Microbial Cell Structure and Morphology
Domain 2 compares bacterial, fungal, algal, protozoan and viral architecture, with emphasis on recognising shapes, arrangements and envelope layers from descriptions rather than images.
Bacterial morphology
Cocci are spherical; arrangements include diplo-, strepto-, staphylo-, tetrad and sarcina. Bacilli are rods, sometimes coccobacilli or streptobacilli. Spirilla are rigid spirals, vibrios are comma-shaped, and spirochaetes are flexible with axial filaments. The envelope is capsule, cell wall, plasma membrane. Peptidoglycan alternates N-acetylglucosamine and N-acetylmuramic acid. Gram-positive walls are thick with teichoic acids and no outer membrane; Gram-negative walls are thin with an outer membrane containing lipopolysaccharide and a periplasm. Bacteria have 70S ribosomes, a nucleoid, plasmids and inclusion bodies but no membrane-bound nucleus. Flagella arrangements are monotrichous, amphitrichous, lophotrichous and peritrichous. Pili mediate attachment and conjugation. Endospores of Bacillus and Clostridium are survival structures formed by sporulation.
Eukaryotic microbes and viruses
Fungi have chitin walls, septate or coenocytic hyphae, mycelium, budding yeasts and dimorphism. Algae have cellulose walls and photosynthetic pigments; diatoms have silica frustules. Protozoa move by cilia, flagella or pseudopodia and may form cysts. Viruses have DNA or RNA, never both, a capsid of capsomeres, and an envelope in some; bacteriophages have head-tail symmetry.
Worked example
A purple cluster of cocci after Gram staining. Reasoning: purple means thick peptidoglycan retained crystal violet–iodine; clusters indicate staphylococci. If pink, the thin wall was decolourised and safranin counterstained the cells; report Gram-negative.
Traps
Viruses are acellular and unaffected by antibiotics. Fungal walls are chitin, not peptidoglycan. Endospores are for survival, not reproduction. Capsules are not universal. Do not call every rod a typical bacillus if it is a coccobacillus.