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:

Microbes and Man
by J.R. Postgate.
A classic introduction to the fascinating world of microbes, now been extensively updated to include such topical issues as BSE, food poisoning, genetic engineering and gene therapy. It deals with the ways in which microbes affect peoples' lives in everyday matters, such as in disease, health, hygiene and nutrition, and also discusses their part in socio-economic areas such as agriculture, biotechnology, genetic engineering and the stability of the environment. This popular account will appeal to students of microbiology as well as to the general reader.
(Amazon.co.UK)

Microbiology
by L.M.Prescott et al.
A balanced, comprehensive introduction to all major areas of microbiology. The sixth edition has been updated extensively to reflect the latest discoveries in the field.
(Amazon.co.UK)