Bacterial morphology refers to the size, shape, and arrangement of bacterial cells. Along with staining characteristics and biochemical tests, morphological features are fundamental to the initial identification and classification of bacterial isolates in clinical and environmental microbiology.
Cell Shapes
Bacteria exhibit several distinct cell shapes. Cocci are spherical or ovoid cells whose arrangements include diplococci (pairs, e.g., Neisseria meningitidis), streptococci (chains, e.g., Streptococcus pyogenes), staphylococci (clusters, e.g., Staphylococcus aureus), tetrads (groups of four), and sarcinae (cubes of eight). Bacilli are rod-shaped, cylindrical cells with varying length-to-width ratios, arranged as single bacilli (e.g., Escherichia coli), diplobacilli (pairs), or streptobacilli (chains, e.g., Bacillus cereus). Spirilla are rigid spiral-shaped bacteria, typically motile with polar flagella (e.g., Spirillum volutans). Vibrios are curved or comma-shaped rods (e.g., Vibrio cholerae), while spirochetes are flexible, helical bacteria with internal flagella (axial filaments) for motility (e.g., Treponema pallidum, Borrelia burgdorferi).
Cell Arrangements
The plane of cell division determines the arrangement. Cocci dividing in one plane form chains; dividing in two perpendicular planes form tetrads; dividing in three planes form sarcinae; and dividing irregularly form clusters. Pleomorphic bacteria (e.g., Mycoplasma, Corynebacterium) exhibit variable shapes depending on growth conditions. Filamentous bacteria (e.g., Streptomyces, Nocardia) form branching filaments resembling fungal hyphae.
Cell Size
Most bacteria range from 0.5 to 5 µm in length, approximately 10–100 times smaller than eukaryotic cells. The smallest known bacteria (Mycoplasma genitalium) are 0.2–0.3 µm, while the largest (Epulopiscium fishelsoni) can reach 600 µm, visible to the naked eye. Size affects nutrient uptake (surface area to volume ratio), diffusion rates, and susceptibility to predation.
Gram Stain and Cell Wall Classification
Gram-positive bacteria have a thick peptidoglycan layer, stain purple, and include Staphylococcus, Streptococcus, Bacillus, Clostridium, and Lactobacillus. Gram-negative bacteria have a thin peptidoglycan layer with an outer membrane, stain pink or red, and include Escherichia, Pseudomonas, Salmonella, Neisseria, and Helicobacter. Acid-fast bacteria such as Mycobacterium and Nocardia have waxy mycolic acid cell walls, stain with carbolfuchsin, and resist decolorization with acid-alcohol.
Specialized Staining Techniques
The capsule stain uses India ink negative staining to visualize polysaccharide capsules surrounding bacterial cells (e.g., Klebsiella pneumoniae, Streptococcus pneumoniae). The flagella stain uses a mordant to thicken flagella, making them visible under light microscopy for motility assessment. The spore stain (Schaeffer-Fulton method) uses malachite green to stain endospores green and safranin to counterstain vegetative cells pink. The acid-fast stain (Ziehl-Neelsen) forces carbolfuchsin into the cell wall by heat, after which cells resist decolorization with 3% HCl in ethanol.
Colony Morphology
Colony morphology on solid agar encompasses size, shape (circular, irregular, rhizoid), margin (entire, undulate, filiform, curled), elevation (flat, raised, convex, umbonate), surface texture (smooth, rough, mucoid), color, opacity, and odor. Hemolysis on blood agar is categorized as α-hemolysis (green discoloration, partial hemolysis), β-hemolysis (clear zone, complete hemolysis), or γ-hemolysis (no hemolysis). Colony morphology combined with Gram stain is the first step in the identification algorithm for clinical isolates.
Biochemical Identification
The catalase test differentiates Staphylococcus (positive) from Streptococcus (negative). The oxidase test differentiates Neisseria and Pseudomonas (positive) from Enterobacteriaceae (negative). The coagulase test distinguishes Staphylococcus aureus (positive) from other staphylococci. API strips and automated systems such as VITEK and MALDI-TOF MS provide rapid, comprehensive biochemical identification.
