Types of Microorganisms

Microorganisms, or microbes, are microscopic living organisms that can only be seen with the aid of a microscope. Although not all microbes are harmful, some can cause diseases in humans, animals, and plants. This chapter focuses on the main types of microorganisms that are relevant to field epidemiology, as they play a vital role in the emergence, transmission, and control of infectious diseases.

Bacteria

Bacteria are single-celled prokaryotic microorganisms that lack a nucleus and other membrane-bound organelles. They exist in various shapes, such as spheres (cocci), rods (bacilli), spirals (spirilla), and filaments. Bacteria reproduce by binary fission, in which one cell divides into two identical daughter cells.

Many bacteria are harmless or even beneficial, such as those that live in the human gut and help with digestion. However, some bacteria can cause diseases, such as tuberculosis (Mycobacterium tuberculosis), strep throat (Streptococcus pyogenes), and Lyme disease (Borrelia burgdorferi). Bacterial infections are typically treated with antibiotics, but the emergence of antibiotic-resistant strains is a growing concern in public health.

Viruses

Viruses are non-cellular infectious agents composed of a nucleic acid genome (DNA or RNA) enclosed within a protein coat called a capsid. Some viruses also have an outer lipid envelope. Viruses are unable to replicate on their own, requiring a host cell to reproduce.

Viruses cause a wide range of human diseases, including the common cold (rhinoviruses), influenza (influenza viruses), HIV/AIDS (human immunodeficiency virus), and COVID-19 (severe acute respiratory syndrome coronavirus 2, or SARS-CoV-2). Antiviral medications are available for some viral infections, but vaccines remain the most effective means of prevention and control.

Fungi

Fungi are eukaryotic organisms, which means their cells contain a nucleus and other membrane-bound organelles. They exist as unicellular yeasts or multicellular molds and mushrooms. Fungi reproduce by releasing spores, which can be carried by air, water, or other organisms.

Although most fungi are harmless, some can cause infections in humans, particularly in immunocompromised individuals. Examples of fungal infections include candidiasis (Candida species), aspergillosis (Aspergillus species), and histoplasmosis (Histoplasma capsulatum). Fungal infections are treated with antifungal medications.

Protozoa

Protozoa are single-celled eukaryotic organisms that are usually motile, using structures like flagella, cilia, or pseudopodia for movement. Protozoa are found in a variety of habitats, including water, soil, and inside other organisms as parasites.

Some protozoa can cause human diseases, such as malaria (Plasmodium species), amoebic dysentery (Entamoeba histolytica), and giardiasis (Giardia lamblia). Protozoan infections are typically treated with antiprotozoal medications.

Helminths

Helminths are multicellular parasitic worms, including roundworms (nematodes), flatworms (platyhelminths), and flukes (trematodes). Helminths have complex life cycles, often involving multiple hosts.

Helminthic infections can cause various diseases in humans, such as ascariasis (Ascaris lumbricoides), schistosomiasis (Schistosoma species), and tapeworm infections

References

• This text was originally written by ChatGPT4.0 on 6 April 2023 and edited by Arnold Bosman
• Mandell, G. L., Bennett, J. E., & Dolin, R. (Eds.). (2019). Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases (9th ed.). Elsevier.