Plant Growth-Promoting Bacteria (PGPB) Liquid Culture Experiment: Methods, Testing Items, and Applic

The plant growth-promoting bacteria (PGPB) liquid culture experiment is a core laboratory procedure for evaluating the fermentation performance, biomass yield, and functional metabolite production of beneficial microorganisms intended for agricultural bioinoculants. By systematically optimizing medium composition, culture conditions, and bioreactor parameters, laboratories can determine whether a candidate strain meets the viable count and purity requirements specified in standards such as GB 20287-2006 and ISO 7218. This article provides a comprehensive technical overview covering sample types, testing items, analytical methods, applications, and frequently asked questions related to PGPB liquid culture experiments.

Technical Overview

Plant growth-promoting bacteria (PGPB), also known as plant growth-promoting rhizobacteria (PGPR), are beneficial microorganisms that enhance plant growth through direct mechanisms such as nitrogen fixation, phosphate and potassium solubilization, and phytohormone production, as well as indirect mechanisms including siderophore secretion, antibiotic production, and induced systemic resistance. Commonly studied genera include Bacillus, Pseudomonas, Azotobacter, Azospirillum, Rhizobium, Burkholderia, and Enterobacter.

The liquid culture experiment is the fundamental laboratory technique used to cultivate these strains in submerged (liquid) fermentation systems, quantify their growth characteristics, and evaluate their functional metabolite output. The workflow typically progresses from strain revival and seed culture preparation, through shake-flask fermentation, to controlled bench-scale bioreactor fermentation, followed by systematic quality and functional verification of the resulting fermentation broth.

The objectives of a PGPB liquid culture experiment include establishing a reproducible growth curve, determining the maximum viable cell concentration, optimizing medium composition and physical culture parameters (temperature, pH, agitation, aeration), confirming strain purity, and verifying that the final broth satisfies the quality thresholds established in agricultural inoculant standards such as GB 20287-2006, NY 884, and the general microbiological guidance of ISO 7218.

Testing Samples

Fresh Liquid Cultures of Candidate Strains

Newly isolated or laboratory-maintained PGPB strains, typically preserved as glycerol stocks at ultra-low temperature or as lyophilized ampoules, are revived on appropriate solid media and propagated into fresh liquid culture for evaluation. Each candidate strain is cultured under conditions appropriate to its physiology, for example nutrient broth for Bacillus species, King's B medium for fluorescent Pseudomonas, Ashby medium for Azotobacter, and yeast extract-mannitol medium for Rhizobium.

Seed Culture and Fermentation Process Samples

Intermediate samples withdrawn at defined time points during seed culture and main fermentation, including samples from the lag phase, exponential phase, stationary phase, and decline phase. These process samples are essential for constructing growth curves, calculating specific growth rates, and monitoring substrate consumption and metabolite accumulation.

Commercial Liquid Inoculant Products

Finished liquid biofertilizer and microbial inoculant products submitted for quality verification, including viable cell count determination, contamination assessment, pH measurement, and shelf-life stability testing in accordance with GB 20287-2006.

Scaled-Up Fermentation Broth

Bulk fermentation broth produced in pilot-scale or production-scale bioreactors, evaluated prior to formulation to confirm that biomass and purity targets have been achieved before downstream processing such as carrier adsorption, stabilization, or packaging.

Testing Items

The testing scope of a PGPB liquid culture experiment covers growth quantification, physicochemical parameters, functional metabolites, purity, and stability. Typical testing items include:

• Viable plate count (CFU/mL) and total cell concentration determination
• Optical density measurement (OD600) and dry cell weight (DCW) analysis
• Growth curve construction, lag phase duration, maximum specific growth rate, and doubling time calculation
• Endospore formation rate for Bacillus species
• pH, oxidation-reduction potential, and conductivity of the fermentation broth
• Residual reducing sugar and nitrogen source consumption analysis
• Indole-3-acetic acid (IAA) and gibberellin production quantification
• Siderophore production assessment
• Phosphate and potassium solubilization activity
• 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity
• Nitrogenase activity and free-living nitrogen fixation capacity
• Ammonia and hydrogen cyanide (HCN) production screening
• Exopolysaccharide (EPS) and biofilm formation capability
• Purity verification, miscellaneous microorganism (contamination) rate, and colony morphology characterization
• Strain identity confirmation by 16S rRNA gene sequencing
• Shelf-life and storage stability of the formulated product
• Safety indicators, including absence of pathogenic organisms such as Escherichia coli, Salmonella, and Staphylococcus aureus

Testing Methods

The liquid culture experiment follows a staged methodology aligned with general microbiological practice under ISO 7218 and agricultural inoculant requirements under GB 20287-2006.

Strain Revival and Seed Culture Preparation

Preserved strains are revived by streaking onto suitable agar medium and incubating at the optimal temperature, typically 28 to 37 degrees Celsius depending on the species. A single colony is inoculated into fresh liquid medium and incubated with shaking until the culture reaches the mid-exponential phase, generally an OD600 of 0.6 to 1.0. This actively growing seed culture serves as the standardized inoculum, usually transferred at 1 to 10 percent (v/v) into the main culture medium.

Shake Flask Fermentation

Seed culture is inoculated into Erlenmeyer flasks filled to 20 to 30 percent of nominal volume to maintain adequate oxygen transfer. Cultures are incubated in orbital shakers at 150 to 220 rpm under controlled temperature. Sampling is performed at regular intervals, typically every 4 to 8 hours over a 24 to 72 hour period, to monitor OD600, pH, viable count, and substrate consumption. Shake flask experiments enable rapid parallel screening of medium formulations and culture conditions.

Bench-Scale Bioreactor Fermentation

Promising conditions identified in shake flasks are validated in stirred-tank bioreactors of 5 to 50 liter working volume with online monitoring of dissolved oxygen, pH, temperature, agitation speed, and aeration rate. Typical control parameters include 28 to 32 degrees Celsius for most PGPB species, pH 6.5 to 7.5 maintained by automatic acid or base addition, and aeration of 0.5 to 1.0 vvm. Batch, fed-batch, or continuous strategies may be applied to maximize viable cell density and metabolite yield.

Medium and Condition Optimization Design

Optimization is conducted using a factorial approach, beginning with one-factor-at-a-time screening of carbon sources, nitrogen sources, trace elements, and initial pH, followed by statistical design methods such as Plackett-Burman screening, Box-Behnken design, or central composite design combined with response surface methodology (RSM). This approach efficiently identifies the optimum medium composition and interaction effects between variables to maximize biomass or target metabolite production.

Biomass and Viability Determination

Viable counts are determined by serial dilution and pour or spread plating in accordance with the general principles of ISO 4833-1 and GB 4789.2, with results expressed as CFU per milliliter. Growth is simultaneously monitored spectrophotometrically at 600 nanometers, and dry cell weight is obtained by centrifugation and constant-weight drying. For spore-forming Bacillus strains, spore counts are determined after heat treatment at 80 degrees Celsius for 10 minutes to inactivate vegetative cells.

Functional Metabolite Assays

IAA production is quantified colorimetrically using Salkowski reagent with spectrophotometric reading at approximately 530 nanometers. Siderophore secretion is assessed using chrome azurol S (CAS) agar or liquid CAS assays. Phosphate solubilization is evaluated on Pikovskaya medium and quantified by measuring soluble phosphorus via the molybdenum blue method. ACC deaminase activity is determined by quantifying alpha-ketobutyrate production. Nitrogenase activity may be measured by the acetylene reduction assay using gas chromatography or confirmed by amplification of the nifH gene marker. Ammonia and HCN production are screened using Nessler reagent and picric acid filter paper methods, respectively.

Purity, Identification, and Contamination Verification

Colony morphology, Gram staining, and microscopic examination verify culture purity, supported by 16S rRNA gene sequencing for definitive strain identification. The miscellaneous microorganism rate is determined by plating on non-selective media and comparing colony types against the target strain, following the quality criteria of GB 20287-2006, which requires contamination rates within defined limits for liquid formulations.

Shelf-Life and Stability Verification

Formulated products are stored at ambient and elevated temperatures, with viable counts measured at defined intervals across the labeled validity period, commonly six months or longer. Accelerated storage studies support extrapolation of shelf-life performance and confirmation that viable counts remain above the minimum thresholds specified in GB 20287-2006 throughout the labeled period.

Applications

PGPB liquid culture experiments underpin both research and industrial practice in agricultural microbiology and biofertilizer manufacturing. Key applications include:

• Industrial biofertilizer process development, including scale-up from shake flask to production bioreactor for liquid microbial inoculants
• Strain screening and ranking programs for selecting high-yield, multi-trait PGPB candidates from environmental isolates
• Seed coating, root drench, and foliar inoculant product development with verified viable count and shelf life
• Biocontrol formulation support, where in vitro antagonism and metabolite profiles are correlated with fermentation performance
• Quality control testing of commercial liquid inoculants against GB 20287-2006 and related national and industry standards in accredited third-party laboratories
• Bioremediation and soil amendment programs utilizing functional microbial consortia cultivated by submerged fermentation
• Academic research and teaching in microbial physiology, fermentation kinetics, and plant-microbe interactions

FAQ

What is the purpose of a PGPB liquid culture experiment?

The experiment determines whether a beneficial bacterial strain can be cultivated to a sufficiently high viable cell density in liquid medium while retaining its plant growth-promoting traits, and it establishes the reproducible process parameters needed for inoculant production and quality control.

What viable count must the final fermentation broth achieve?

Under GB 20287-2006, liquid bacterial inoculant formulations are commonly required to contain viable counts at or above 2.0 × 10⁸ CFU/mL at the end of the labeled shelf life, with the miscellaneous microorganism rate maintained within the standard-specified limit. Product-specific targets are often set higher by manufacturers to provide a stability margin.

How long does a complete liquid culture experiment take?

A single growth cycle, from seed culture to stationary-phase harvest, typically requires 24 to 72 hours depending on the strain. Including strain revival, medium optimization, replicate runs, functional assays, and data analysis, a complete experimental program generally spans two to six weeks.

What is the difference between shake flask and bioreactor culture?

Shake flasks offer simple, low-cost parallel screening of media and conditions but provide limited control and monitoring of oxygen transfer and pH. Stirred-tank bioreactors enable precise online control of dissolved oxygen, pH, temperature, and feeding strategy, delivering higher cell densities and process data required for industrial scale-up.

How is contamination detected and controlled?

Contamination is detected through microscopic examination, colony morphology comparison, plating on non-selective media, and molecular identification of suspicious isolates. Control measures include aseptic technique, medium sterilization at 121 degrees Celsius for 15 to 20 minutes, pure seed culture verification, and in-process bioreactor integrity testing.

Can filamentous beneficial fungi be evaluated with the same workflow?

Yes, beneficial fungi such as Trichoderma species can be cultured in liquid fermentation, with adjustments to medium composition, inoculum form (spore suspension), agitation intensity, and enumeration method, following guidance such as ISO 21527 for yeast and mold enumeration where applicable.