• Cell wall: Thin peptidoglycan layer with an outer membrane containing lipopolysaccharides.

• Stains: Pink/red in Gram stain.

• Shape: May be bacilli (rod-shaped), cocci (spherical), or spirilla (spiral).

• Reproduction: Binary fission.

• Habitat: Found in soil, water, and inside animal intestines.

• Antibiotic resistance: More resistant to antibiotics due to outer membrane barrier.

• Special feature: Outer membrane with lipopolysaccharides (LPS) can cause strong immune responses.
  
  

Examples:

• Escherichia coli (E. coli) – found in human intestine.

• Salmonella typhi – causes typhoid fever.

• Vibrio cholerae – causes cholera.

• Neisseria gonorrhoeae – causes gonorrhea.

• Pseudomonas aeruginosa – common in soil and water, can infect wounds.

• Cell wall: Thick peptidoglycan layer, no outer membrane.

• Stains: Purple/violet in Gram stain.

• Antibiotic sensitivity: Usually more sensitive to antibiotics (e.g. penicillin).

• Endospore formation: Some can form resistant resting cells called endospores.

• Role: Can be beneficial (used in food fermentation and decomposition) or pathogenic (cause diseases).
  
  

Examples:

• Bacillus subtilis – soil bacterium, harmless.

• Staphylococcus aureus – can cause skin infections.

• Streptococcus pneumoniae – causes pneumonia.

• Clostridium tetani – causes tetanus.

• Cell wall: Similar to Gram-negative bacteria.

• Special feature: Photosynthetic (contain chlorophyll a), produce oxygen.

• Autotrophic: Perform oxygenic photosynthesis (like plants).

• Pigments: Contain chlorophyll a, phycocyanin, and phycoerythrin.

• Photosynthesis site: Have thylakoid membranes (but no chloroplasts).

• Nitrogen fixation: Some can fix nitrogen using specialized cells called heterocysts.

• Habitat: Aquatic (freshwater and marine), often form colonies or filaments.

• Importance: Contributed to oxygen production in early Earth’s atmosphere.
  
  

Examples:

• Anabaena – has heterocysts for nitrogen fixation.

• Nostoc – forms jelly-like colonies.

• Oscillatoria – filamentous, glides in water.

• Spirulina – used as health food supplement.