Structure, forms and nutrition of bacteria

Biology · Kingdom Prokaryotae. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Bacteria are prokaryotes: single cells with no nuclear membrane and no membrane-bound organelles. Their DNA is one circular chromosome lying in a region called the nucleoid. Leeuwenhoek first saw them with his simple microscope. They live almost everywhere: soil, water, air, food and inside other organisms.

Most have a cell wall of peptidoglycan (murein). The Gram stain divides them into two groups by the thickness of this wall. Extra structures include a capsule, flagella, pili, mesosomes, plasmids and endospores.

Bacteria are classified by shape and by the arrangement of flagella, and they show every mode of nutrition and respiration.

Rules to remember

Gram-positive: thick peptidoglycan wall, keeps crystal violet, looks purple. Gram-negative: thin wall plus an outer membrane, loses the dye in alcohol, looks pink.
Shapes: cocci (spheres), bacilli (rods), spiral forms (vibrio = comma, spirillum, spirochete).
Flagella: atrichous (none), monotrichous (one at one end), lophotrichous (tuft at one end), amphitrichous (at both ends), peritrichous (all over).
Pili: attachment and conjugation. Mesosomes: infoldings of the cell membrane that help in DNA replication, cell division and respiration.
Plasmids: small circular DNA outside the chromosome, often with antibiotic resistance genes. Endospores: resistant resting bodies, e.g. Bacillus and Clostridium.
Nutrition: heterotrophs (saprotrophs and parasites); autotrophs (photosynthetic and chemosynthetic). Photosynthetic bacteria use H₂S, not water, so they release sulphur and no oxygen.
Respiration: obligate aerobes, obligate anaerobes, facultative anaerobes and microaerophilic bacteria (need low oxygen).

Common mistake

Do not mix up capsule and capsid, or endospores and reproductive spores. A capsule is a slimy layer outside a bacterial wall, and one bacterium forms one endospore for survival, not for multiplication.

Worked example

A smear from a sore throat is Gram stained. Under the microscope the cells are purple spheres joined in long chains. Name the group and describe the wall.

Purple after washing with alcohol means the crystal violet was kept, so the cells are Gram-positive.
Gram-positive cells have a thick peptidoglycan wall and no outer membrane.
Spheres are cocci, and cocci in chains are streptococci.

Answer: Gram-positive streptococci with a thick peptidoglycan wall.

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Chapter 6: Kingdom Monera

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