Salmonella species that causes food poi-soning (Salmonella Typhimurium) has two different kinds of H antigen - one H antigen stimulates an immune system response and one does not. When S. typhimurium is outside of the body it expresses the H antigen that can stimulate immune responses, but as soon as it infects its host, it switches to the H-antigen that cannot stimulate the immune system. In this way it is able to infect its host and avoid detection.


Chemotaxis –The movement of a bacterium based on the chemicals

in their environment


LESSON MATERIALS

Figure 1.3.4: Swimming allows bacteria to chemotax toward or away from a stimulus

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What is the primary purpose of flagella?___________________________________________________________________________________________________________________________________________Why would movement be a useful adapta-tion?____________________________________________________________________________________________________________________________________________

When would a flagellum spin clockwise?____________________________________________________________________________________________________________________________________________

When would a flagellum spin counter clock wise?

____________________________________________________________________________________________________________________________________________

When might having a flagellum be a disad-vantage?

__________________________________________________________________________________________________________________________________________

What is the difference between the two H antigens?

____________________________________________________________________________________________________________________________________________


Pili - How bacteria stick aroundSome sites in the host are very inhospitable to infection. For instance the nasal cavity is continually being cleansed by sneezing, while swallowing washes contents of the mouth down into the harsh acid environment of the stomach. Bacteria that need to survive these challenges use special structures – pili to attach to cells of the host or other bacteria.

Pili are used to stick to surfaces. Like flagella, pili also protrude from the cell wall, but unlike flagella they are used to attach to specific surfaces and not for movement. Pili are shorter than flagella and often distributed in large numbers over the entire sur-face of the bacteria. During attachment the tip of the pilus attaches to cells or other bacteria, in a process that resembles the use of grappling hooks or Velcro. In addition to these “common pili” some bacteria also use sex pili to link donor (male) and recipient (female) cells before transferring DNA between them.

How pili impact disease. Like flagella, pili are often recognized by the immune system, so many bacteria also continually vary the types of pili they produce. This enables them to keep one step ahead of the immune system. For example, Gonococci that cause gonorrhea have a large number of genes that code for different pili proteins. However, they only make a few at any given time. The immune system rapidly makes antibodies against that pilus protein and destroys the bacterium that is producing it. As a result there will be a rapid selection in favor of any gonococci bacteria that are making pili that cannot be recognized by the antibodies. In this quick-change scenario the bacteria keep one step ahead of the immune system. This is why attempts to immunize against gonococci using a vaccine that stimulates production of only one kind of antibody have failed so far.

Spores- How bacteria stay aliveWhen environmental stresses become too harsh simply sticking to a surface is not enough. Some bacteria wait out harsh stresses such as starvation, desiccation, heat and radiation until more favorable conditions appear. They do this by forming a spore by condensing its cell within a protective coat. Each bacterium usually makes only one spore

How spores impact disease. Some particularly important pathogenic bacteria form spores. The Clostridium species that cause tetanus and botulism are key ex-amples. Tetanus spores can lie dormant in the soil for many years before becoming reactivated in the presence of a skin wound, while botulinum spores are resistant to heat and can survive food canning processes. They can cause food poisoning when the food is eaten,

Figure 1.3.5: A picture of a spore within a Clostridium botulinum bac-terium. Each bacterium makes only one spore, about 1 micron in size.

LESSON MATERIALS


Tetanus- a syndrome character-ized by uncontrolled muscle con-tractions, caused by a neurotoxin

produced by Clostridium tetani.

Botulism -a rare but serious para-lytic illness caused by the botulism

toxin, produced by Clostridium botulinum.


Why are the nose and mouth difficult envi-ronments for bacteria? And how do bacteria overcome this? _____________________________________________________________________________________________________________________________________________________________________________________________________________

What are two different functions of pili? _______________________________________________________________________________________________________________________________________________________________________________

How do bacteria prevent their pili from being recognized by the immune system?______________________________________________________________________________________________________________________________________________________________________________

What is a spore?__________________________________________________________________________________________________________________________________________________________________________________________________________________When might a bacterium form a spore?____________________________________________________________________________________________________________________________________________



Nucleoid-a region within pro-karyotes that contains the nuclear

material, such as the DNA. It is not enclosed by a membrane.

Plasmid- a DNA molecule that is separate from and can replicate independently from the chromo-

somal DNA.


What is one characteristic of bacteria that allows them to replicate faster than prokary-otes? __________________________________________________________________________________________________________________________________________________________________________________________________________________What is the nucleoid? __________________________________________________________________________________________________________________________________________________________________________________________________________________

What are plasmids?_____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

What functions can plasmids play?___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

LESSON MATERIALS

Nucleoid and plasmid - what about their genes?Bacterial genes are in the nucleoid.

Bacteria do not have a nucleus like eukaryotic cells. Instead there is a DNA-rich area in the cytoplasm called the nucleoid. Also unlike eukaryotic cells that store DNA in many thousands of genes organized in several structures called chromosomes, bacteria nucleoids contain genes that are usually in only one chromosome. Because no nuclear membrane separates DNA transcription from protein synthesis, both processes can occur at the same time and as a result bacteria can divide very rapidly. This adaptation makes bacterial growth and replication extremely efficient, and so bacteria can quickly take advantage of a favorable environment. Bacterial genes obviously play a key role in their ability to cause disease because they determine whether they are Gram positive or negative, have multiple types of flagellae or pili and whether they can form spores.

Bacterial genes are also found in plasmids.

Unlike eukaryotic cells whose genes are confined to chromo-somes, bacteria also have small circular pieces of DNA called plasmids. Although these plasmids are not critical to the life of the bacterium itself, they play important functions in disease. For example, some plasmids contain genes that produce toxins, such as those secreted by Streptococcus species that cause food poisoning. In fact, the two most potent toxins known to man - tetanus and botulinum toxin - are produced from plasmids that are found in different Clostridium species.

Plasmids can also contain genes that make the bacteria resistant to specific antibiotics. The advantage of plas-mid DNA, rather than chromosomal DNA, is that plasmids are relatively easy to transfer between bacteria, using their sex pili. This exchange of advantageous genes is of great benefit to the bacteria but causes a public health nightmare. It is easy to imagine how bacteria swapping drug resistance genes can rapidly become problematic. Vibrio cholera, which causes cholera, routinely swaps genes via plasmids. Recently two species – one of which was highly infectious and one of which displayed significant drug resistance were found to have combined to make a single highly infectious species that was resistant to many more antibiotics. This new cholera species is now producing epidemics throughout the third world.

Figure 1.3.6: Nucleoid in the cyto-plasm cytoplasm

Figure 1.3.7: Plasmids are circular pieces of DNA found in the cytoplasm


STUDENT RESPONSES

Remember to identify your sources.

■ Bacterial structure ■ Description ■ How is it advantageous?

■ Capsule

■ Gram-positive cell wall

■ Gram-negative cell wall

■ Plasma membrane

■ Murein

■ LPA

■ LPS

■ Flagella

■ Pili

■ Spore

■ Nucleoid

■ Plasmid

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