These ubiquitous unicellular microorganisms can induce corrosion by a variety of mechanisms. A basic, simplified classification is into the aerobic (thrive under oxygenated conditions) and anaerobic types (thrive under de-aerated conditions). 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.

References:

MTI Publication No.13, "Microbiologically Influenced Corrosion: A State-of-the-Art Review", Second Edition, 1989.

Links:

Link to Scientific American article

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