Are Culture Methods still reliable for monitoring Microbial Contamination in Fuel and Oil?

Microbiology has made rapid advances over the past few decades and modern Molecular Microbiological Methods (MMM), such as qPCR and Fluorescent In-Situ Hybridisation (FISH), have made us realise that microbial communities are far more complex than we originally thought.

This is as true of fuel and oil systems as it is of any other environment where microbes flourish.  An often quoted deficiency of traditional culture methods is that they only recover a very small percentage of the microorganisms which may be present in any given sample.  However, a point that is often missed is that the percentage recovery usually refers to the diversity of species present and not to the total number of cells, nor amount of microbial biomass nor level of microbial activity present.

It may well be that molecular techniques will pick up the presence of, say, 50 or more microbial species, many of which are not detected by culture assessment.  But much of the microbial DNA recovered by molecular methods will be from microbes which are inactive and/or present simply as incidental contaminants; these microbes will have no bearing on fuel system operations, corrosion or fitness for use of the fuel or oil.  An understanding of the full diversity of microbes present may be of academic interest to a researcher of microbial ecology but not necessarily of interest to fuel quality and operations managers.

In ECHA’s many years of experience of examining fuel and oil samples, direct observation, microscopy and culturing of deposit samples will highlight that the majority of biomass fouling fuel tanks and filters will be due to a relatively few fungal species.  These species are readily cultured by standard laboratory methods (e.g. IP 385 and ASTM D6974) and by simple diagnostic field tests such as MicrobMonitor2 (IP 613 / ASTM D7978).  Ironically, injudicious use of molecular techniques has in the past completely overlooked the importance of fungi and instead focused on diversity of bacterial species present.  In certain cases, of course, bacteria may be of importance and in some cases standard culture methods may overlook some bacterial contaminants.  However, in the vast majority of fuel and oil contamination cases we see, the bulk of bacterial and fungal contaminants (i.e. those actually causing the problem) are detected by culture methods and, most importantly, the level of contamination detected reflects the severity of the problem.

Given the high levels of expertise and cost required to conduct molecular microbiological methods, and given the advantages of relative ease of use and simplicity of culture methods, the latter will have a very important role to play in field monitoring and investigation of microbial contamination in fuel and oil systems for many years to come.  It is notable that culture methods still remain the “gold standard” technique for microbiological testing in applications where microbial contamination is of critical importance such as quality testing of potable water quality, pharmaceuticals and cosmetics.   For all the wonderful insights Molecular Microbiological Methods bring, there has, in our view, been a tendency in some quarters to “throw the baby out with the bath water” with regards to culture methods.

As with any test method, the user should be aware of its limitations and, in this respect, culture methods should be seen as indicators of fuel/oil tank microbiological status.  And in our view, they remain the most practical and reliable monitoring tool available.

For more information on our research of molecular and culture techniques for testing of fuel, conducted in conjunction with Cardiff University, see Judith White et al, Culture-Independent Analysis of Bacterial Fuel Contamination Provides Insight into the Level of Concordance with the Standard Industry Practice of Aerobic Cultivation, APPLIED AND ENVIRONMENTAL MICROBIOLOGY, July 2011, Vol. 77, No. 13, p. 4527–4538.