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Diesel Bug Explained

Diesel Bug (Bacteria Contamination of Diesel)

Bacteria in diesel is a well known problem to anyone who works with diesel engines, so what is this bug and why does it contaminate diesel?
Diesel is an organic fuel so it provides an ideal environment for microscopic fungi, yeast and bacteria to feed and grow.

Enlargement - Diesel Testing KitNew Sample Kit - Diesel

Order your diesel fuel sampling from any Goughs branch parts department (dial 0800 93 39 39).

This environment provides:

  • dissolved water for germination
  • carbon for food
  • oxygen and sulphur for respiration
  • trace elements for growth and propagation.

As many as twenty seven (27) varieties of bacteria are responsible for the majority of problems with diesel engines and their performance. There are many differing types of bacteria which can infect systems and form bio-films on steel surfaces. Accelerated corrosion can also occur wherever the bio-film settles, usually in pits or crevices. Unlike general corrosion, it is an attack on a very specific area.

It is very difficult to determine when a system is first contaminated, but once contaminated diesel enters the fuel system, it is very difficult to eradicate.

Diesel bug can originate from the air or moisture, or during tank filling and/or expansion and contraction of storage tanks, the bacteria cover themselves in a protective film (slime) to protect against biocides and can lie dormant in the minute crevices of the metal, rubber and polyurethane coatings of the fuel tanks and fuel systems.

Then, when water is present (a droplet is a lake to a microbe) and the environment hits the right temperature range, they begin reproduction in the area of fuel/water interface.

Microscopic in size, they can develop into a mat easily visible to the naked eye very rapidly. A single cell, weighing only one millionth of a gram can grow to a biomass of 10 kilograms in just 12 hours, resulting in a biomass several centimetres thick across the fuel/water interface.

Each species has its own characteristics:

Bacteria utilise hydrocarbons and reproduce asexually by binary fission; swelling in size as they feed, they then separate into two cells. In this way, microbes double their numbers every 20 minutes, one spore converting to 262,144 in 6 hours.

SRB's are a specific group of bacteria utilising simple carbon, not hydrocarbons, and require the activity of other microbes in a consortium. Aerobic (in the presence of oxygen) or anaerobic (without oxygen) bacteria have a combined effect. The aerobic bacteria (sulphate oxidising) create a film to consume the oxygen first. This allows the anaerobic (sulphate reducing) bacteria to thrive.

SRB's reduce sulphates and produce hydrogen sulphide (a lethal gas). They are directly involved with many microbial corrosion reactions and can cause sulphide souring of stored distillate products. Their action changes the Ph creating an acidic environment, conducive to accelerated corrosion. They attach themselves to the steel as a film and go to work. They derive their nutrition from the surrounding environment and multiply. They are particularly difficult to deal with and produce a sludgy by-product with a strong sulphur odour similar to rotten eggs (hydrogen sulphide).

These also contribute to corrosion, eating steel and reducing ferrite to an oxide through a chemical reaction.

Yeasts prefer acidic environments, such as produced by SRB's. They bud on the parent cell, eventually separating. Reproduction takes several hours.

Fungi grow in the form of branched hyphae, a few microns in diameter, forming thick, tough, intertwined mycelia mats at fuel/water interfaces.

All of these can and do cause damage to the fuel system.

Maintain the fuel system by draining water very regularly, keep the tank as full as possible, (especially over-night) and try to ensure your supplier maintains his system well.

Clean the entire system with a cleaning agent available from or recommend by your diesel supplier.

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