Classification of Microorganisms

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Classification of Microorganisms

TL;DR

Microorganisms are sorted into three huge domains—Bacteria, Archaea, and Eukarya—based on cell structure and genetics. Within each domain, we use increasingly specific categories: kingdom, phylum, class, order, family, genus, and species. The key dividing line is whether cells have a nucleus; everything else branches from there.

1. The Mental Model

Think of microorganism classification as a filing system where you start with the broadest folder (domain) and drill down through narrower ones until you land on one specific organism. The first split asks: "Does this cell have a nucleus?" That answer determines almost everything else about how it's built and how it evolved. Once you know the domain, you zoom in through kingdom, phylum, and class—each step adds detail about body plan and metabolism. You end at species: organisms that can breed together and share a common ancestor. The domain is the tree; everything else is branches on it.

2. The Core Material

The Three Domains

All microorganisms fall into one of three domains. This framework—proposed by Carl Woese in 1977—replaced the old "prokaryote vs eukaryote" model because it turns out two of those groups are wildly different from each other.

Bacteria are single-celled prokaryotes (no nucleus). Their DNA floats loose in the cytoplasm. They have cell walls made of peptidoglycan, a unique polymer you won't find elsewhere. They're metabolically super flexible—some eat sugar, some eat rock, some make their own food from sunlight. They live everywhere: soil, oceans, your gut, hot springs, Antarctic ice.

Archaea also lack a nucleus and look bacterial under a microscope, but genetically they're closer to eukaryotes than to bacteria. Their cell walls are made of different stuff (not peptidoglycan). Many live in extreme environments: boiling hot springs, salt lakes, oxygen-free swamps. Some produce methane; others love acid or freezing cold. They're as common as bacteria in nature, but we notice them less because they're often in harsh places.

Eukarya have a nucleus. Their DNA is wrapped in chromosomes inside that nucleus. They include protists (like amoebas and algae), fungi (yeasts and moulds), plants, and animals. Within the microorganism world, you mostly care about protists (single-celled) and fungi (usually single-celled at the cellular level, though some form multicellular networks).

Why Domain Matters

The domain split reflects evolution. Bacteria and Archaea diverged about 3.5 billion years ago. Eukarya split off around 2.5 billion years ago, and the evidence is written in their DNA and RNA sequences. When you sequence the ribosomal RNA (rRNA) of an organism, you can build a family tree—sequences that are more similar came from a common ancestor more recently. Domain is the deepest split on that tree.

graph TD
    A["All Life<br/>(LUCA)"] --> B["Domain Bacteria"]
    A --> C["Domain Archaea"]
    A --> D["Domain Eukarya"]

    B --> B1["Cyanobacteria<br/>(photosynthetic)"]
    B --> B2["E. coli<br/>(heterotrophic)"]

    C --> C1["Methanogens<br/>(produce CH₄)"]
    C --> C2["Thermophiles<br/>(heat-loving)"]

    D --> D1["Kingdom Protista<br/>(Amoeba, algae)"]
    D --> D2["Kingdom Fungi<br/>(Yeast, moulds)"]

The Taxonomic Ranks Below Domain

Once you know the domain, you narrow down using seven standard ranks. Kingdom is the broadest subdivision within a domain. For Eukarya, we distinguish:
- Protista: single-celled eukaryotes (amoebas, paramecia, dinoflagellates)
- Fungi: absorb nutrients by secreting enzymes onto food, then ingesting (yeasts, moulds)

For Bacteria and Archaea, classification is trickier—they don't have a universally agreed kingdom system yet, so you often jump straight to phylum based on genetics and metabolism.

Phylum groups organisms by major body plan. In microorganisms, this usually means cell wall type, flagella structure, or metabolic strategy. For example:
- Firmicutes (Bacteria): thick peptidoglycan walls, often form spores
- Proteobacteria (Bacteria): gram-negative, huge metabolic diversity
- Ascomycota (Fungi): produce spores in sacs

Class, Order, Family, Genus, Species get progressively narrower. Most of the time in a biology course, you'll use Genus and Species because that's the binomial name: Escherichia coli, Saccharomyces cerevisiae, Plasmodium falciparum. The genus is capitalised; the species is lowercase. Together they're italicised.

How We Actually Identify Microorganisms

In practice, we use three main tools:

Morphology: Shape (coccus = sphere, bacillus = rod, spirillum = spiral), size, and arrangement (diplo- = pairs, strepto- = chains, staphylo- = clusters). Under a light microscope, you can narrow down fast. A gram stain adds colour and reveals cell wall type.

Genetics: Sequence the 16S rRNA gene (bacteria/archaea) or 18S rRNA gene (eukaryotes). BLAST it against a reference database. The database tells you which organisms have nearly identical sequences—those are your closest relatives. This is the gold standard now.

Metabolism and Environment: Does it need oxygen? Does it produce acid? Does it ferment lactose? Does it grow at 37 °C or 65 °C? Each answer eliminates possibilities. A flow chart of yes/no questions can narrow a mystery bacterium down to species in an afternoon.

Modern labs combine all three. You culture a sample, stain it, sequence its rRNA, and run metabolic tests. Together they give you a species name and enough confidence to publish.

3. Worked Example

You isolate a bacterium from a hot spring in Iceland (68 °C). Here's how you'd classify it:

Step 1: Morphology. Under the microscope, it's a short rod, stains gram-negative (pink), no spores visible. This tells you it's likely a proteobacterium and definitely not a gram-positive firmicute.

Step 2: Extract DNA and sequence 16S rRNA. You send the sample to a sequencing facility. The result comes back: 1,542 base pairs. You BLAST it and get a match: 98% identical to Thermus aquaticus.

Step 3: Verify with metabolism. You culture it aerobically (with oxygen) at 68 °C. It grows. You test for catalase (enzyme that breaks down hydrogen peroxide): positive. You test for oxidase (electrons in a chain): positive. It's chemoheterotrophic—it eats organic matter and uses oxygen. This all matches Thermus aquaticus.

Conclusion: Your isolate is Thermus aquaticus (or a strain very close to it), phylum Deinococcetes, class Deinococci. The rRNA match was the clincher; morphology and metabolism just confirmed it.

4. Key Takeaways

4.1 Most Important Concepts

1. Domain is the primary split. Bacteria vs Archaea vs Eukarya—this reflects the deepest branches of the tree of life and determines cell structure.

2. Prokaryotes lack a nucleus; eukaryotes have one. This single difference cascades: prokaryotes are simpler, smaller, and reproduce asexually; eukaryotes are bigger, have compartments, and often reproduce sexually.

3. 16S/18S rRNA sequencing is the modern standard. It gives you a genetic fingerprint that tells you evolutionary relatedness better than anything else.

4. Binomial nomenclature (genus + species) is universal. Escherichia coli means the same thing in Tokyo, São Paulo, and Stockholm.

5. Microorganisms are classified by cell structure, genetics, and metabolism. No single trait is enough; you need all three for confidence.

6. Archaea are not just "weird bacteria." They're genetically distinct and often thrive in extreme conditions where bacteria struggle.

7. Within microorganisms, protists and fungi are eukaryotes; bacteria and archaea are prokaryotes. This is the deepest practical split you'll use in most labs.

4.2 Common Misconceptions

4.3 Compare & Contrast

5. Now Try It

Exercise: Classify an Unknown Microorganism (20 minutes)

You've isolated a microorganism from a yogurt culture. Here's what you observe:

• Under a light microscope: oval-shaped, no visible nucleus, ~5 μm wide
• Gram stain: purple (gram-positive)
• Growth test: grows aerobically at 37 °C, does not grow at 4 °C or 70 °C
• Metabolic tests: ferments lactose, produces acid, catalase-positive
• 16S rRNA BLAST: 99% identical to Lactobacillus bulgaricus

Your task:
1. Is this organism prokaryotic or eukaryotic? How do you know?
2. What domain is it?
3. What kingdom and phylum would you assign it?
4. Write its full binomial name. Should it be italicised?
5. Why was the 16S rRNA result the most reliable data you collected?

Write out your answers in full sentences. For question 5, explain in 2–3 sentences why genetics beats morphology and metabolism in a tie.

Success looks like: You correctly identify it as a gram-positive bacterium in domain Bacteria, classify it as Lactobacillus bulgaricus (italicised), and explain that rRNA sequencing directly compares evolutionary history, whereas shape and metabolism can be misleading in closely related species.

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