Micro-organisms

Groups of micro-organisms

Micro-organisms are living things too small to see clearly without a microscope. Antonie van Leeuwenhoek was the first to observe them with his simple microscope in the 17th century.

Microbial cell structure: prokaryotes (bacteria, actinomycetes, cyanobacteria) have no nuclear membrane, no membrane-bound organelles, 70S ribosomes and a cell wall containing peptidoglycan. Eukaryotes (fungi, algae, protozoa) have a true nucleus and organelles.

• Bacteria: unicellular prokaryotes. Shapes: cocci (spherical), bacilli (rod), spirilla (spiral), vibrio (comma). Grouped as Gram-positive or Gram-negative by staining. Reproduce mainly by binary fission. Examples: Rhizobium, Azotobacter, Bacillus.
• Actinomycetes: filamentous bacteria that look like fungi. They give moist soil its earthy smell and many are sources of antibiotics; Streptomyces produces streptomycin. They help decompose resistant organic matter.
• Fungi: eukaryotic, non-photosynthetic, with cell walls of chitin. Body of thread-like hyphae forming a mycelium (yeast is unicellular). Reproduce by spores. Roles: decomposers, mycorrhiza, many plant diseases (rusts, smuts, blights), yeast in fermentation.
• Algae: photosynthetic organisms with chlorophyll. Blue-green algae (cyanobacteria) such as Anabaena and Nostoc are actually prokaryotes and fix atmospheric nitrogen in rice fields.
• Protozoa: unicellular eukaryotes, animal-like and motile (Amoeba, Paramecium). In soil they feed on bacteria; some cause disease (e.g. Eimeria causes coccidiosis in poultry).
• Viruses: acellular, made of nucleic acid (DNA or RNA) inside a protein coat. They multiply only inside living host cells (obligate parasites). Example: tobacco mosaic virus; bacteriophages infect bacteria.

Key points

• Prokaryotes: bacteria, actinomycetes, blue-green algae.
• Fungal cell wall: chitin; bacterial cell wall: peptidoglycan.
• Streptomyces → streptomycin; earthy smell of soil from actinomycetes.
• Viruses are acellular, obligate parasites.

Exam tips

Questions ask for shape names, cell wall material, examples of each group, and the special features of viruses and actinomycetes.

Common mistakes

• Calling blue-green algae eukaryotes; they are cyanobacteria.
• Thinking viruses can grow on ordinary culture media.