Bacterial enzymes that break down proteins sit at the heart of detergents, food processing, leather treatment and pharmaceutical manufacturing, and proteases account for roughly sixty percent of the global industrial enzyme market. A new open-access study in Bioresources and Bioprocessing reports a statistical screening exercise that identified the medium and process variables most strongly governing extracellular protease production by Bacillus sp. MTCC 511, a strain obtained from the Microbial Type Culture Collection and Gene Bank in Chandigarh, India. Using a compact 12-run Plackett–Burman Design, the researchers evaluated ten variables at once under four different mutagenic stress conditions, and the best combination they uncovered raised enzyme output more than twenty-fold above an unsupplemented basal medium.
The ten screened variables were dipotassium phosphate, monopotassium phosphate, ammonium sulphate, sodium nitrate, magnesium sulphate heptahydrate, calcium chloride, the carbon source, the nitrogen source, agitation speed and pH. Rather than testing each factor one at a time, the Plackett–Burman approach is a two-level fractional factorial design that estimates the main effect of every variable from a small matrix of runs, in this case generated and analysed with MINITAB software. Fermentation took place in stirred-tank bioreactors with a basal nutrient broth supplemented across defined ranges: ammonium sulphate from 0.01 to 0.05 M, calcium chloride from 0.002 to 0.006 M, the two phosphates from 0.01 to 0.05 M each, magnesium sulphate from 0.002 to 0.006 M, sodium nitrate from 0.1 to 0.5 M, and glucose or starch at one percent by weight, paired with beef extract or yeast extract as nitrogen sources.
A distinctive feature of the work was its side-by-side comparison of mutagenic treatments within a single screening framework. Overnight cultures were exposed to four conditions: a water-bath heat shock at 70 °C for ten minutes, the same treatment at 80 °C, and ultraviolet irradiation at 254 nanometres for either two or three minutes at a distance of thirty centimetres. The authors chose the thermal range from preliminary kill-curve experiments indicating sub-lethal stress, and the ultraviolet durations from published Bacillus mutagenesis protocols. Surviving cells were inoculated into the twelve screening media, and validation fermentations were run at 37 °C rather than the usual 48 °C to accommodate the potentially reduced thermotolerance of mutagenized cells.
Protease activity was quantified with a modified Folin–Ciocalteu assay against a tyrosine standard curve that was linear from 1.4 to 65.0 micrograms per millilitre, with a regression coefficient of 0.998. One activity unit corresponds to one microgram of tyrosine released per millilitre per minute, measured with casein as the substrate in glycine–NaOH buffer at pH 10. Across the twelve screening runs, activity varied widely under every treatment, and the pattern differed strikingly by stressor. Under the 70 °C treatment, run 2 peaked at 8.0 U/mL with a mean of 1.996 U/mL across runs; at 80 °C the peak fell to 4.72 U/mL with a tighter spread, suggesting the harsher heat narrowed the enzyme response window. The two-minute ultraviolet treatment shifted the peak to a different run entirely, at 5.33 U/mL, while run 2 yielded only 1.01 U/mL under that condition.
The three-minute ultraviolet exposure produced the overall highest activity of the study: 8.66 U/mL in run 2, corresponding to a medium containing starch at one percent, yeast extract at 0.1 percent, dipotassium and monopotassium phosphate each at 0.05 M, ammonium sulphate at 0.05 M, sodium nitrate at 0.1 M, magnesium sulphate heptahydrate at 0.002 M and calcium chloride at 0.002 M, held at pH 9 with agitation at 120 rpm. Relative to the unsupplemented basal medium, which produced 0.42 U/mL, this represented a 20.6-fold improvement, while the best 70 °C formulation gave a 19.0-fold increase. In volumetric terms, the ultraviolet optimum reached 0.361 U/mL per hour over the 24-hour fermentation. Starch consistently outperformed glucose, and ammonium sulphate supplementation did not suppress output, observations the authors link to the alleviation of carbon catabolite repression and the weak repressive effect of ammonium salts.
Statistical adequacy differed among the treatments. First-order models fitted well for three conditions, with R² values of 98.05 percent at 70 °C, 98.55 percent at 80 °C and 97.99 percent for the three-minute ultraviolet treatment, but the two-minute ultraviolet model was statistically inadequate at an R² of 56.52 percent and was not interpreted biologically. The adjusted R² values, between roughly 78 and 84 percent, were notably lower than the raw R² values because a 12-run design screening ten variables leaves only one residual degree of freedom; in such saturated designs two-factor interactions are aliased with main effects, so some estimated main effects may partly absorb confounded interaction contributions. The authors stress this as an intrinsic limitation of screening designs and a reason follow-up response surface work is needed.
Pareto ranking and main-effects analysis singled out agitation speed, sodium nitrate, magnesium sulphate heptahydrate and the carbon source as the leading factors across the three adequate models. Sodium nitrate and magnesium sulphate together accounted for over 54 percent of the variance in the 70 °C response, while agitation and carbon source dominated the ultraviolet response with more than 50 percent combined. Sodium nitrate showed a negative effect under thermal treatments, meaning the lower tested concentration of 0.1 M favoured higher titres, a pattern the authors interpret as consistent with nitrogen catabolite repression, in which the global regulator CodY dampens transcription of the major protease genes when preferred nitrogen sources are plentiful. Magnesium sulphate also had a negative effect in all treatments, most pronounced at 80 °C, and the preferred 0.002 M level may balance cofactor supply against reported interference of elevated magnesium with protein secretion. Agitation was consistently positive, plausibly reflecting improved oxygen transfer for aerobic metabolism and ATP-dependent protein export, and alkaline pH 9 was favourable throughout, matching the pH optimum of Bacillus alkaline serine proteases.
A second layer of analysis examined whether different physical stressors produce correlated enzyme responses. Pearson correlations were strong between the 70 °C and 80 °C responses at 0.827, between 70 °C and three-minute ultraviolet at 0.775, and between 80 °C and three-minute ultraviolet at 0.718, while the two-minute ultraviolet responses were essentially uncorrelated with the thermal treatments. The authors note that both heat shock and prolonged ultraviolet exposure have been reported to activate the sigma-B-dependent general stress regulon, a network of more than 150 genes in Bacillus covering protein quality control, oxidative stress defence and extracellular enzyme production, and that both stress classes induce Clp proteases that could enlarge the amino acid pool available for enzyme synthesis. They are careful to frame these as plausible, unverified hypotheses: no gene expression or proteomic measurements were performed, and the negligible correlations for the two-minute ultraviolet condition likely reflect sub-threshold damage that failed to trigger a uniform regulatory response.
The screening also carried practical validation. All twelve trial runs were independently reproduced in a 15-litre bioreactor with dissolved oxygen held at 25 percent saturation, and under the consensus run 2 conditions the first-order model predicted 7.84 U/mL against an observed 8.66 U/mL, a prediction error of 9.5 percent. Independent triplicate fermentations at that setting gave a coefficient of variation of 8.3 percent. The maximum activities reported here, 8.66 U/mL under ultraviolet mutagenesis and 8.0 U/mL after heat shock, are comparable to published Plackett–Burman results in other Bacillus strains, including 7.2 U/mL for Bacillus mojavensis and 8.4 U/mL for Bacillus cereus, and the entire exercise required only 48 assay points to evaluate ten variables, a marked economy over one-variable-at-a-time experimentation.
The authors are explicit about the study’s limits. No quantitative survival analysis by colony-forming counts was performed after mutagenesis, so differences in protease output cannot be cleanly separated from differential cell survival; the triplicate measurements were technical replicates from the same culture supernatant rather than independent biological replicates; and each screening run employed a single fermentation batch. All mechanistic interpretations, from CodY and CcpA regulation to sigma-B activation, remain hypotheses grounded in prior literature rather than conclusions from molecular experiments. Within those constraints, the work delivers a compact, statistically grounded map of which levers matter most for protease yield in this strain, and sets the stage for response surface optimization and molecular characterization of the mutagenized lines.
Subject of Research: Statistical screening of media components for enhanced extracellular protease production by Bacillus sp. MTCC 511 under thermal and UV mutagenic stress
Article Title: Screening and identification of significant media components using Plackett–Burman design for enhanced extracellular protease production by Bacillus sp. MTCC 511 under mutagenic stress conditions
Article References: Sharma, A. K., Kumar, A., & Jena, M. K. (2026). Screening and identification of significant media components using Plackett–Burman design for enhanced extracellular protease production by Bacillus sp. MTCC 511 under mutagenic stress conditions. Bioresources and Bioprocessing, 13(1), Article 145. https://doi.org/10.1186/s40643-026-01134-0
Image Credits: AI Generated
DOI: 10.1186/s40643-026-01134-0
Keywords: Bacillus, protease, Plackett–Burman design, media optimization, UV mutagenesis, heat shock, fermentation, bioprocessing, catabolite repression, industrial enzymes, statistical screening, sigma B regulon
News Source: Drew Townsend. (October 29, 2026). Plackett–Burman Screening Pinpoints Key Media Factors Boosting Bacillus Protease Output. Scienmag.
