These microorganisms are ubiquitous unicellular and can induce corrosion by a variety of mechanisms. In basic, and simplified classification, these microorganisms are two types:

• Aerobic (thrive under oxygenated conditions)
• Anaerobic (thrive under de-aerated conditions)

For example, potentially corrosive species produced by various types of bacteria include sulfuric acid, organic acids, enzymes and sulfur containing compounds. Given the right conditions, bacteria populations can undergo explosive growth, as can be illustrated in a simplistic growth model.

The sulfate reducing bacteria (SRB) are particularly well known for inducing corrosion damage on ferrous metals, including stainless steels. These anaerobic bacteria reduce sulfate to sulfide as part of their metabolism.

Corrosion of concrete sewer pipelines due to Bacteria
The degradation of concrete sewer pipelines, are caused by Bacteria (shown schematically in the diagram below).

The acidic conditions produced on the concrete sewer walls by relatively complex microbial interactions. Heresulfate reducing bacteria and sulfur oxidizing bacteria, both  attacked the concrete.

References
MTI Publication No.13, "Microbiologically Influenced Corrosion: A State-of-the-Art Review", Second Edition, 1989.
From- http://www.corrosion-club.com/