Structure of Micro-organisms

Micro-organisms, commonly called microbes, are organisms or infectious agents that are too small to be seen clearly with the unaided eye. They are found in soil, water, air, food, animals, plants, the human body and healthcare environments. Some are useful or harmless; some cause disease when they enter the wrong site, multiply excessively, produce toxins or overcome host defences.

This topic explains what microbes are made of, how they differ structurally and why the structure of a microbe matters in microscopy, staining, treatment, infection prevention and nursing care.

• Define micro-organism, pathogen, commensal and opportunistic pathogen.
• Classify microbes as cellular prokaryotes, cellular eukaryotes or acellular infectious agents.
• Describe the major structures of bacteria, fungi, protozoa and viruses.
• Relate bacterial morphology and cell-wall structure to laboratory identification and patient care.
• Apply microbiology structure knowledge to infection prevention and safe nursing practice.

A micro-organism is a microscopic living organism. In health sciences, the term is commonly used for bacteria, fungi, protozoa and microscopic parasites. Viruses are also studied in microbiology although they are acellular: they are not made of cells and can reproduce only inside a living host cell.

Colonisation means microbes are present and multiplying without tissue invasion or symptoms. Infection means invasion and multiplication in the host; disease occurs when infection produces tissue damage or symptoms. A nurse must collect specimens correctly and interpret results with the patient's clinical condition, not merely the presence of an organism.

• Viruses: consist of nucleic acid enclosed in a protein coat, sometimes with a lipid envelope. They depend on host cells to replicate.
• Prions: infectious misfolded proteins that contain no nucleic acid. They are rare but important because they are unusually resistant to ordinary decontamination.
• Viroids: small infectious RNA molecules mainly associated with plant disease; they are not a routine cause of human disease.

Microbes differ greatly in size. A typical bacterium is around 1 micrometre (µm), while many viruses are measured in nanometres (nm). Most protozoa and fungal cells are larger than bacteria, but they still require microscopy for detailed examination.

• 1 millimetre (mm) = 1,000 micrometres (µm).
• 1 micrometre (µm) = 1,000 nanometres (nm).
• Therefore, a 100 nm virus is far smaller than a 1 µm bacterium.

A bacterium is a complete living prokaryotic cell. Although small, it can take in nutrients, make proteins, generate energy, grow, respond to its environment and reproduce independently by binary fission.

A bacterial endospore is a survival structure formed inside one bacterial cell under unfavourable conditions. One vegetative cell forms one endospore and, when conditions improve, one vegetative cell emerges. It does not increase bacterial numbers. Fungal spores are commonly reproductive structures.

Morphology means the form, shape and structural arrangement of an organism. In the laboratory, shape and arrangement give early clues but do not identify a species by themselves.

The cell envelope includes the cell membrane and structures outside it. Gram staining divides bacteria into Gram-positive and Gram-negative groups according to differences in their cell-wall envelope. This is one of the first and most useful laboratory tests in bacteriology.

• It is a rapid classification test, not a final species identification.
• Old, damaged or poorly stained cultures may give unreliable results.
• Mycobacterium has a waxy cell wall and is better assessed with acid-fast staining.
• Mycoplasma lacks a cell wall, so it does not Gram stain in the usual way.

Fungi are eukaryotic organisms. Their cell wall contains chitin and glucans, and their cell membrane contains ergosterol rather than cholesterol. These structural differences are important targets for antifungal therapy.

• Yeasts: unicellular fungi, often round or oval; many reproduce by budding. Example: Candida.
• Moulds: multicellular fungi formed by branching filaments called hyphae; a mass of hyphae is called a mycelium. Example: Aspergillus.
• Dimorphic fungi: can change form, often existing as moulds in the environment and yeasts/tissue forms in the host depending on conditions.
• Fungal spores: may be reproductive and dispersal structures; they are different from bacterial endospores.

Protozoa are single-celled eukaryotes. They have a nucleus, cytoplasm, cell membrane and specialised structures for movement, feeding or attachment. Depending on the group, they may move by pseudopodia, flagella or cilia. Many parasitic protozoa have a feeding/multiplying trophozoite stage and a resistant/transmission cyst stage.

• Plasmodium: malaria parasite with stages in mosquitoes and human liver/red cells.
• Entamoeba histolytica: may form cysts and trophozoites; can cause amoebic dysentery.
• Giardia duodenalis: has flagella and an attachment disc; causes diarrhoeal disease.

Helminths are multicellular parasitic worms. Their eggs and larval forms are often examined microscopically in stool, urine, blood or tissue specimens. They include nematodes (roundworms), cestodes (tapeworms) and trematodes (flukes).

A virus is not a cell. The complete infectious particle is called a virion. It contains a genome of either DNA or RNA, never both as its genomic material, enclosed by a protein coat called a capsid. Some viruses also have a lipid envelope derived from host-cell membranes.

• Helical: capsid proteins arranged in a spiral around nucleic acid.
• Icosahedral: roughly symmetrical capsid with 20 triangular faces.
• Complex: more complicated structure, for example bacteriophages that infect bacteria.
• Enveloped or non-enveloped: enveloped viruses have a lipid envelope; non-enveloped viruses have only nucleic acid and capsid.

• Specimen collection: a correct specimen from the correct site, collected with aseptic technique and labelled accurately helps the laboratory distinguish true infection from contamination.
• Hand hygiene: breaks transfer of bacteria, fungi, viruses and parasites between patients, staff, equipment and the environment.
• Device care: capsules and biofilms help some bacteria adhere to catheters, cannulae and other devices; follow aseptic insertion and maintenance bundles.
• Decontamination: bacterial spores are highly resistant; use approved sterilisation/disinfection methods rather than assuming all organisms are equally easy to kill.
• Antimicrobial stewardship: bacterial structures provide drug targets, but unnecessary antibiotics select resistant bacteria. Antibiotics do not treat viral infections.
• Patient education: explain that not every “germ” causes disease, but hygiene, safe water/food, immunisation, correct medicine use and early care-seeking reduce infection risk.

A patient with a urinary catheter develops fever and cloudy urine. The nurse should not simply report “infection.” Consider the device as a potential surface for bacterial adherence and biofilm formation; assess the patient, obtain specimens correctly when ordered, maintain aseptic technique, review catheter need, monitor vital signs and escalate signs of sepsis promptly.

• Writing that viruses are prokaryotes. Viruses are acellular, not prokaryotic or eukaryotic cells.
• Confusing bacterial endospores with fungal reproductive spores.
• Stating that all microbes cause disease. Many are harmless, beneficial or opportunistic only under certain conditions.
• Forgetting that Gram-negative bacteria have an outer membrane and thin peptidoglycan.
• Calling every round bacterium “staphylococcus.” Shape and arrangement are clues, not a full identification.
• Using antibiotics for viral illness without a bacterial indication.

1. Classify bacteria, fungi, protozoa, helminths and viruses as prokaryotic, eukaryotic or acellular.
2. Draw and label a typical bacterial cell, stating one function of each structure.
3. Differentiate a bacterial endospore from a fungal spore.
4. Compare Gram-positive and Gram-negative bacterial cell envelopes.
5. Describe the structures of an enveloped virus and explain why the envelope matters for infection prevention.
6. Explain how microbial structure influences specimen collection, antibiotic use and decontamination.

• Supplied teaching slides: Structure and Classification of Microbes
• OpenStax Microbiology: Types of Microorganisms
• OpenStax Microbiology: Unique Characteristics of Prokaryotic Cells
• CDC: Factors Affecting Efficacy of Disinfection and Sterilization