• Method adopted/last revised: 2019
• Method reapproved: 2020
• REF/ISBN: IP385-7251830
• Status: Current
• First printed in STM books: February 2013

This standard describes two procedures, A and B, for the determination of the viable aerobic microbial content of middle distillate fuels and
associated waters. Viable aerobic bacteria and viable aerobic fungi (yeasts and moulds) are evaluated by this method.

Procedure A should be used for the enumeration of viable aerobic microbial units in middle distillate fuels and is suitable for enumeration of viable aerobic mould colony forming units up to 50,000 per litre, and viable aerobic yeast and bacterial colony forming units up to 100,000 per litre separately.

Procedure B should be used for the enumeration of viable aerobic microbial units in fuel associated water and is suitable for enumeration of viable aerobic mould colony forming units up to 5 × 108 per millilitre, and viable aerobic yeast and bacterial colony forming units up to 1 × 109 per millilitre separately.

The procedures employ two selective microbiological growth media which nominally allow separate enumeration of bacteria and fungi.
But as the media are not exclusively selective, it is possible that the determination of viable microbial units from the medium which favours bacterial growth will also include some fungi and vice versa. The microbiological procedures for distinguishing bacterial growth and for distinguishing between yeasts and moulds within the fungal content are not within the scope of this standard.