Starved bacteria

Another series of experiments used sandstone cores previously injected with starved bacteria to investigate the ability of the bacteria to grow within rock cores when given a suitable nutrient Berea sandstone cores of 200 and 400 millidarcy (md) permeabilities were used as they were considered to be more representative of reservoir conditions than the glass bead cores. The sandstone cores were injected with 300 to 450 pore volumes of 10 /ml starved bacteria until the cores contained an even distribution of bacteria (Fig. 3A B) and the core permeabilities were between 13% and 18%. SCM nutrient was injected through the cores (Fig. 3C) until the core permeability fell to 0.1%, this required 360 pore volumes of SCM. 

The starved bacteria resuscitated by utilizing the SCM and grew within the sandstone forming a deep bacterial plug composed of cells... 

Figure 3. Diagrammatic representation of the plugging of rock cores with resuscitated starved bacteria. See text for details...

The starved bacteria were still tiny and singular with little or no biofilm. 

We also investigated several cost reduction exercies, such as, a) giving the starved bacteria short bursts of nutrient (less than 50 pore volumes) instead of a continuous flow and b) injecting fewer starved bacteria into the core (150 pore volumes) before nutrient injection. Both still resulted in deep bacterial plugs when SCM was used as a nutrient. 

In summary, care must be taken to inject nutrients that do not encourage rapid growth as undesirable shallow bacterial plugs form (Fig. 3F). With the correct nutrient package, such as SCM in this instance, a deep plug will form throughout the strata (Fig. 3D). In conclusion, our laboratory based studies demonstrate that starved bacteria may be used to physically block rock strata already drained of oil. Further recovery operations can then deal with strata still containing oil and thus enhance recovery rates. 

Weinberg (26), who found that the bacterial death rate in synthetic media was accelerated by iron deprivation. The survival of serum-exposed bacteria incubated at 4°C makes it most unlikely that the iron-starvation per se is responsible for the killing of bacterial cells. Griffiths (5) found that the inhibition of E. coli by serum is accompanied by the appearance of abnormal bacterial phenylalanyl t-RNA. Such abnormal t-RNA was not found in iron-supplemented serum. It is possible, therefore, that permanent lesions develop in iron-starved bacteria which not only stop bacterial multiplication but eventually cause bacterial death. 

Fig. 1 The effect of poly[(f )-3-hydroxybutyrate] (PHB) on survival capability of starved bacteria. Cells of Azospirillum brasilense Sp7 (filled triangles) and phaC mutant (filled circles) were grown on a medium with a high carbon to nitrogen ratio for 24 h and transferred to phosphate buffer, where they were incubated for 12 days. Bacterial density was determined using dilution plating (Reproduced from Kadouri et al. 2002, with kind permission from the American Society for Microbiology)...

Calorimetric studies of this kind, in combination with specific techniques, may be valuable in resolving metabolic responses of non-growing/starved microorganisms. For an example see section 3.2.4., where a combination of calorimetry and respirometry on starved bacteria is presented. 

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