Why Some Bacteria Turn Purple Under the Microscope and Others Turn Pink

A drop of pond water holds bacteria, single-celled algae and tiny animal-like cells, all far too small to see. Deciding which of these are related, and how they differ from plants, animals and fungi, is the job of biological classification.

This lesson covers the three domains and five kingdoms, bacterial structure and types, the reproduction and uses of bacteria, and Kingdom Protista.

All organisms fall into three domains — Archaea, Bacteria and Eukarya — based on cell type and molecular differences, while the five-kingdom system places them in Monera, Protista, Fungi, Plantae and Animalia using cell structure, body organisation, nutrition and reproduction.

The three domains:

- Archaea — prokaryotes with unusual cell walls and membrane lipids, often found in extreme habitats
- Bacteria — true bacteria, prokaryotes with peptidoglycan cell walls
- Eukarya — every organism whose cells have a nucleus bounded by a membrane

The five kingdoms:

- Monera — prokaryotic and mostly unicellular; cell wall without cellulose
- Protista — eukaryotic and unicellular
- Fungi — eukaryotic, mostly multicellular and heterotrophic, with chitin cell walls
- Plantae — multicellular and autotrophic, with cellulose cell walls
- Animalia — multicellular and heterotrophic, without cell walls

An everyday example. The bacteria that set curd, the yeast that raises bread dough and the neem tree outside a school belong to three different kingdoms: Monera, Fungi and Plantae.

The substance. Archaea and Bacteria are both prokaryotes, yet Archaea resemble Eukarya in several molecular features — which is why a single kingdom Monera hid a deep split.

A bacterial cell is prokaryotic, with a cell wall, a cell membrane, cytoplasm with 70S ribosomes, a nucleoid of circular DNA and often plasmids and flagella; bacteria are classified by shape, nutrition and respiration, and the Gram stain splits them by cell-wall type.

Shapes:

- Coccus — spherical
- Bacillus — rod-shaped
- Vibrio — comma-shaped
- Spirillum — spiral

Nutrition. Autotrophs make their own food by photosynthesis or chemosynthesis; heterotrophs depend on organic matter, as saprotrophs, parasites or symbionts.

Respiration. Aerobic bacteria need oxygen, anaerobic bacteria grow without it, and facultative bacteria can live either way.

Gram staining:

- Gram-positive — a thick peptidoglycan wall holds the crystal violet dye and stains purple, as in Bacillus and Staphylococcus
- Gram-negative — a thin peptidoglycan layer with an outer membrane loses the dye when washed with alcohol and takes up the red counterstain, appearing pink, as in Escherichia coli

An everyday example. A hospital laboratory testing a throat swab often begins with a Gram stain, because the result helps doctors choose an antibiotic quickly.

The substance. The outer membrane of Gram-negative bacteria also keeps out many antibiotics — so the colour of the stain hints at how hard an infection may be to treat.

Bacteria multiply mainly by binary fission, survive harsh conditions as endospores, and exchange genes by conjugation, transformation and transduction; they are vital in food, farming, medicine and industry, though some cause disease.

Reproduction and gene transfer:

- Binary fission — the cell copies its DNA and splits into two
- Endospores — tough resting structures formed under unfavourable conditions
- Conjugation — DNA passes from one cell to another through a connecting bridge
- Transformation — a cell takes up free DNA from its surroundings
- Transduction — a virus carries DNA from one bacterium to another

Useful bacteria:

- Lactobacillus turns milk into curd
- Rhizobium in root nodules and free-living Azotobacter fix nitrogen
- Streptomyces yields antibiotics such as streptomycin
- Methanogens, which are archaebacteria, produce biogas from cattle dung

Harmful bacteria. They cause cholera, typhoid and tetanus in humans, and citrus canker in plants.

An everyday example. A gobar gas plant in a village runs on methanogens breaking down dung without oxygen.

The substance. An endospore is not a way of multiplying — one cell forms one endospore, which only helps it survive until conditions improve.

Kingdom Protista contains single-celled eukaryotes — the photosynthetic chrysophytes, dinoflagellates and euglenoids, the fungus-like slime moulds, and the animal-like protozoans.

General features. Mostly aquatic, with a membrane-bound nucleus; many move by flagella or cilia; they reproduce both asexually and sexually.

Groups:

- Chrysophytes — diatoms and desmids; diatom cell walls contain silica and fit together in two overlapping halves, like a soapbox
- Dinoflagellates — mostly marine, with two flagella and stiff cellulose plates; Gonyaulax multiplies rapidly to cause red tides
- Euglenoids — mostly freshwater, with a flexible protein layer called a pellicle instead of a cell wall; Euglena photosynthesises in light but feeds on other organisms in the dark
- Slime moulds — saprophytic; they form a spreading plasmodium and, in poor conditions, fruiting bodies with tough spores

Protozoans:

- Amoeboid — Amoeba, and the parasitic Entamoeba
- Flagellated — Trypanosoma, which causes sleeping sickness
- Ciliated — Paramoecium, with a gullet that takes in food
- Sporozoans — Plasmodium, which causes malaria

An everyday example. Polishing powders and filters made from diatomaceous earth use the silica walls of countless dead diatoms.

The substance. Euglenoids are a boundary case — plant-like in light and animal-like in the dark, which is why single-celled eukaryotes were given a kingdom of their own.