Bacterial Morphology

30 August, 2025

Bacterial Morphology

Introduction

Bacterial morphology refers to the shape, size, and structural arrangement of bacteria. Understanding morphology is crucial in microbiology because it helps in identification, classification, pathogenesis study, and diagnosis of bacterial infections.

Size of Bacteria

Average size: 0.5 – 5.0 µm
Smallest: Mycoplasma (~0.2 µm, filterable)
Largest: Epulopiscium fishelsoni (~200–700 µm long)

Basic Shapes of Bacteria

Cocci (spherical/oval)
Arrangements:
Diplococci – in pairs (e.g., Neisseria gonorrhoeae)
Streptococci – in chains (e.g., Streptococcus pyogenes)
Staphylococci – in grape-like clusters (e.g., Staphylococcus aureus)
Tetrads – groups of four (e.g., Micrococcus)
Sarcinae – cubic arrangement of eight (e.g., Sarcina)
Bacilli (rod-shaped)

Arrangements of Bacteria

Single rods – e.g., Escherichia coli
Diplobacilli – e.g., Klebsiella pneumoniae
Streptobacilli – in chains, e.g., Streptobacillus moniliformis
Coccobacilli – short oval rods, e.g., Haemophilus influenzae
Palisades / Chinese letter arrangement – e.g., Corynebacterium diphtheriae
Spiral Forms
Vibrio – comma-shaped (e.g., Vibrio cholerae)
Spirilla – rigid spiral with external flagella (e.g., Spirillum minus)
Spirochetes – flexible, corkscrew-shaped with axial filaments (e.g., Treponema pallidum)

Other Morphological Types

Filamentous bacteria – long thread-like forms (Actinomyces, Nocardia)
Pleomorphic bacteria – variable shape (Mycoplasma, Corynebacterium)
Square bacteria – unusual flat, rectangular forms (Haloarcula, rare)

Bacterial Structures Related to Morphology

Cell Wall
Determines shape and rigidity.
Gram-positive → thick peptidoglycan.
Gram-negative → thin peptidoglycan + outer membrane.
Capsule & Slime Layer
Surrounds cell wall, affects colony morphology.
Flagella
Provides motility, classified as monotrichous, lophotrichous, amphitrichous, or peritrichous.
Fimbriae & Pili
Influence bacterial adherence and arrangement.

Importance of Morphology in Medicine

Identification & Classification – first step in microbiology lab.
Pathogenesis – morphology linked to virulence (e.g., capsule in Klebsiella).
Diagnostic Clues – arrangement helps rapid microscopic diagnosis (e.g., Streptococcus vs. Staphylococcus).
Antibiotic Target – cell wall differences determine antibiotic sensitivity.

Conclusion

Bacterial morphology is a fundamental tool in clinical microbiology, providing essential information for the identification, diagnosis, and treatment of bacterial infections. Understanding bacterial shape, size, and arrangement is a foundation for medical students and clinicians.

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