Fig. 2 depicts the bacterial growth, biosurfactant production,
and crude oil removal assay under optimal culture conditions.
There was a significant increase in protein content and cell density
in the culture broth as compared to the control after 2 weeks of
incubation, indicating that B. methylotrophicus USTBa can utilize
crude oil hydrocarbon as the sole carbon and energy source. The
protein content of strain USTBa increased gradually and reached
the maximum of 3.8 g/L in 12 days. This strain removed nearly
80% of the crude oil from the liquid culture after 8 days of incubation,
corresponding to the exponential phase growth of the bacteria.
After 8 days, there was a slight increase in crude oil removal,
which was further correlated with an increase in BATH and decrease
in ST. After 12 days, there was a 92% crude oil removal
and the highest BATH and ST were 62% and 28 mN/m, respectively.In fact, there is good correlations of crude oil removal with the decrease
in surface tension (spearman coefficients of 0.988) and the
increase in BATH (spearman coefficients of 0.986). These results
indicate that the produced biosurfactant is able to improve the bioavailability
of crude oil in the liquid culture so as to speed up its
degradation. Similar observation where Penicillium sp. was used
to produce biosurfactant was reported by Luna-Velasco et al.
[29]. In this work, the crude oil removal efficiency by Bacillus sp.
USTBa was much higher than reported in the literature [30].