Arthrobacter

Arthrobacter is a common genus of soil bacteria. All species in this genus are Gram-positive, obligate aerobes. Like many soil bacteria, this genus is metabolically versatile, producing many different enzymes allowing it to grow on a wide range of substrates. The cells can be quite tough and resist desiccation and starvation. Arthrobacter species have been isolated a few times from patients with immunodeficiencies but most strains do not appear to be pathogenic.

A distinctive feature of this genus is that the shape of the cells changes during the growth cycle, typically forming rods during exponential growth and cocci in the stationary phase:

Arthrobacter sp., phase contrast microscopy, cells ~1-2 µm diameter, variable length:

Here is the same specimen viewed using darkfield microscopy, a techniques which makes details of cell morphology more easily visible:

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