1.3 Types of Microorganisms

Types of Microorganisms

Microorganisms span a huge range of forms, from simple bacteria to complex fungi, and even non-living infectious agents like viruses and prions. Classifying them correctly is one of the first skills you'll build in microbiology, because how an organism is structured determines how it causes disease, how it reproduces, and how we can fight it.

Types of Microorganisms

The broadest way to organize microorganisms is by whether they're made of cells and, if so, what kind.

Cellular Microorganisms

Prokaryotes include bacteria (e.g., E. coli) and archaea (e.g., Thermococcus).

• They lack a membrane-bound nucleus. Instead, their single circular chromosome sits in a region called the nucleoid.
• They also lack other membrane-bound organelles (no mitochondria, no endoplasmic reticulum).
• Their ribosomes are 70S, which is smaller than eukaryotic ribosomes. This size difference is actually why certain antibiotics can target bacteria without harming your own cells.

Eukaryotes include fungi (e.g., yeast), protozoa (e.g., Amoeba), and algae (e.g., Chlorella).

• They possess a membrane-bound nucleus plus other membrane-bound organelles like mitochondria and endoplasmic reticulum.
• Their DNA is organized into multiple linear chromosomes housed within the nucleus.
• Their ribosomes are 80S, larger than prokaryotic ribosomes.

Acellular Infectious Agents

Viruses (e.g., influenza virus, HIV) are not considered living organisms because they can't reproduce on their own.

• A virus consists of genetic material (either DNA or RNA, never both) enclosed in a protein coat called a capsid.
• Some viruses also have a lipid envelope surrounding the capsid.
• They must hijack a host cell's machinery to replicate.

Viroids (e.g., Potato spindle tuber viroid) are even simpler than viruses. They're just naked, circular, single-stranded RNA molecules with no capsid at all. They primarily infect plants.

Prions (e.g., the agent causing Creutzfeldt-Jakob disease) are misfolded proteins that contain no genetic material whatsoever. They cause disease by inducing normal proteins in the brain to misfold, leading to neurodegenerative conditions.

Microorganisms vs. Infectious Agents

This distinction trips people up, so here's the key difference: cellular microorganisms can metabolize and reproduce independently, while viruses and other infectious agents cannot.

Archaea vs. Bacteria

Both archaea and bacteria are prokaryotes, so they look similar under a microscope. But they differ in several important ways at the molecular level.

• Cell wall composition: Bacteria use peptidoglycan (e.g., Streptococcus). Archaea use pseudopeptidoglycan, an S-layer, or have no cell wall at all (e.g., Methanococcus). This is why antibiotics that target peptidoglycan (like penicillin) don't affect archaea.
• Membrane lipids: Bacteria have ester-linked lipids. Archaea have ether-linked lipids, which are more stable at extreme temperatures and pH levels.
• RNA polymerase: Bacteria have a single type. Archaea have multiple types, more similar to eukaryotic RNA polymerases. This is one reason archaea are considered more closely related to eukaryotes.
• Metabolic diversity: Both groups are metabolically diverse, but archaea have unique pathways like methanogenesis (producing methane, as in Methanosarcina). Many archaea are extremophiles, thriving in environments like hot springs, salt lakes, and deep-sea vents.

Scope of Microbiology

Microbiology covers all the groups discussed above: bacteria, archaea, fungi, protozoa, algae, and viruses. These organisms show up across nearly every area of science and industry.

• Human health: Pathogenic microbes cause disease (Streptococcus pneumoniae causes pneumonia), while beneficial ones maintain health (Lactobacillus acidophilus in your gut aids digestion). Understanding both sides drives vaccine development and antibiotic discovery.
• Ecosystem balance: Microorganisms drive nutrient cycling. Nitrogen-fixing bacteria (e.g., Rhizobium) convert atmospheric nitrogen into forms plants can use. Fungi decompose dead organic matter, recycling nutrients back into the soil.
• Biotechnology: Penicillium fungi produce the antibiotic penicillin. Bacillus subtilis is used to manufacture industrial enzymes. Microbes are also engineered to produce biofuels and recombinant proteins.
• Food production: Fermentation depends on microbes. Saccharomyces cerevisiae (baker's yeast) makes bread rise and produces alcohol in beer and wine. Lactococcus lactis is used in cheese production.

Microbial Interactions and Adaptations

Even at the introductory level, it's worth knowing a few concepts about how microbes relate to their environments and to us.

• Microbiome refers to the collective genetic material of all microorganisms living in a particular environment. The human gut microbiome, for example, contains trillions of microbes that influence digestion, immunity, and even mood.
• Symbiosis describes close, long-term interactions between different species. It comes in three main forms: mutualism (both benefit), commensalism (one benefits, the other is unaffected), and parasitism (one benefits at the other's expense).
• Pathogenicity is an organism's ability to cause disease. This depends on specific virulence factors like toxins, adhesion molecules, or capsules that help the microbe evade the immune system.
• Antibiotic resistance occurs when microorganisms evolve to withstand antibiotics. Resistance genes can spread between bacteria through horizontal gene transfer, making this one of the most pressing challenges in modern medicine.