bacteria

One of the first organisms to evolve on earth was a unicellular organism, similar to the modern-day bacteria. Life then evolved into many different life forms over millennia. However, we trace our ancestry back to a single celled organism.

LEARNING OBJECTIVES

By the end of this lesson, you should be able to:

• Describe what is bacteria
• Explain the structure of bacteria
• Explain the classification of bacteria
• Explain reproduction in bacteria
• Explain the harm and benefits of bacteria

Bacteria refers to unicellular organisms that belong to the prokaryotic group. Organisms of this group (prokaryotic) do not have a true nucleus and they lack a few organelles.

Most bacteria are harmful to humans. However, some bacteria have mutual relationship with humans and they are important for our survival. Let us start by looking at the structure of bacteria.

The diagram below is of bacteria. It shows its structure with different parts.

The structure of bacteria is a simple body design. Bacteria are microorganisms that are single celled and without the nucleus and other cell organelles. Organisms like these are called prokaryotes. A bacteria cell includes:

• Capsule. This is a layer in some bacteria found on the outside of a cell wall.
• Cell wall. This is a layer made up of peptidoglycan polymer. It gives bacteria its shape. It is found outside the plasma membrane. Gram positive bacteria have a thicker cell wall.
• Plasma membrane. This is found in the cell wall. It generates energy and transports chemicals. Substances can pass through this membrane as it is permeable.
• Cytoplasm. This is the substance found inside the plasma membrane. It contains genetic material and ribosomes.
• DNA. This is the carrier of genetic instructions used in function and development of the bacterium. It is found inside the cytoplasm.
• Ribosomes. This is the site where proteins are made. These are complex particles that are made up of granules rich in RNA.
• Flagellum. Bacteria move with the help of flagella. They are used to propel some bacteria. Some bacteria have more than one flagellum.
• Pili. These are hair like appendages found on the outside of the cell. They allow the bacterium to stick to surfaces and transfer genetic material to other cells. This contributes to disease spread in humans.

Bacteria have the ability to survive in extremely tough conditions.

Another unique feature about bacteria is their cell wall. It is made up of a protein known as peptidoglycan and it is used for protection. This protein is only found in the cell walls of bacteria. However, a few bacteria lack this cell wall, and some have a third layer of protection known as the capsule. On the outer layer of bacteria, one or more flagella is attached. Flagella are used for locomotion. Some bacteria have pili instead of flagella. Pili help some bacteria while attaching themselves to the host’s cells. Bacteria do not have many cell organelles like plant or animal cells apart from ribosomes.

Ribosomes are the sites where proteins are synthesized. Apart from this DNA, ribosomes have an extra circular DNA known as plasmid. Plasmids help some bacteria strains become resistant to antibiotics.

CLASSIFICATION OF BACTERIA

Bacteria can be classified into different categories on the basis of their characteristics and features. The main basis of classification of bacteria include:

• Shape
• Mode of nutrition
• Cell wall composition
• Mode of respiration
• Environment

CLASSIFICATION OF BACTERIA ON THE BASIS OF SHAPE

• Rod-shaped. They are called bacillus. Escherichia coli (E. coli) is an example of this type of bacteria.

• Spiral. They are called spirilla. Spirillum volutans is an example of this type of bacteria.

• Spherical. They are called coccus. Streptococcus pneumoniae is an example of this type of bacteria.

• Comma-shaped. They are called vibrio. Vibrio cholerae is an example of this type of bacteria.

CLASSIFICATION OF BACTERIA ON THE MODE OF NUTRITION

• Autotrophic bacteria. These bacteria make their own food. They can do this through photosynthesis (using carbon dioxide, sunlight and water) or chemosynthesis (using water, carbon dioxide and chemicals like sulfur, nitrogen and ammonia). An example of this type of bacteria is cyanobacteria.

• Heterotrophic bacteria. These bacteria get their energy by consuming organic carbon. All disease-causing bacteria fall under this category.

CLASSIFICATION OF BACTERIA ON BASIS OF COMPOSITION OF THE CELL WALL

• Peptidoglycan cell wall. These are bacteria that have their cell walls made of the protein peptidoglycan. Gram-positive bacteria fall under this category.
• Lipopolysaccharide cell wall. These are bacteria that have their cell wall made of lipopolysaccharide. Gram-negative bacteria fall under this category.

CLASSIFICATION OF BACTERIA ON THE BASIS OF MODE OF RESPIRATION

• Aerobic bacteria. These are bacteria that respire aerobically (they require oxygen). An example is mycobacterium.

• Anaerobic bacteria. These are bacteria that respire anaerobically (without oxygen). An example is actinomyces.

CLASSIFICATION OF BACTERIA ON THE BASIS OF ENVIRONMENT

• Thermophiles. These are bacteria that survive in extremely high temperatures.
• Acidophiles. Bacteria that survive in extremely acidic conditions.
• Alkaliphiles. Bacteria that survive in extremely alkaline conditions.
• Halophiles. Bacteria found in salty environments.
• Psychrophiles. Bacteria found in cold temperatures like in glaciers.
• Extremophiles. Bacteria that can survive extremely tough conditions.

REPRODUCTION IN BACTERIA

The mode of reproduction of bacteria is asexual. It is known as binary fission. One bacterium divides into two cells called daughter cells. These cells are identical to each other as well as to the parent cell. DNA replication in the parent bacterium marks the start of the fission. Eventually, the cell elongates and divides to form two daughter cells.

The timing and rate of reproduction depends on conditions such as temperatures and the availability of nutrients. Under favorable conditions, E. coli produces approximately 2 million bacteria every seven hours.

The reproduction of bacteria is strictly asexual, but in some rare cases, it is sexual. Genetic combination in bacteria can happen through transduction, transformation or conjugation. In such cases, it is possible for bacteria to become resistant to antibiotics. This is enabled by the variation in genetic material, unlike in asexual reproduction where the same genetic material remains in generations.

USEFUL BACTERIA

Despite most bacteria being harmful, some bacteria are beneficial to humans in different ways. Benefits of bacteria include:

• Fermenting food products. Bacteria are used when fermenting food products like when making yogurt. Bacillus and streptococcus bacteria are used for fermentation.
• Helping in digestion and improving the immunity system of the body. These bacteria include Proteobacteria, Actinobacteria, Bacteroidetes and Firmicutes.
• Production of antibiotics used for the treatment and prevention of other bacterial infections. For example, soil bacteria.
• Fixation of nitrogen in plants. Nitrogen is among the essential plant nutrients. Rhizobium bacteria helps fix nitrogen in the soil to be used by plants.

HARMFUL BACTERIA

Most bacteria are harmful and can cause diseases. They are responsible for many infectious diseases such as pneumonia, syphilis, tuberculosis, tooth decay and diphtheria. Their effects can be treated by taking antibiotics or prescribed medication. However, prevention is more effective. Most of these bacteria can be eliminated by disinfecting surfaces or sterilizing tools. This can be achieved by different methods like the application of heat, UV radiations, disinfectants and pasteurization.

SUMMARY

We've learnt that;

• Bacteria refers to unicellular organisms that belong to the prokaryotic group.
• Most bacteria are harmful to humans but others have a mutual relationship with humans.
• Bacteria are microorganisms that are single celled and without the nucleus and other cell organelles.
• Bacteria have the ability to survive in extremely tough conditions.
• Bacteria can be classified on the basis of shape, mode of nutrition, mode of respiration, cell wall composition, and environment.