## 1 Introduction  
Petroleum hydrocarbons (PHs) are common fuels used in extremely large quantities and represent a serious problem when released into the environment [1; 2; 3] because they contain n-alkanes, i.e., aliphatic and aromatic compounds [4]. Accidental release of PHs usually results in their persistence in soil [5; 6; 7] for decades because their biodegradation process occurs very slowly due to their recalcitrant nature [8]. The major sources of PH soil contamination are accidents during petroleum transfer, refinery wastes released into the environment, or petroleum leakage from pipelines [9]. [PERSON] et al. [10] have reported that many PHs are usually highly toxic, carcinogenic, and mutagenic for both soil microorganisms and humans. The danger posed by the presence of PHs in soil is connected with their inherent properties: volatility and solubility [11]. PHs represent bioresistant organic compounds that can inhibit the metabolism of aerobic bacteria [12]. Consequently, they can exert a toxic effect on soil biodiversity and a negative impact on plants and soil health 1 [2; 13]. Therefore, effective removal of PHs from the environment is crucial [11; 14]. The risk posed by PH soil contaminants can be reduced by the application of indigenous microorganisms dedicated to bioremediation processes [3; 13; 15]. Being in intimate contact with the soil, microorganisms are known to be the best indicators of soil pollution [13]. To date, various methods have been recommended to remediate polluted soil, i.e., chemical oxidation, thermal treatment, and bioremediation [11]. Soil microorganisms are economically feasible and are an effective solution for soil clean up [1; 3; 4]. Soil microorganisms break down complex compounds (i.e., PHs) and release inorganic nutrients that are available to plants [14; 16]. In ideal conditions, PHs are completely biodegradable to carbon dioxide and water (products that are innocuous to humans and the environment); however, more frequently, biodegradation is not complete [4]. Generally, microorganisms in contaminated soil should adapt to environmental conditions as an effect of genetic mutations [3] and induction of specific enzymes, i.e., oxygenases, hydrolases, and oxidoreductases [6; 17]. Nevertheless, it should be remembered that introduced microorganisms do not often survive when added to soil [1]. Therefore, introduction of new microflora dedicated to bioremediation should always be preceded by identification of autochthonic bacteria inhabiting the soil environment that use hydrocarbon pollution as a carbon source for their metabolic pathway [1; 18; 19]. The best-known bacteria that are able to degrade PHs include the following genera: _Arthobacter_, _Acinetobacter_, _Bacillus_, _Brachybacterium_, _Corynebacterium_, _Geobacillus_, _Nocardiia_, _Klebsiella_, _Mycobacterium_, _Mycobacterium_, _Pseudomonas_, _Microoccus_, and _Rlodococcus_[19; 20; 21; 22; 23; 24]. Similarly, _Ralstonia_ and _Burkholderia_ have been identified as PH degraders [25]. [PERSON] et al. [7] have suggested that the precise estimation of the effect of petroleum substances on the natural environment is very difficult. The main reason is that the proportions between fast- and slow-growing bacteria in PH-contaminated soils are usually variable [26], and knowledge of microbial succession is necessary for soil purification from petroleum contaminants [7]. The Biolog EcoPlate technique is recommended to indicate viable microorganisms and to perform metabolic fingerprinting [16; 27; 28]. Since microorganisms usually react quickly to any environmental disturbances and stresses, changes in their metabolic activity could be an early signal of alterations in the ecosystem [16]. The Biolog EcoPlate is a redox method for which the variability in the rate of colour development depending on the microbial community structure indicates the rate of utilization of carbon substrates preferred by soil microbes [16].