Microbiology: Types of Microorganisms and Infection

Study Notes

Microbiology is the study of organisms that are not visible to the naked eye and requires microscopy for observation and study.
Microorganisms include bacteria, viruses, fungi, and protozoa, which are widespread in the environment and can also exist within the human body, providing health benefits.

Microorganisms are named using a two-part binomial system, with the first word representing the genus and the second word specifying the species.
The two-part name is written in italics, e.g., Escherichia coli (E. coli).

Bacteria are unicellular, prokaryotic organisms (cells have no nucleus) with a diameter of about 1 micrometer.
They can cause disease if they invade the human body.
Examples of bacteria include E. coli and Staphylococcus aureus.

Viruses are smaller than bacteria, measuring in nanometers, and are non-living particles that require a host cell to replicate.
They cannot be seen using a light microscope and require an electron microscope for observation.
Viral diseases are caused by the replication of viruses within host cells, and viruses can only survive and replicate within other organisms.
Examples of viruses include HIV, rhinovirus, and Epstein-Barr virus.

Fungi are multicellular, eukaryotic organisms that feed on decaying or dead material.
They can be observed using an electron microscope.
Examples of fungi include Penicillium, Candida, and Aspergillus.

Protozoa are eukaryotic organisms that can be unicellular or multicellular.
They can be found in both fresh and saltwater and can cause infections in humans, such as dysentery.
Examples of protozoa include Amoeba, Flagellates, and Ciliates.

Bacteria can thrive on and in the human body, while viruses require a host cell to replicate.
Fungi can produce metabolites that can have antibacterial properties, such as penicillin.
Protozoa can move using pseudopods or flagella.

Bacteria can be classified as gram-positive or gram-negative based on the staining properties of their cell walls.
Gram-positive bacteria have a thick cell wall that retains the primary stain (crystal violet) during the decolourising step.
Gram-negative bacteria have a thin cell wall, and the crystal violet washes away during the decolourising step.
Examples of bacterial morphologies include cocci (spherical), bacilli (rod-shaped), diplococci (two spheres joined together), and streptococci (chains of cocci).

Infections can be acquired through endogenous or exogenous means.
Endogenous infections occur when an infectious agent is already present in the body but has previously been dormant.
Exogenous infections are acquired from the environment, with the most important source being other humans.

Inhalation of aerosols can lead to the entry of pathogens, such as rhinovirus or mycobacteria.
Ingestion of contaminated food and water can lead to the entry of pathogens, such as salmonella.
Inoculation, often enabled by an insect vector, can cause African sleeping sickness and HIV.
Congenital transmission can occur from mother to unborn child.

Good hygiene reduces infections.
Three means of preventing infections are:
1. Cleaning: physical removal of dirt, blood, and sputum that may allow the growth of microorganisms.
2. Disinfection: reduces the number of microorganisms, limiting the possibility of growth.
3. Sterilisation: destroys all living organisms, including fungal spores, and is used for surgical procedures.