What are the key characteristics and classifications of major bacterial groups as outlined in the textbook?

Here is a clear, systematic breakdown of each topic from the textbook page: --- ### **1. Diversity and Ecology of Bacteria** * **Overview:** Prokaryotes evolved around $3.5$ billion years ago and can survive in extreme habitats across the planet. * **Classification:** Margulis and Schwartz classified prokaryotes into 16 distinct phyla within Domain Bacteria. --- ### **2. Omnibacteria** * **Definition:** Rigid, rod-shaped, Gram-negative, aerobic heterotrophic bacteria that do not form spores. * **Key Features & Example:** Most possess flagella for motility. A prominent example is *Escherichia coli* (*E. coli*), alongside *Vibrio* species. --- ### **3. Cyanobacteria** * **Definition:** Photosynthetic, oxygen-producing prokaryotes containing chlorophyll-$a$ alongside accessory pigments (carotenoids and phycobilins). * **Key Features & Role:** They played a historical role in oxygenating Earth's atmosphere and fix nitrogen in specialized cells called **heterocysts**. They frequently form algal/colourful blooms in water and mats on soil or marine sediments. --- ### **4. Mycoplasmas and Spiroplasmas** * **Definition:** Unique prokaryotes that completely lack a cell wall, rendering them naturally resistant to penicillin and cell-wall-inhibiting antibiotics. * **Pathogenicity:** Mycoplasmas cause mammalian infections (such as human pneumonia), while Spiroplasmas cause plant diseases (such as lethal yellowing of coconuts). --- ### **5. Spirochaetes** * **Definition:** Long, spiral-shaped (spirilla) Gram-negative bacteria driven by 2 to over 100 flagella. * **Key Example:** *Treponema*, which causes syphilis, a major sexually transmitted infection. --- ### **6. Pseudomonads** * **Definition:** Straight or curved Gram-negative rod-shaped bacteria with one or more polar flagella at one end. * **Ecological & Biological Role:** Found abundantly in soil and water; they break down organic matter, participate in denitrification, and include species like *Pseudomonas aeruginosa*.