Lesson 5:
Bacteria
Objective
In this lesson we will learn the following concepts:
- The structure of bacteria.
- Stages of bacterial growth.
- The different types of respiration.
- How to identify and classify bacteria.
Reading Assignment
In addition to the online lecture, read chapter 4 in Wastewater Microbiology.
Lecture
Introduction
Bacteria colonize the human environment: homes and businesses, the soil, even the human body. Tiny as they are, bacteria can pose enormous threats to public health if conditions allow them to thrive and multiply. Although all bacteria share certian structural, genetic, and metabolic characteristics, important biochemical differences exist among the many species of bacteria. These differences permit bacteria to live in many different, and sometimes extreme, environments. For example, some bacteria recycle nitrogen and carbon from decaying organic matter, then release these gases into the atmosphere to be reused by other living things. Other bacteria cause diseases in humand and animals, help digest sewage in treatment plants, or produce the alcohol in wine, beer and liquors. Still others are used by humans to break down toxic waste chemicals in the environment, a process called bioremediation.
Parts of Procaryotic Cells
A procaryotic cell is a cell that does not have a true nucleus. The nuclear structure is called a nucleoid, which contains most of the cell's genetic material and is usually a single circular molecule of DNA. A procaryotic organism, such as bacteria, is a cell that lacks a membrane-bound nucleus or membrane-bound organelles. The exterior of the cell usually has glycocalyx, flagellum, fimbriae, and pili.
Glycocalyx
Glycocalyx is a sticky, sugary envelope composed of polysaccharides and/or polypeptides that surround the cell. Glycocalyx can be firmly attached to the cell's surface, called capsule, or loosely attached, called slime layer. A slime layer is water-soluble and is used by the procaryotic cell to adhere to surfaces external to the cell.
Flagella
Flagella are made of protein and appear "whip-like". They are used by the procaryotic cell for mobility. Flagella propel the microorganism away from harm and towards food in a movement known as taxis.
Flagella can exist in the following forms:
- Monotrichous: One flagellum
- Lophotrichus: A clump of flagella, called a tuft, at one end of the cell.
- Amphitrichous: Flagella at two ends of the cell.
- Peritrichous: Flagella covering the entire cell.
- Endoflagellum: A type of amphitrichous flagellum that is tightly wrapped around spirochetes. A spirochete is a spiral-shaped bacterium that moves in a corkscrew motion. Borrelia burgdorferi, which is the bacterium that causes lyme disease, exhibits an endoflagellum.
Fimbriae
Fimbriae are proteinaceous, sticky, bristle-like projections used by cells to attach to each other and to objects around them. Fimbriae may be responsible for the clinging of cells that leads to biofilms and other thick aggregates of cells on the surface of liquids and for the microbial colonization of inanimate solids such as rocks and glass.
Pili
Pili are tubules that are used to transfer DNA from one cell to another cell, similar to tubes used to fuel an aircraft in flight. Some are also used to attach one cell to another cell. The tubules are made of protein and are shorter in length than flagella and longer than fimbriae.
Cell Wall
The procaryotic cell's cell wall is located outside the plasma membrane and gives the cell its shape and provides rigid structural support for the cell. The cell wall also protects the cell from its environment. Pressure within the cell builds as fluid containing nutrients enters the cell. It is the job of the cell wall to resist this pressure the same way that the walls of a balloon resist the build-up of pressure when it is inflated. If pressure inside the cell becomes too great, the cell wall bursts, which is referred to as lysis.
The cell wall of many bacteria is composed of peptidoglycan, which covers the entire surface of the cell. It is made up of a combination of peptide bonds and carbohydrates. The wall of a bacterium is classified in two ways:
- Gram-positive. A gram-positive cell wall has many layers of peptidoglygan that retain the crystal of violet dye when the cell is stained. This gives the cell a purple color when seen under a microscope.
- Gram-negative. A gram-negative cell wall is thin. The inside is made of peptidoglycan. The outer membrane is composed of phospholipids and lipopolysaccharides.
The cell wall does not retain the crystal of violet dye when the cell is stained. The cell appears pink when viewed with a microscope.
Cytoplasmic Membrane
The procaryotic cell has a cell membrane called the cytoplasmic membrane that forms the outer structure of the cell and separates the cell's internal structure from the environment. This membrane provides a selective barrier, allowing certain substances and chemicals to move into and out of the cell.
The cytoplasmic membrane regulates the flow of molecules (such as nutrients) into the cell and removes waste from the cell by opening and closing passages called channels. In photosynthetic procaryotes, the membrane functions in energy production by collecting energy in the form of light. This membrane is selectively permeable because it permits the transport of some substances and inhibits the transport of others. Two types of transport mechanisms are used to move substances through the cytoplasmic membrane. These are passive transport and active transport.
Passive Transport
Passive transport moves substances into and out of the cell down a gradient similar to how a rock rolls downhill, following the gradient. There are three types of passive transport:
- Simple diffusion: Simple diffusion is the movement of substances form a higher-concentration region to a lower-concentration region. Large molecules are too large to enter the cell.
- Facilitated diffusion: Facilitated diffusion is the movement of substances from a higher-concentration region to a lower-concentration region with the assistance of an integral protein across a selectively permeable membrane.
Active Transport
Active transport is the movement of a substance across the cytoplasmic membrane against the gradient by using energy provided by the cell. This is similar to pumping water against gravity through a pipe. Energy must be spent in order for the pump to work. A cell makes energy available by removing a phosphate (P) from adenosine triphosphate (ATP). ATP contains chemical potential energy that is released by a chemical reaction within the cell. It is this energy that is used to change the shape of the integral membrane protein-enabling substances inside the cell to be pumped through the membrane. For example, active transport is used to pump sodium from a cell.
Reproduction
Procaryotic cells reproduce asexually. Asexual reproduction is a process through which one parent gives rise to genetically identical offspring - in other words, two clones of the parent.
Asexual reproduction in microorganisms is often known as binary fission since it consists of a cell splitting in half. As you can see in the animation above, the microorganism first makes a second copy of its DNA in a process known as replication . Next, the cell begins to constrict in the middle, leaving one set of DNA and organelles on each side of the constriction. Eventually, the cell splits apart into two identical daughter cells. Once these daughter cells enlarge to adult size, each one is ready to split into two more daughter cells. The physical and chemical requirements for growth can vary widely among different species of bacteria, but in general, the physical requirements include proper temperatures, pH and osmotic pressure. Most bacteria thrive only within narrow ranges of these conditions, however extreme those ranges may be.
Stages of Bacterial Growth
The term "bacterial growth" generally refers to growth of a population of bacteria, rather than of an individual cell. Under optimal conditions, a bacterium can divide into two daughter cells every 15 to 30 minutes. After another 15 to 30 minutes, the two daughter cells can each divide into two more cells. These four cells then divide into eight cells, and so on. As you can see, microorganism populations have the potential to grow tremendously within a very short period of time.
You might expect for a microorganism population to continue to grow indefinitely. However, in a closed system (an environment such as a test tube or a batch of sewage which is separate from the outside world), microorganism populations usually follow a predictable pattern of growth and death shown in the diagram above.
When microorganisms are first introduced to a new environment, they go through a lag phase. The lag phase is a time when the microorganisms do not reproduce, so the number of microorganisms in the population remains constant. During the lag period, microorganisms are adjusting to their new environment.
After a short time, the microorganisms begin to reproduce. At first, they reproduce relatively slowly, but the reproduction rate quickly speeds up as they pass out of the accelerated growth phase and into the logarithmic growth phase .
As the microorganisms grow, they begin to use up the food and oxygen in their environment. They also excrete wastes which pollute their environment. Eventually, the environmental conditions degrade to a point where the bacterial growth begins to slow - the decelerated growth phase . Then the number of organisms found in the population levels off as the death rate equals the birth rate in the stationary phase .
The environment continues to be degraded, and soon the microorganism death rate exceeds the birth rate and the population size begins to fall in the accelerated death phase . At the maximum death rate, the population is in the logarithmic death phase in which the population shrinks very quickly until nearly all of the cells are dead. At this point - the survival phase - the population will level off at a relatively small size.
The wastewater operator needs to be familiar with this population growth curve since it will influence the functioning of an activated sludge system. During conventional treatment, the activated sludge is held for a sufficient time for the microorganism population to enter the logarithmic death phase.
