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The mechanism of microbial corrosion inhibition cannot be linked to a single biochemical reaction or particular species or group of microbes. Some microorganisms are able to both cause and inhibition corrosion. Studies on the effect of Bacillus thuringiensis on the corrosion behaviour of mild steel were carried out using gravimetric and atomic force microscopy (AFM) analysis. The mild steel coupons 2 x 2x 2 cm in size were suspended with a cotton thread which passes through a hole in each coupon, inoculated with the bacterium and incubated aerobically. The coupons were retrieved at 10 days intervals progressively for 60 days and analyzed. The result revealed that B. thuringiensis inhibited the corrosion of mild steel. The corrosion rate showed clear decrease in rate from 0.45 mpy after 10 days to 0.03 mpy after 60 days of exposure to B. thuringiensis when compared to a significant increase in corrosion rate observed (from 0.67mpy after 10 days to 3.98 mpy after 60 days) for mild steel not exposed to the bacterium respectively. The AFM analysis showed a wavy pattern of corrosion on the surface of the metal not exposed to the bacterium coupled with some peaks and valley formed as a result of uneven deposition of corrosion products. B. thuringiensis was very effective in inhibiting mild steel corrosion in aerobic environment.

Amylase production by Citrobacter sp. isolated from potato was optimized in batch culture studies under shake flask conditions. Effects and interactions of best sources and levels of carbon and nitrogen estimated by 4 x 5 and 4 x 4 factorial experimental arrangements were significant (P < 0.01) on amylase production. Optimal alpha-amylase yield was obtained in a medium containing sorghum flour (2.0 % w/v) and a mixture of (NH4)2SO4 + soybean meal (1.5% w/v) with an initial medium pH of 8.0. Under optimum conditions, amylase yield was maximal (0.499 U/ml) after 60h incubation at room temperature (28oC ± 2oC). Characterization studies showed that the enzyme had maximum activity at 60oC, retained 100% of its original activities at 60oC for 2h, was maximally active at pH 7.0 and retained 100% of original activities at pH 9.0 for 2h. Enzyme activity was stimulated by urea, Mg2+, Ca2+ and Zn2+ but inhibited by Hg2+.

The impacts of Hg2+, Cd2+ and Zn2+ on the activities of periplasmic nitrate reductase (NAP) and dehydrogenase (DHA) enzymes of three organisms isolated from soil and sediment-water interface were analysed in liquid culture studies. NAP and DHA activities were estimated from nitrite and triphenyl formazan were produced respectively after 4h incubation at 28 ± 2oC. Hg2+ completely inhibited NAP activity in Escherichia and Pseudomonas spp at all the concentrations (0.2 – 1mM) while progressive inhibitions of NAP activity were observed in Escherichia and Pseudomonas spp with increasing concentrations of Zn2+ and Cd2+. Both metals were stimulatory to NAP of Acinetobacter sp at 0.2 – 1mM. Apart from stimulation of DHA activity by Zn2+ (0.2 – 1mM) in Escherichia sp, Cd2+ (0.4 -1.0mM) in Acinetobacter sp and (1.0mM) in Pseudomonas sp, all the metals progressively inhibited DHA activities in the three organisms. In Escherichia sp, the activities of the two enzymes were negatively correlated on exposure to Zn2+ (r = -0.91) and positively correlated (r = >0.90) on exposure to Cd2+ and Hg2+. Based on IC50 values of the metals for the DHA and NAP enzymes, the most resistant of the three organisms were Escherichia sp and Acinetobacter sp respectively. Quantitatively, NAP with its lower IC50 values than DHA was a more sensitive toxicity measure for Hg2+ in all the organisms. The sensitivity of microbial metabolic enzymes to the toxic effects of metals varies with the type of enzyme, metal and the microorganism involved.