• Kingdom Monera refers to all species of bacteria.
  • Bacteria are ubiquitous.
  • They are the most numerous living organisms on Earth. Their biomass is more than the combined biomass of all the other living organisms put together!
  • Bacteria are prokaryotes – meaning they do not have a membrane-bound nucleus nor membrane-bound organelles (see Chapter 6: Cell Structure).

Structure of Bacteria:

All bacteria have a cell wall and cell membrane, but no organelles (they are prokaryotic). Many bacteria have an additional layer of protection called the capsule and flagella (for movement).

  • Capsule: slime layer for protection.
  • Cell wall: structure and protection.
  • Cell membrane: selectively permeable controlling what enters and leave the cell.
  • Flagellum: movement.
  • Chromosome: DNA and protein carrying genes.
  • Ribosome: protein synthesis.
  • Cytoplasm: liquid portion of the cell in which all metabolic reactions occur.
  • Plasmid: circular piece of DNA that gives the bacterium special traits such as antibiotic resistance.

Types of bacteria:

Spherical (coccus/cocci)

These bacteria are round and spherical in shape; e.g. Staphylococcus aureus (bacterium normally present on human skin).

Spiral (spirillum/spirilla)

These bacteria are a spiral or helical shape; e.g. Helicobacter pylori (bacterium that often causes stomach ulcers).

Rod (bacillus/bacilli)

These bacteria are a elongated rod shape; e.g. Lactobacillus casei (bacterium found in milk).

Bacterial Reproduction:

Binary fission

Binary fission: asexual reproduction in bacteria.

Binary fission

Endospore formation

Endospore: thick, tough-walled, dormant and dehydrated bacterial cell formed during unsuitable conditions.

An endospore

Process of endospore formation:

  • Conditions become unfavourable for the bacterial cell.
  • Cell undergoes asymmetrical binary fission with the smaller cell being engulfed by the larger cell.
  • Thick wall, called the cortex, forms around the engulfed cell.
  • Outer coat forms around the cortex

Nutrition in bacteria:

  • Nutrition: way in which organisms obtain and use food.

There are two types of nutrition: autotrophic and heterotrophic nutrition.

Autotrophic nutrition:

Autotrophic nutrition: way in which organisms make their own food.

There are two types of autotrophic nutrition in bacteria: chemosynthetic and photosynthetic nutrition.

Chemosynthetic nutrition:

Chemosynthetic nutrition: way in which bacteria make their own food from inorganic chemicals; e.g. nitrifying bacteria produce nitrates (used in protein synthesis) from ammonia.

Photosynthetic nutrition:

Photosynthetic nutrition: way in which bacteria use sunlight to make their own food; e.g. purple sulphur bacteria produce carbohydrates from carbon dioxide using light energy.

Heterotrophic nutrition:

Heterotrophic nutrition: way in which organisms obtain their food from other organisms.

There are two types of heterotrophic nutrition in bacteria: saprophytic and parasitic nutrition.

Saprophytic nutrition:

Saprophytic nutrition: way in which bacteria feed off dead organic matter; e.g. bacteria of decay.

Parasitic nutrition:

Parasitic nutrition: way in which bacteria feed directly off living organisms; e.g. E. coli.

Factors affecting bacterial growth

  • Temperature: lower temperatures mean lower enzyme activity and a much slower rate of bacterial binary fission.
  • Oxygen concentration: most bacteria need oxygen to survive (aerobic bacteria). There are bacteria that are killed by oxygen called obligate anaerobic bacteria. There are bacteria that cannot use oxygen but tolerate its presence called aerotolerant anaerobic bacteria. There are bacteria that can use oxygen but do not depend on it called facultative anaerobic bacteria.
  • pH: changes in pH affect enzyme function (see Chapter 9: Enzymes) meaning pH values outside of optimum mean that certain bacteria cannot survive. Most bacteria require a neutral pH. There are bacteria that can survive in very acidic conditions called acidophiles. There are bacteria that can survive in very alkaline conditions called alkaliphiles.
  • External solute concentrations: the concentration of the solution surrounding the cell affects the rate of osmosis (see Chapter 8: Movement through cell membranes). If osmosis occurs too quickly in either direction (in or out of the cell) then the cell may die.
  • Pressure: fluid pressure affects all living organisms. High pressures are used by many companies in the food industry to sterilise (kill microorganisms) the food. High pressure kills microorganisms by breaking the cell wall and cell membranes.

Growth curve of microorganisms

  1. Lag phase: microorganisms are adjusting to a newly colonised environment.
  2. Log phase: microorganisms start to reproduce exponentially.
  3. Stationary phase: nutrients begin to run out/toxins begin to build up causing reproduction to slow down and death to increase.
  4. Decline phase: death of microorganisms is at a higher rate than reproduction due to build up of toxins.
  5. Survival phase: some microorganisms produce endospores and can survive the harsh conditions.
Microorganism growth curve

Economic importance of bacteria

For leaving cert biology you must be able to give two examples of beneficial bacteria and two examples of harmful bacteria.

Beneficial bacteria:

  1. Lactic acid bacteria (Lactobacillus casei) – bacteria found in dairy products that help in the production of many dairy products. They also colonise our digestive systems producing lactic acid and inhibiting the growth of harmful bacteria.
  2. E. coli – bacteria of the large intestine are responsible for producing vitamins.
Lactic acid bacteria are present in dairy products

Harmful bacteria:

  1. Strep throat bacteria (Streptococcus pyogenes) – cause of sore throat and scarlet fever.
  2. Tuberculosis bacteria (Mycobacterium tuberculosis) – cause of tuberculosis.


Antibiotics: chemicals produced by microorganisms that inhibit the growth of, or kill, other microorganisms.

Overuse of antibiotics:

Overuse of antibiotics has led to the emergence of antibiotic resistance among bacterial strains such as MRSA (multi-resistant Staphylococcus aureus). Resistance to antibiotics occurs when one bacterium in a population develops a mutation that enables it to avoid the effects of the antibiotic. This one bacterium then divides to form a new colony of resistant bacteria that are unaffected by the antibiotic.

Food processing

Food processing: taking raw ingredients/substrates and making food/products fit for consumption.

Food processing is a type of bioprocessing (see Chapter 9: Enzymes). Bioprocessing is the use of microorganisms/enzymes to make useful products.

Batch food processing:

  • Measured amount of nutrients are added to a bioreactor.
  • Bioreactor inoculated with a culture of bacteria.
  • Bacteria act on the substrate going through the lag, log and stationary phases of the growth curve.
  • Product is removed before the decline phase is reached.
  • Bioreactor is washed, cleaned and sterilised in preparation for another batch.

Continuous flow food processing:

  • Nutrients are continuously added to the bioreactor.
  • Bacterial culture is maintained in the log phase of growth.
  • Product is continuously removed from the bioreactor.