Bacteria — Revision Notes

⚡ 30-Second Revision

Prokaryotic: — No membrane-bound nucleus or organelles.
Genetic Material: — Single circular $DNA$ (nucleoid), often plasmids.
Cell Wall: — Peptidoglycan (murein) unique to bacteria.
Ribosomes: — 70S type.
Shapes: — Cocci (spherical), Bacilli (rod), Spirilla (spiral), Vibrios (comma).
Reproduction: — Binary fission (asexual).
Gram Staining: — Gram-positive (purple, thick peptidoglycan), Gram-negative (pink/red, thin peptidoglycan + outer membrane).
Nutrition: — Photoautotrophs (e.g., Cyanobacteria), Chemoautotrophs (e.g., Nitrifying bacteria), Heterotrophs (saprophytes, parasites, symbionts).
Key Roles: — Decomposers, Nitrogen fixers (*Rhizobium*, *Azotobacter*), Pathogens.
Survival: — Endospores (highly resistant dormant structures).

2-Minute Revision

Bacteria are fundamental prokaryotic organisms, meaning they lack a true nucleus and other membrane-bound organelles. Their genetic material is a single circular DNA molecule in the nucleoid, often supplemented by plasmids.

A defining feature is their cell wall, made of peptidoglycan, which is crucial for Gram staining. Gram-positive bacteria have a thick peptidoglycan layer and stain purple, while Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane, staining pink/red.

They reproduce primarily by binary fission. Bacteria exhibit vast metabolic diversity: some are photoautotrophs (like cyanobacteria), others are chemoautotrophs (like nitrifying bacteria), and many are heterotrophs, acting as decomposers, parasites, or symbionts.

Key ecological roles include nitrogen fixation (e.g., *Rhizobium* in legumes) and nutrient cycling. Some form highly resistant endospores for survival in harsh conditions. Understanding their structure, classification by shape and Gram stain, and metabolic roles is essential for NEET.

5-Minute Revision

Bacteria are single-celled prokaryotes, characterized by the absence of a membrane-bound nucleus and organelles. Their genetic material, typically a single circular chromosome, resides in the nucleoid, and many also carry plasmids – small, circular DNA molecules often conferring antibiotic resistance.

The bacterial cell wall, composed of peptidoglycan, is a critical structural feature. This cell wall forms the basis of Gram staining: Gram-positive bacteria, with their thick peptidoglycan layer, retain crystal violet and appear purple, while Gram-negative bacteria, having a thin peptidoglycan layer and an outer membrane (containing lipopolysaccharides), do not retain the stain and appear pink/red after counterstaining.

Common shapes include cocci (spherical), bacilli (rod-shaped), spirilla (spiral), and vibrios (comma-shaped). Reproduction is predominantly asexual via binary fission, leading to rapid population growth.

Bacteria display remarkable metabolic versatility: photoautotrophs (e.g., cyanobacteria) use light, chemoautotrophs (e.g., nitrifying bacteria like *Nitrosomonas*) oxidize inorganic compounds, and heterotrophs obtain nutrients from organic matter (as saprophytes, parasites, or symbionts).

They are vital for ecological processes such as decomposition and biogeochemical cycles, notably nitrogen fixation (*Rhizobium* in legumes, *Azotobacter* free-living). Some bacteria, like *Bacillus* and *Clostridium*, form highly resistant endospores to survive extreme conditions.

For NEET, focus on distinguishing prokaryotic features, Gram staining, key metabolic types with examples, and their ecological/pathogenic significance.

Prelims Revision Notes

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Prokaryotic Nature: — Bacteria are prokaryotes. Key features: no true nucleus (genetic material in nucleoid), no membrane-bound organelles (mitochondria, ER, Golgi), 70S ribosomes.
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Cell Wall: — Composed of peptidoglycan (murein). Provides shape and protection. Target for many antibiotics.
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Gram Staining:

* Gram-positive: Thick peptidoglycan layer. Retain crystal violet, appear purple. Lack outer membrane. E.g., *Staphylococcus*, *Streptococcus*, *Bacillus*, *Clostridium*. * Gram-negative: Thin peptidoglycan layer. Outer membrane with Lipopolysaccharides (LPS). Do not retain crystal violet, counterstained pink/red. E.g., *E. coli*, *Salmonella*, *Pseudomonas*.

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Shapes:

* Cocci: Spherical (e.g., *Streptococcus* - chains, *Staphylococcus* - clusters). * Bacilli: Rod-shaped (e.g., *E. coli*). * Spirilla: Rigid spiral (e.g., *Spirillum minor*). * Vibrios: Comma-shaped (e.g., *Vibrio cholerae*). * Spirochetes: Flexible spiral (e.g., *Treponema pallidum*).

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Other Structures:

* Flagella: For motility (chemotaxis). * Pili/Fimbriae: For attachment; sex pili for conjugation (genetic transfer). * Capsule/Slime layer: Outer protective layer, aids adhesion, prevents desiccation/phagocytosis. * Plasmids: Extrachromosomal DNA, carry genes like antibiotic resistance. * Endospores: Highly resistant dormant structures for survival (e.g., *Bacillus*, *Clostridium*).

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Reproduction: — Primarily binary fission (asexual). Genetic recombination via transformation, transduction, conjugation.
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Nutrition:

* Autotrophs: Make own food. * Photoautotrophs: Light energy (e.g., Cyanobacteria). * Chemoautotrophs: Chemical energy from inorganic compounds (e.g., Nitrifying bacteria: *Nitrosomonas* ($NH_3 \to NO_2^-$), *Nitrobacter* ($NO_2^- \to NO_3^-$)). * Heterotrophs: Obtain food from organic compounds. * Saprophytes: Decomposers. * Parasites: Cause disease. * Symbionts: Mutualistic relationships (e.g., *Rhizobium* in legume root nodules for nitrogen fixation).

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Ecological Roles: — Decomposers, nutrient cycling (N-fixation, nitrification, denitrification).
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Pathogens: — Examples: *Vibrio cholerae* (cholera), *Salmonella typhi* (typhoid), *Mycobacterium tuberculosis* (TB), *Clostridium tetani* (tetanus).

Vyyuha Quick Recall

For bacterial shapes, remember: Cute Boys Sing Very Sweetly. Cocci (spherical) Bacilli (rod) Spirilla (rigid spiral) Vibrios (comma) Spirochetes (flexible spiral)