FR2880035A1 - Method of detecting biocontamination in a liquid fuel, comprises detecting micro-organisms in the liquid fuel by using an adenosinetriphosphate-metry technique - Google Patents

Abstract

Method of detecting biocontamination in a liquid fuel, comprises detecting microorganisms in the liquid fuel by using an adenosinetriphosphate (ATP)-metry technique.

Description

METHOD FOR DETECTION OF BIOCONTAMINATION OF A

LIQUID FUEL

DESCRIPTION

TECHNICAL AREA

  The present invention relates to a method for detecting the biocontamination of a liquid fuel.

  It applies to all liquid fuels, including domestic fuels such as fuel oil, and fuels, such as gas oil, automotive gasoline, and fuels for aircraft and missiles.

  The invention is particularly applicable to the detection of the biocontamination of jet fuels and more particularly kerosene, and also exo-THDCPD (exotetrahydrocyclopentadiene).

  STATE OF THE PRIOR ART It is known that all jet fuel is contaminated with microorganisms. The spores of the latter generate a viscous "bioglea" which can lead to harmful effects such as: - the increase in the consumption of jet fuel, the reduction of its energy performance, - the clogging of the circuit filters. feeding jet fuel and blocking gauges, which can lead to misinformation about the amount of fuel in the tanks, and - deterioration of the protective coatings of jet fuel tank bottoms and attack of structural tanks made of light alloys by intercrystalline corrosion.

  The control of the biocontamination of aircraft tanks makes it possible to define preventive or curative treatments that should be applied during the maintenance of these aircraft.

  Currently, the detection of biocontamination of jet fuels is carried out by means of a reference method, which is an experimental bioanalytical laboratory method.

  This reference method consists in isolating and culturing strains of microorganisms, which are present in a jet fuel such as kerosene, on the surface of a microporous and sterile filter membrane, screen type.

  This process requires investigative means such as sterilization, filtration, incubation and microscopic observation devices, which are found in laboratories but almost not with the airlines or control bodies of the latter. .

  In addition, the implementation of this reference method requires experimenters capable of performing microbiological analyzes and microscopic observations of colonies of strains of microorganisms. In addition, this process requires, at least, 48 to 72 hours of incubation, to obtain the first results of analysis, and two to three weeks of incubation to obtain the final results.

  Certainly, this process can be simplified by using the filtration system which is commercially available from the Millipore Company under the reference Milliflex-100. Nevertheless, this method remains unsuitable for use in the immediate vicinity of an aircraft and requires the intervention of people with skills in microbiology.

  SUMMARY OF THE INVENTION The subject of the present invention is an inexpensive method that can be implemented in the immediate vicinity of an aircraft or a storage tank and which enables the rapid detection of the presence of microorganisms in the environment. a jet fuel.

  Note that only the detection of this presence is important and that it is not necessary to identify microorganisms.

  The method which is the subject of the invention then makes it possible not only to regularly monitor the degree of contamination of the jet fuels but also to perform early interventions on the tanks of such fuels, to carry out preventive or curative treatments, by adding biocidal products ( fungicides and bactericides) in these reservoirs.

  More generally, the invention solves the problem of detecting, in a fast and inexpensive manner, the presence of microorganisms in a liquid fuel.

  To solve this problem, the invention uses the technique of ATPmetry and bioluminescence (in English bioluminescent ATPmetry), more simply called ATPmetry or assay of ATP, which will be discussed later.

  The inventor has found that ATPmetry is compatible with a liquid fuel such as a jet fuel. By using commercially available ATPmetry systems, he has indeed found that an enzyme, luciferase, was not annihilated by a jet fuel such as Jet A-1 kerosene.

  The ATPmetry systems marketed by the Prodemat, Merck and preferably Euralam companies are interesting for the implementation of the invention.

  As will be seen better later, the invention is simple to implement, provides a rapid response (usually in less than 15 minutes) and can detect the main microorganisms capable of living in a liquid fuel such as a jet fuel , for example a kerosene, and exo-THDCPD, also called JP-10.

  In addition, the invention is directly applicable where the fuel tank is (for example in the immediate vicinity of an aircraft whose kerosene is to be controlled) and does not require the intervention of persons with microbiological skills.

  The invention also makes it possible to overcome a drawback which is presented by known ATPmetry techniques.

  Indeed, these techniques use a sampling pen provided with a swab and, to carry out a direct analysis of a fuel, such as kerosene, by ATPmetry in accordance with the invention, one is led to immerse this swab in a small volume of kerosene, generally of the order of 1 cm3 or less, while the microorganisms are only present in the aqueous phase of decantation of kerosene, phase which can represent only a very small percentage of the total volume of fuel.

  The present invention overcomes this disadvantage by increasing the volume of fuel analyzed.

  Preferably, a volume greater than 50 cm 3, for example 100 cm 3, 0.5 dm 3 or even 1 dm 3, is then used.

  In addition, the strains of microorganisms present in the kerosene are then isolated by filtration on the surface of a sterile and microporous membrane, and an ATPmetry technique is used after having brushed the retention surface of the membrane. sterile membrane after filtration, using the swab mentioned above or any other method that allows the recovery of microorganisms on the microporous membrane.

  The results obtained are comparable to the colony counts and their invasion, which is obtained by implementing the microbiological reference method by filtration-incubation on a Petri dish membrane.

  Thus, according to an advantageous aspect of the present invention, an ATPmetry and bioluminescence technique is proposed, optimized by introducing a microfiltration of the fuel before the ATPmetry itself.

  Specifically, the present invention relates to a method for detecting the biocontamination of a liquid fuel, this method being characterized in that an ATPmetry technique is used to detect the presence of microorganisms in the liquid fuel.

  According to a particular mode of implementation of the method which is the subject of the invention - ATP metering equipment is used - a volume of liquid fuel is filtered on a sterile microporous membrane, so as to isolate strains of microorganisms which are susceptible to to be present in the liquid fuel, the strains of microorganisms thus isolated are collected, and the presence of the microorganisms in the strains thus collected is detected by means of the ATPmetry equipment in order to determine the ATP.

  According to a particular embodiment of the invention, the equipment comprises a sampling member of a sample to be analyzed with this equipment and the strains are collected by means of the sampling member.

  Preferably, the volume of liquid fuel that is filtered is approximately equal to 100 cm 3.

  According to a preferred embodiment of the invention, the sampling member constitutes a brush by means of which the microporous membrane retention surface is brushed to collect the microorganism strains.

  Preferably, the retention surface of the microporous membrane is brushed along two perpendicular directions.

  Preferably, in the case of the embodiment of the method which is the subject of the invention and which uses the microporous membrane: first, a white light intensity corresponding to the sampling member alone is measured by means of the equipment; Before collecting the microorganism strains by means of this sampling organ, at least one measurement of luminous intensity d) is made using the equipment, for the collected strains, the difference is calculated. white and the contamination is determined from this difference.

  According to a preferred embodiment of the invention: - several light intensity measurements are made for the strains, - the average arithmetic average of these measurements is calculated, - the difference is calculated (Dre,, d) white and - one determines the contamination from this difference (Drei (Dblanced) The ATPmetry equipment preferably comprises: - a swab which constitutes the sampling organ, - a container which contains components allowing the implementation of the technique of ATPometry and in which the swab is immersed after collecting the strains of microorganisms by means of this swab, so that these strains react with the components, and - a luminometer for measuring the luminous intensity emitted by the strains having thus reacted.

  According to a particular application of the present invention, the liquid fuel is contained in a reservoir and a preventive treatment of this reservoir is carried out if the ATP metering technique used leads to measuring a luminous intensity greater than or substantially equal to a previously fixed threshold. depending on the intended application. This threshold is preferably expressed in relative units of light intensity.

  DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS The following shows the advantage of using ATPmetry to detect the biocontamination of a liquid fuel, such as a jet fuel, and the advantage of optimizing ATP metering by pre-microfiltration.

  Recall that the technique of ATPmetria is an instantaneous technique of ATP dosing, that is to say, adenosine triphosphate (English adenosine triphosphate).

  This technique, which is based on the principle of bioluminescence, implements an enzymatic reaction that translates a quantity of ATP into a quantity of light.

  ATP is present in varying concentrations in all living organisms. The present ATP indicates a staining by organic compounds. Wherever it is present, there is the risk of bioburden. In addition, the ATP molecule is used by cells for storage of cellular energy and as a co-enzyme.

  The presence of ATP in an inert medium, such as a jet fuel, indicates a biological contamination of microbial and / or fungal origin.

  To carry out ATPmetry, a reagent is added which induces luminescence (in English luminescence inducing reagent) to the reaction medium to be analyzed. This reagent includes an enzyme, luciferase, and its substrate, luciferin, a nitrogenous substance that is present in some animals and whose oxidation causes luminescence.

  In the presence of ATP, luciferin is immediately converted to oxyluciferin by luciferase. The reaction is accompanied by light emission, of a greenish yellow color, which is proportional to the amount of initial ATP.

  The procedure of the ATPmetry technique consists in adding a known quantity of luciferin and luciferase to a known volume of the medium to be analyzed, allowing to act for a few seconds (incubation) and then measuring the intensity of the light emitted at medium of a luminometer.

  The evaluation of microbial biomass, by assaying ATP, tends to become widespread. This constituent is an important intermediary of cellular metabolism, since it is at the center of energy metabolism. The ATP content of the cells varies from one organism to another. For the same organism, this variation is a function of its physiological state, hence the interest of its dosage, especially if one works with a single type of cell.

  Since ATP is also absent from the constituents of the culture medium, its determination by bioluminescence reaction is relatively simple, rapid and sensitive.

  Once the cell is dead, ATP is quickly destroyed. This is why the level of ATP in a sample can be related to the population of cells, or living bacteria, that are present.

  ATPmetry seems well suited for the search for microorganisms, without identification, in jet fuels, because it gives results in a few minutes. It can be used "at the foot of an airplane", by a staff with no particular qualification in microbiology.

  Initially, it was ensured that the luciferase was not inhibited by the aerobic endothermic fuel compounds, mainly by the Jet A-1 kerosene compounds and the JP-10 fuel, and that the ATPmetry was applicable to such reaction media.

  In addition, among commercially available ATP kits, the one best suited to aircraft control, practiced in the field, namely the Euralam kit, was selected.

  This ATP kit from Euralam is called Pocket Swab. It comprises a pen-shaped device, comprising a sampling swab (small brush) and a reaction vessel, which contains the lysis compounds (destruction of tissues or bacteria by physical, chemical or biological agents), a solution buffer (in English buffer solution), luciferin and luciferase.

  The swab is used for sampling and then dipped into the reagent tank. After a few seconds of incubation, this incubation being based on the enzymatic reaction using the luciferin / luciferase light emitting pair, the emission of photons is quantitatively detected by means of a luminometer. One uses the one marketed by Charm Sciences under the reference Luminator T (registered trademark).

  The latter is able to statistically process the results, store them, display them on their screen and then transfer them later to a printer or a computer, which allows the on-site testing and restitution of the data, to another moment and in another place, either a printer or a processing software.

  It is specified that the content of ATP is expressed in relative units of light intensity (English Relative Light Units.

  The brighter the light, the higher the measured values and the higher the ATP content. High RLU values clearly indicate microbiological contamination.

  ATPmetry has never been implemented with kerosene. It must therefore be ensured that luciferase is not inhibited by one of the compounds of these fuels.

  The study of the compatibility of ATPmetry with kerosenes was carried out with Euralam's ATP kit on Jet A-1 kerosene.

  The ability of this ATP kit to detect, one by one, the presence of five mold strains, namely Aspergillus piger, Paecilomyces varioti, Aureobasidium pullulans, Chaetomium globosum and Cladosporium resinae, and an aerobic bacterium, namely Pseudomonas aeruginosa, among those most frequently found in kerosene.

  However, it must be remembered that the weak point of the method is that the direct analysis is carried out by immersing the swab in a small volume of fuel, about 1 ml or less, while the microorganisms are present only in the aqueous phase. decantation of kerosene which may represent only a very small percentage of the total fuel volume.

  It is therefore desirable to optimize the method by increasing the volume of fuel analyzed. A volume at least equal to 100 cm 3 is preferably used.

  It has been found that it is advantageous to optimize the ATPmetry by prior microfiltration, by comparison with the reference method, by microfiltration-incubation in a petri dish. We will return to this reference method at the end of the present description.

  An example of the method optimized by filtration and brushing is given later.

  The results (bioluminescent intensity values) obtained by this method are directly comparable, from 100ml microfiltered, to colony counts and their invasion, expressed in% SRF (growth percentage at the Filter Retention Surface), exploited during the implementation of the microbiological reference method by microfiltration and incubation of the membrane Petri dish, which refers to 100 cm3 of filtered fuel.

  In order to verify the effectiveness of a bactericidal and fungicidal preventive treatment of a biocontaminated kerosene, the reaction of ATPmetry and bioluminescence against dead organisms has been identified. For this, a sample of kerosene was used which was inoculated with the five strains of mold and the aerobic bacterium studied. Biocontaminated kerosene was treated with a deadly solution of microorganisms, namely sodium hypochlorite (bleach).

  The reaction was identified using the Euralam kit, using the optimized microfiltration and brushing method. Originally, the inoculated kerosene gives an intensity of 344144 RLU. Immediately after adding 500 ml of bleach, the bioluminescent intensity noted is 4536 RLU. Then, depending on the time of death from 1 hour to 24 hours, the bioluminescent intensity has an asymptotic monotone decrease up to 1151 RLU.

  Knowing that the bioluminescent intensity of bleach is of the order of 4600 RLU, it can be considered that ATPmetry is not influenced by the presence of dead organisms and that it reacts only vis-à- live organisms.

  In addition, the possibility of implementing ATP metering and bioluminescence at the foot of an aircraft, at an operating site or in a holding zone under operational conditions has been shown.

  Specifically, this technique has been validated as a method for controlling biocontamination of jet fuels in the field when identification of microorganisms is not required.

  For any aircraft and any storage tank that has been studied, the following protocol has been implemented with the Pocket Swab technique of Euralam.

  At the foot of the aircraft, using a sampling tool connected to the purge or kerosene sampling point, the fuel is sampled with the aqueous phase of decantation.

  The volumes collected are at least cm3. They are packaged in sterile vials.

  The fuel samples are stirred manually in order to homogenize the dispersion of the colonies of microorganisms and their distribution during the fractionation of the kerosene test samples.

  The on-site ATPmétrie tests are then carried out immediately, with 100 cc samples of fuel, microfiltered at 0.45 μm absolute retention, by the Milliflex-100 system, in order to brush the retention surface. the sterile membrane using the swab of the sampling pen.

  The remaining sample volume is used to carry out the microbiological reference tests by membrane microfiltration and incubation in petri dishes in contact with a global nutrient medium MTGE. These tests make it possible, in addition to comparing the values obtained with the reference biocontamination levels, to validate the use of the biocontamination control technique "at the foot of the aircraft".

  On the other hand, all the comparative experimental results obtained are also used to define the preventive and curative treatments of reservoirs (see below).

  To validate the ATPMetry method, we chose to quantify: - the biocontamination of jet A-1 kerosene stored for about 4 months in the structural tanks of three aircraft, and - the biocontamination of Jet A-1 kerosene stored in two tanks of 20000 liters each.

  The rapid ATPmetry and bioluminescence method was used and the results were compared to the reference method by microfiltration and membrane incubation.

  For each aircraft, the fuel was sampled at the purge points of the three central, right and left structural tanks, according to the rules of the art.

  For each tank, half a liter of jet fuel was withdrawn, conditioned and fractionated in two flasks each having a volume of 250 cm3. One of the flasks was used for on-site microbiological expertise. The other vial was used for further investigations and reference tests by microfiltration and membrane incubation.

  On some 250 cm3 samples taken from airplanes, the optimized ATPmetric method was performed in which, using the brush of the sampling pen swab, the sterile membrane retention surface used to microfilter 100 cm3 Jet A-1 kerosene.

  The experimental achievements are specified below and are an example of the method optimized by filtration and brushing.

  - Before each test, the white (zero) of the sampling pen is determined: the luminescent intensity of the swab of this pen is identified before brushing the filter. The virgin swab is introduced into the luminometer and the white light intensity is read (in empirical value RLU). Three measurements are performed in chronological order.

  On a sterile microporous membrane, of screen type, of at least 0.45 μm absolute retention, 100 cc of Jet A-1 kerosene is microfiltered. For this operation, a separator and a Milliflex unit (sterile filtration unit) and the corresponding Milliflex-100 vacuum pump are used.

  It should be noted, however, that other commercially available micro-filtration systems could also be used.

  The analysis is conducted as follows.

  - A separator and a Milliflex unit are placed on the support of the Milliflex-100 vacuum pump.

  100 ml of kerosene sample are poured into the funnel and the pump is evacuated to filter the sample.

  - Then, two successive rinsing of the filter vessel and the filter are carried out using the following sterile solutions: 1) 100 cm3 of Triton X-100 diluted to 0.1% by volume in deionized water, and 2) 30 cm3 of physiological saline containing 0.85% of NaCl diluted in deionized water.

  - With the swab of the sampling pen, carefully brush the entire retention surface of the filter membrane in two perpendicular directions that follow those of the printed squares on the membrane. After brushing, the membrane is discarded. In no case, by this method, the membrane can be used to be incubated in contact with any nutrient medium.

  - The swab is put in contact with the luciferin and luciferase reagents.

  After a few seconds of incubation, the swab is introduced into the luminometer and the relative bioluminescent intensity reading is read (in empirical value RLU). Three measurements are performed in chronological order.

  The value of the relative bioluminescent intensity (Dre / is defined by the arithmetic mean of these three measurements: = 3 (01 + (I) 2 + (133). The value of the average absolute bioluminescent intensity is determined. ,,, which is equal to the average relative bioluminescent intensity minus the value of the luminous intensity corresponding to the virgin pen (white), before brushing the retention surface of the filtering membrane: nhs (1) rel hlanc In addition for each 250 cm3 sample taken from airplanes, microbiological analyzes are carried out using the reference method by microfiltration and membrane incubation.

  To do this, on a sterile microporous membrane, screen type, 0.45} g of absolute retention, 100 cm3 of Jet A-1 kerosene was microfiltered. The membrane is rinsed with 100 cm3 of Triton X-100 solution and then with 30 cm3 of physiological saline. The membrane is then incubated, in a petri dish, in contact with a swab soaked in a MTG-E nutrient medium.

  This method makes it possible to interpret the results of bioluminescence expressed in RLU, causing the establishment of a correlation with the enumeration of strains and their degree of invasion (expressed in SRF%).

  The levels of biocontamination obtained from the microbiological reference method by microfiltration on a membrane and incubation in a Petri dish in contact with a global nutrient medium MTG-E make it possible to define preventive and curative treatments of the structural reservoirs of the aircraft, or reservoirs. similar or identical, depending on the different biocontamination levels exploited by ATPmetry and bioluminescence.

  In practice, if a detection by ATPmetry, preferably by means of a kit from Euralam, and by previously using a filtration and brushing, gives values greater than about 500 RLU, this detection should trigger the usual preventive treatments . Curative treatments are to be studied on a case by case basis from the moment when the preventive treatments are no longer effective.

  In conclusion, the ATPmetry and bioluminescence technique is perfectly compatible with Jet A-1 kerosene. It is very interesting because it is simple to implement. It provides a response very quickly, in less than 15 minutes, and can detect the main viable organisms in jet fuels. In addition, it is directly applicable in the field and does not require the intervention of personnel with special skills in microbiology.

  In the following, reference is made to the so-called reference process by microfiltration and incubation.

  In the case of this process, the microorganisms which are present in the jet fuel, essentially in the kerosenes, are isolated and cultured on the retention surface of a filter membrane, sterile, microporous, screen type.

  The analysis is conducted as follows.

  A solution of Triton X-100, available from the company Rohm and Haas, diluted to 0.1% by volume in deionized water, and a solution of physiological saline, at 0.85% by weight of NaCl, is prepared. diluted in deionized water.

  For these two solutions, the permutated water is microfiltered by a membrane of 0.45 pm absolute retention and, after distribution in 10 cm3 screw vials, the solutions are sterilized by autoclaving for 20 minutes at 121 C.

  100 cm3 of jet fuel removal are filtered under vacuum using a sterile, microporous membrane of 0.45 μm absolute retention.

  - The filter vessel and filter membrane are rinsed successively with the aid of the following sterile solutions: 1) 100 cm3 of Triton X-100, and 2) 30 cm3 of physiological saline.

  The filter is transferred to a petri dish which contains a sterile absorbent pad, soaked in and saturated with 2 cm 3 of MTG-E broth. This broth is a global nutrient medium for the development of bacteria, fungi and actinomycetes, an intermediate stage between bacteria and fungi of the class of filamentous bacteria, living in Jet A-1 kerosene jet fuels.

  The filter is examined, before incubation, under a binocular biological microscope, at a magnification of 40 and by incident light, in order to identify any non-biological solid particulate pollution.

  The entire inverted Petri dish is incubated at a temperature of 30 ° C. to 35 ° C. and for a minimum of 48 hours. During this incubation, the filter is in contact with the impregnated buffer, which is located on the membrane, so that the MTG-E broth comes into contact with the microorganisms, by capillarity through the pores.

  - During incubation and after incubation, biological binocular microscope filters, at magnifications of 40, 100 and 200, and incident light are examined for detection, enumeration and identification. colonies and germs of microorganisms.

  Developments of microorganisms are quantified, depending on their nature and incubation time, as percent growth at the Filter Retention Surface (% SRF).

  The microscopic observations under magnification of 200 are made at the end of the incubation, the focal length requiring a mounting of the filter membrane between two glass slides.

  It is specified that all these operations of filtration and preparation of the solutions and filters as well as the microscopic operations being incubated are carried out within a controlled dust controlled enclosure, with laminar flow.

  Before and after these manipulations, all the glassware used for the preparation of the solutions and the filtration is sterilized by autoclaving for 20 minutes at 121 ° C.

  We are now returning to the detection of biocontamination of jet fuels using the method that uses the "Milliflex-100" filtration system.

  In the case of this method, the microorganisms, which are present in kerosene, are isolated and cultured on the surface of a filter membrane, microporous, screen type.

  The analysis is conducted as follows.

  - A separator and a Milliflex unit are placed on the support of a Milliflex vacuum pump or a standard vacuum filtration line.

  - 100 ml of a kerosene sample is poured into a funnel and the pump is turned on to filter the sample.

  - The Milliflex unit is placed on a cassette whose buffer has been soaked with a global nutrient medium for the culture of microorganisms.

  - Manual pressure is applied to separate the funnel.

- Close the cassette.

  The cassette is incubated at 35 ° C. for a minimum of 48 hours.

  Filter membranes are examined under a microscope at magnifications of 40, 100 and 200.

  The examples of the invention, which have been given, relate to the detection of the biocontamination of a jet fuel. However, the method of the invention is not limited to jet fuels. Those skilled in the art can adapt the examples given to the detection of the biocontamination of any liquid fuel.

  In addition, in the present invention, microorganisms likely to be present in the liquid fuel can also be recovered by any recovery technique, such as filtration, and directly analyze the microorganisms thus recovered in an ATP metry cell.

Claims (10)

  1. A method for detecting the biocontamination of a liquid fuel, this method being characterized in that an ATPmetry technique is used to detect the presence of microorganisms in the liquid fuel.   2. Method according to claim 1, in which ATP metering equipment is used, a volume of liquid fuel is filtered on a sterile microporous membrane, so as to isolate strains of microorganisms that are likely to be present in the fuel. liquid, - the strains of microorganisms thus isolated are collected, and the presence of the microorganisms in the strains thus collected is detected by means of the ATPmetry equipment in order to determine the ATP.   5. The method of claim 2, wherein the equipment comprises a sample collection member to be analyzed with this equipment and the strains are collected by means of the sampling member.   6. The method of claim 3, wherein the volume of liquid fuel that is filtered is approximately equal to 100 cm3.   7. Method according to any one of claims 3 and 4, wherein the sampling member is a brush by means of which brushing the retention surface of the microporous membrane to collect strains of microorganisms.   6. The method of claim 5, wherein brushing the microporous membrane retention surface in two perpendicular directions.   7. Method according to any one of claims 3 to 6, wherein first a white light intensity corresponding to the sampling member alone, by means of the ATPmetry equipment, before collecting the strains of microorganisms by means of this sampling device, at least one measurement of luminous intensity 0 is carried out by means of the equipment, for the strains collected, the difference is calculated and the contamination is determined from this difference.   8. Method according to claim 7, in which several light intensity measurements are made for the strains, the arithmetic mean of these measurements is calculated, the difference 0re / Obland and the The method according to any one of claims 3 to 8, wherein the ATP metry equipment comprises: - a swab which constitutes the sampling organ, - a container which contains components that allow the implementation of the ATPometry technique and in which the swab is immersed after collecting the microorganism strains with this swab, so that these strains react with the components, and - a luminometer to measure the luminous intensity emitted by the strains having thus reacted.   10. Process according to claim 1, in which the microorganisms are recovered by any recovery technique, such as filtration, and the microorganisms thus recovered are analyzed directly in an ATP metry cell.   11. A method according to any one of claims 1 to 10, wherein the liquid fuel is contained in a reservoir and the preventive treatment of this reservoir is carried out if the ATP metering technique used leads to measuring a higher luminous intensity or substantially equal to a threshold previously set according to the intended application.