Car parts manufacturer Eaton Ltd did not correctly manage water cooling systems at its plant in Thorns Road, Brierley Hill, West Midlands, leaving workers and members of the public at potential risk of exposure.
Water systems can be breeding grounds for bacteria such as Legionella and some strains of E.coli if they are not maintained properly.
The multinational, whose headquarters are in Fareham, Hampshire, was fined £80,000 and told to pay costs of £45,000 after admitting breaching Section 2(1) of the Health and Safety at Work etc Act 1974 and Section 3(1) of the Act.
Aegis Ltd, of Felspar Road, Amington Industrial Estates, Tamworth, Staffordshire, had been contracted in to offer water treatment services, and was also found in breach of regulations.
The company, which now trades as Aegis Water Treatment Ltd, was convicted of breaching Section 3(1) of the Act at a trial in May and must pay a fine of £40,000 and £80,000 costs.
The prosecution at Wolverhampton Crown Court was brought by the Health and Safety Executive after an investigation in 2006.