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CSIM2.2: BACTERIAL SHAPES, SIZES AND STRUCTURES
20/01/09
BASIC CLASSIFICATION OF ORGANISMS
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An organism is classified as any living thing, which includes both plants and animals
All organisms are made up of one or more cells
Organisms can be classified according to a fundamental structural aspect of their cell or cells:
o Eukaryotes are organisms whose cells contain nuclei
o Prokaryotes are those whose cells do not contain nuclei
The prokaryotes are further divided into two domains:
o Archaebacteria – primitive, often found in extreme environments, non-pathogenic
o Eubacteria – essentially what are generally referred to as ‘bacteria’
LEARNING OUTCOMES
Describe the major shapes and cellular aggregation patterns of bacteria
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Different types of bacteria can be identified through the properties of shape and aggregation pattern
Common bacterial shapes include:
o Coccus – spherical
o Rod
 Bacillus – straight rods, may be single or in linear chains
 Vibrios – curved rods
o Spirillum – rigid helices
o Spirochete – flexible helices
o Filamentous – long thin threads
o Budding – with a stalk-like appendage
Various prefixes to the cocci define their aggregation patterns:
o Diplococcus – pairs
o Streptococcus – linear chains
o Staphylococcus – grape-like clusters
o Tetrads – groups of four in a square arrangement
o Sarcinae – groups of eight in a cubic arrangement
The two most medically significant types of bacteria are:
o Gram-positive – stain purple due to components of the cell wall retaining dye
o Gram-negative – stain pink after decolourisation due to inability to retain dye