Introduction to Classification
Senior 4 · Biology
Introduction to Classification
Lesson 3 of 5
25 min
A patient arrives at a hospital with a high fever, a stiff neck and a severe headache: possible bacterial meningitis. Treatment must start within hours, before any culture of the bacteria has grown. A laboratory technician spreads a drop of the fluid from around the patient's spinal cord on a slide, stains it, and within about fifteen minutes reports: "pink diplococci". That one line, colour and shape, already narrows down which bacterium is likely and which antibiotics will reach it.
This lesson explains why bacteria stain purple or pink, how they are classified by shape and by other features, how fast they multiply, and why most of them are useful rather than harmful.
Definition
Bacterium (plural bacteria): a single-celled prokaryote of the domain Bacteria, usually 0.5–5 µm long, with a cell wall containing peptidoglycan.
Peptidoglycan (murein): a polymer of sugars cross-linked by short chains of amino acids, which forms a strong mesh around the bacterial cell and stops it bursting by osmosis.
Plasmid: a small, circular loop of DNA separate from the main chromosome, often carrying genes such as those for antibiotic resistance; it can be passed from one bacterium to another.
Gram stain: a staining method that divides bacteria into Gram-positive (stain purple) and Gram-negative (stain pink to red), according to the structure of their cell wall.
Binary fission: asexual reproduction in which the DNA is copied and the cell divides into two identical daughter cells. The time taken for a population to double is the generation time.
Figure 1 shows the parts found in a typical bacterium. Not every bacterium has all of them: capsules, flagella, pili and plasmids are present in some species only.
| Structure | Function |
|---|---|
| Cell wall (peptidoglycan) | Gives shape and stops the cell bursting when water enters by osmosis |
| Capsule (slime layer) | Protects against drying out and against being engulfed by white blood cells; helps cells stick to surfaces |
| Cell surface membrane | Controls what enters and leaves; carries the enzymes of respiration (prokaryotes have no mitochondria) |
| Circular DNA (nucleoid) | One chromosome carrying the genes for all the cell's activities, not enclosed by a membrane |
| Plasmids | Extra genes, for example for antibiotic resistance; copied and shared between bacteria |
| 70S ribosomes | Protein synthesis; smaller than the 80S ribosomes in eukaryotic cytoplasm |
| Flagellum | Rotates like a propeller to move the cell |
| Pili | Attach the cell to surfaces or host cells; a sex pilus joins two cells to transfer a plasmid (conjugation) |
By shape and arrangement (Figure 2). Cocci are spheres, bacilli are rods, vibrios are commas, spirilla are rigid spirals and spirochaetes are flexible spirals. Cocci that stay together after dividing form pairs (diplococci), chains (streptococci) or clusters (staphylococci).
By the Gram stain. A smear of bacteria is stained with crystal violet, fixed with iodine, rinsed with alcohol, and then counterstained with a red dye, safranin.
| Gram-positive | Gram-negative | |
|---|---|---|
| Wall structure | Thick peptidoglycan layer, no outer membrane | Thin peptidoglycan layer covered by an outer membrane of lipopolysaccharide |
| Effect of alcohol | Crystal violet–iodine complex trapped in the thick wall | Outer membrane dissolved, thin wall cannot hold the dye, which washes out |
| Final colour | Purple | Pink to red (safranin) |
| Examples | Staphylococcus, Streptococcus, Lactobacillus | Escherichia coli, Vibrio cholerae, Neisseria, Rhizobium |
| Penicillin | Usually effective: it blocks the cross-linking of peptidoglycan | Often less effective: the outer membrane keeps many antibiotics out |
By respiration: obligate aerobes need oxygen; facultative anaerobes, such as E. coli, use it when it is present but can respire without it; obligate anaerobes, such as Clostridium, are killed by it.
By nutrition: photoautotrophs (cyanobacteria) photosynthesise; chemoautotrophs, such as the nitrifying bacteria in soil, get energy by oxidising inorganic substances like ammonium; heterotrophs feed as saprotrophs (decomposers), parasites (pathogens) or mutualists (such as Rhizobium in bean nodules).
Worked example 1: reading a Gram stain
The smear from the meningitis patient showed pink cocci in pairs. (a) Classify the bacteria by Gram reaction, shape and arrangement. (b) Explain why they are pink. (c) Suggest why a doctor would not rely on penicillin alone until tests are complete.
A bacterium copies its circular DNA, grows, and divides by binary fission. In good conditions some species divide every 20 minutes. Starting from cells, after generations the number of cells is
where is the time elapsed and the generation time. Growth this fast cannot last: food runs out and waste builds up (you will study the bacterial growth curve in the Microbiology unit). Some bacteria, such as Bacillus and Clostridium, survive bad conditions as endospores, tough resting cells that can survive boiling.
Worked example 2: how fast do bacteria multiply?
(a) 100 bacteria are placed in warm milk; their generation time is 20 minutes. How many are there after 3 hours, if growth is unlimited? (b) A culture of 1,000 cells has a generation time of 30 minutes. How long will it take to reach one million cells?
| Useful roles | Harmful roles |
|---|---|
| Decomposition: saprotrophic bacteria break down dead matter and return nutrients to the soil | Disease: cholera (Vibrio cholerae), tuberculosis (Mycobacterium tuberculosis), typhoid, pneumonia, meningitis |
| Nitrogen cycle: Rhizobium in the root nodules of beans and other legumes fixes nitrogen gas into ammonium; nitrifying bacteria make nitrate | Food spoilage and food poisoning, for example by some strains of Salmonella and Staphylococcus |
| Food: lactic acid bacteria such as Lactobacillus sour milk into fermented milk and yoghurt | Denitrification: some soil bacteria turn nitrate back into nitrogen gas, removing it from waterlogged fields |
| Gut bacteria in humans make vitamin K and compete with pathogens; in cattle they digest cellulose | Antibiotic resistance genes spread between bacteria on plasmids |
| Biotechnology: genetically modified E. coli makes human insulin | Plant diseases, such as bacterial wilt of potatoes and bananas |
Amoeba Sisters: "Bacteria (Updated)". Watch for the structures in Figure 1, how binary fission and conjugation differ, and the examples of helpful bacteria.
Warning
Common mistakes
Tip
Memory hook. "Positive = Purple = thick Peptidoglycan." For growth questions, first find the number of generations , then use ; to find a time, use logarithms.
Practice: bacteria
Key points
Sources and credits
Quick check
A Gram stain shows purple spheres in grape-like clusters. How are they described?
