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4. Microbes Shapes and Sizes

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Let's see how bacteria and parasites look. They are both single-cell organisms that come in a variety of shapes and sizes. Both types of microorganisms are very adaptable and can live in almost all environments, partially due to the protective features that help them to survive and thrive.

A typical bacterial cell has a protective wall surrounding it. Some have flagella which act as propellers to let the bacteria swim to find food and some are covered in dozens of tiny hairs called pili to help them stick to surfaces.

Bacteria are classified into five groups according to their basic shapes and they can exist as single cells, in pairs, chains or clusters.

  • round (coccus)
  • rod (bacillus)
  • comma (vibrio)
  • spiral (spirillum)
  • corkscrew (spirochaetes)

The cartoon below shows some of the common shapes.

Common Bacterial Shapes
Common bacterial shapes (Credit to Janice Haney Carr, Center for Disease Control and Prevention and Oregon State University)

Similarly, some parasites also have ability to move and propel forward. For example, amoeba organisms (Eukaryotic) can change their shape by extending little feet-like projections used for feeding (see image).

Scales and Sizes

As some microbes are very small, we use microscopes to look at them. The smallest object the human eye can see without magnification is about 100 µm (µm stands for micrometer) in diameter. For reference, a printed full stop in a newspaper is about 300 µm in diameter.

Units of length we commonly use in biology span a wide range of scale:

Metric unitMeaning of measurements
Centimetre (cm)1/100 m or 0.01 m
Millimetre (mm)1/1000 m or 0.001 m
Micrometre (µm)1/1,000,000 m or 0.000001 m
Nanometre (nm)1/1,000,000,000 m or 0.000000001 m

Activity: Now try our Scales and Sizes worksheet.
An Amoeba Reaching Out of Food
An Amoeba Reaching Out of Food
Scales and Sizes Worksheet
Scales and Sizes (PDF) and Answers

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