Unit 2: Isolation, purification and mass multiplications of entomopathogens

I. Orientation — Entomopathogens as Biological Control Agents

Entomopathogens are microorganisms that infect and kill insects and can therefore suppress agricultural pests. Their development as biopesticides follows a connected sequence: sampling → isolation → purification → identification → pathogenicity testing → maintenance → mass multiplication → quality control.

• Major groups: Bacterial agents include Bacillus thuringiensis (Bt), Bacillus cereus group isolates, Lysinibacillus sphaericus, Serratia spp. and insect-associated bacteria; fungal agents include Beauveria bassiana, Metarhizium anisopliae, Lecanicillium spp. and Isaria/Cordyceps spp.
• Isolation: Separation of a target microorganism from soil, an infected insect or another mixed biological sample.
• Purification: Obtaining a culture derived from a single colony, spore or hyphal tip and free from detectable contaminants.
• Maintenance: Preservation of the isolate’s viability, identity and virulence while minimizing repeated subculture.
• Mass multiplication: Production of large quantities of viable cells, spores, toxins or infective propagules under controlled conditions.
• Aseptic convention: Sterile media, tools and containers are used inside a disinfected workspace or laminar-flow cabinet to prevent contamination and exposure.
• Identity and efficacy: Colony appearance alone is insufficient; microscopy, biochemical or molecular identification and insect bioassays are used together.
• Quality attributes: A useful product requires high viable count, genetic and phenotypic stability, insecticidal activity, purity, storage stability and consistent field performance.
• Safety principle: Unknown isolates are handled under institutionally approved containment. Only strains shown to be safe for workers, crops, vertebrates and non-target organisms should enter production.

II. Bacterial Biocontrol Agents — Recovery, Stabilization and Production

A. Isolation of bacterial biocontrol agents from soil and naturally infected insects

Bacterial isolation aims to recover discrete colonies from complex samples while enriching organisms associated with insect pathogenicity.

• Soil sampling: Approximately 5–10 g of soil is collected aseptically from several points, commonly at a depth of 5–15 cm, pooled when appropriate and stored cool until processing.
  • Rhizosphere soil, insect-rich habitats and soil beneath diseased cadavers are useful sources.
  • Sample labels record location, date, habitat, depth and environmental conditions.
• Serial dilution method: One gram of soil may be suspended in 9 mL sterile saline or buffered diluent to make a (10^{-1}) suspension; successive 1:10 transfers produce higher dilutions.
  • Aliquots are spread on nutrient agar or a selective medium.
  • Well-separated colonies are selected after incubation at a temperature appropriate for the target, often approximately 25–30°C for environmental isolates.
• Spore-former enrichment: Heat treatment can reduce non-spore-forming bacteria and enrich endospore-forming Bacillus.
  • The treatment must be validated because excessive heating may eliminate desired strains.
  • Bt-like colonies are examined microscopically for vegetative cells, endospores and parasporal crystalline inclusions.
• Isolation from naturally infected insects: Fresh insects showing abnormal coloration, flaccidity, septicemia or characteristic odor are collected separately to avoid cross-contamination.
  • The cadaver surface is disinfected briefly, rinsed with sterile water and dissected aseptically.
  • Haemolymph or internal tissue is streaked onto suitable agar; internal sampling helps distinguish pathogens from surface saprophytes.
• Colony selection: Colonies differing in size, margin, elevation, pigmentation and texture are restreaked individually. For Bt, opaque, irregular colonies and crystal-bearing sporulated cells provide presumptive evidence.
• Confirmation of pathogenicity: A standardized suspension is administered through diet, surface treatment or another biologically relevant route to healthy target insects.
  • Mortality is compared with an untreated control.
  • The same bacterium is re-isolated from diseased test insects, linking the culture with the observed disease.
• Critical limitation: Recovery from a dead insect does not itself prove pathogenicity; bacteria may be secondary invaders that multiplied after death.

B. Purification and maintenance of bacterial biocontrol agents

Purification establishes a single-strain culture, while maintenance protects that strain from contamination, mutation and loss of insecticidal activity.

• Repeated streaking: A selected colony is streaked across fresh agar for isolated colonies; this is repeated until colony morphology and cellular appearance are uniform.
• Purity checks: Cultures are evaluated by Gram staining, cell and spore morphology, colony uniformity and growth on differential media.
  • Molecular markers, gene sequencing or Bt toxin-gene detection may support identification.
  • A single test cannot establish both taxonomic identity and virulence.
• Single-colony culture: One isolated colony is inoculated into sterile broth or onto an agar slant to create a working culture.
• Short-term maintenance: Agar slants are sealed and refrigerated, commonly near 4°C, for limited periods. Frequent transfer is avoided because it increases contamination and selection pressure.
• Long-term preservation:
  • Cryopreservation: Cells are stored at very low temperature with a validated cryoprotectant such as glycerol.
  • Lyophilization: Freeze-drying stabilizes suitable cultures in sealed ampoules with protective additives.
  • Spore storage: Purified bacterial spores may remain stable when dry, cool and protected from moisture.
• Culture hierarchy: A master seed lot generates working seed lots, which supply production batches. Production material should not be returned to the master stock.
• Authentication: Each culture receives an isolate code linked to source, passage number, identification profile, preservation date and bioassay record.
• Virulence monitoring: Periodic target-insect bioassays detect loss of activity after storage or repeated laboratory cultivation.
• Contamination indicators: Unexpected pigmentation, mixed cell types, altered odor, poor sporulation or multiple colony forms require rejection or re-purification.

C. Mass multiplication of bacterial biocontrol agents

Bacterial mass multiplication uses controlled fermentation to maximize viable biomass, spores and, where relevant, insecticidal metabolites.

• Inoculum development: A verified working culture is transferred successively from a small starter flask to a seed fermenter and then to the production vessel.
  • Each stage should be actively growing and free from contaminants.
  • Inoculum size and physiological age are standardized between batches.
• Submerged fermentation: Bacteria grow in a sterilized liquid medium containing carbon, nitrogen, minerals and water.
  • Agitation maintains uniformity.
  • Aeration supplies oxygen to aerobic organisms such as Bt.
  • Temperature, pH, dissolved oxygen, foam and fermentation time are monitored.
• Bt production objective: Fermentation is continued through growth and sporulation so that spores and parasporal Cry/Cyt toxin crystals accumulate.
  • Premature harvest reduces spore and crystal yield.
  • Microscopy can monitor the proportion of sporulated cells and released crystals.
• Harvesting: Biomass and spores are concentrated by centrifugation, filtration or sedimentation, depending on equipment and formulation requirements.
• Formulation: Concentrate may be converted into wettable powder, suspension concentrate, granule or oil-based formulation with carriers, dispersants, protectants and stabilizers.
• Viable-count calculation: Countable plates are used to estimate colony-forming units:

CFU/mL = C ÷ (V × D)

• (C) = number of colonies counted.
• (V) = plated volume in millilitres.
• (D) = dilution plated, expressed as a decimal.
  • Quality control: Tests include viable count, absence of unwanted microorganisms, moisture, pH, sporulation, toxin profile, physical dispersibility and target-insect potency.
  • Scale-up constraint: Larger vessels alter oxygen transfer, mixing and heat removal; identical medium composition does not guarantee identical biological yield.

III. Fungal Biocontrol Agents — Isolation, Culture Preservation and Propagule Production

A. Isolation of fungal biocontrol agents from soil and naturally infected insects

Fungal isolation recovers entomopathogenic hyphomycetes as colonies or spores while suppressing rapidly growing contaminants.

• Soil dilution plating: Soil suspensions are serially diluted and spread onto potato dextrose agar, Sabouraud dextrose agar or an entomopathogen-selective medium.
  • Antibiotics may suppress bacteria.
  • Selective fungistatic compounds can restrict competing saprophytic fungi, but concentrations must not inhibit the target.
• Insect-bait method: Moist soil is exposed to susceptible larvae such as Galleria mellonella or Tenebrio molitor.
  • Dead larvae are surface-disinfected and incubated in a humid chamber.
  • Characteristic external sporulation is transferred to culture medium.
• Naturally infected insects: Cadavers bearing white Beauveria-like, green Metarhizium-like or other fungal growth are collected in separate sterile containers.
• Surface disinfection: Brief treatment with an appropriate disinfectant, followed by sterile-water rinses, reduces external contaminants without sterilizing internal infection.
• Humid incubation: Cadavers are placed on moist sterile support without direct contact with free water; high humidity encourages diagnostic outgrowth.
• Primary transfer: Conidia or a small piece from the advancing edge of fungal growth is transferred to agar with a sterile needle.
• Identification: Colony color, growth rate, conidiophore arrangement and conidial shape provide presumptive identification; molecular sequencing can confirm species.
• Pathogenicity verification: Healthy insects are exposed to a measured conidial suspension, and the fungus is re-isolated from cadavers showing mycosis. Control mortality and background contamination must remain low.

B. Purification and maintenance of fungal biocontrol agents

Pure fungal cultures are derived from a single propagule or uncontaminated hyphal region and preserved with minimal passage.

• Single-spore isolation: A dilute conidial suspension is spread on agar, and one germinating conidium is transferred microscopically to fresh medium.
• Hyphal-tip method: A clean tip from the actively growing colony margin is excised and transferred; this is useful when sporulation is poor.
• Purity assessment: Uniform colony sectors, conidial morphology and microscopic hyphae are examined, while bacterial contamination is checked on suitable media.
• Routine culture: Purified fungi are maintained on PDA, SDA or another validated medium at an appropriate temperature, commonly around 25°C.
• Short-term storage: Refrigerated agar slants or sterile-water storage may preserve cultures for limited periods, depending on species.
• Long-term storage: Cryopreservation, lyophilization where suitable, or storage of spores on inert carriers reduces genetic drift and degeneration.
• Passage control: Repeated growth on rich artificial media may reduce sporulation or virulence; cultures are periodically renewed from the master stock.
• Quality records: Isolate identity, host or soil source, morphology, conidial germination, passage history and insect bioassay results are documented.

C. Mass multiplication of fungal biocontrol agents

Fungal production seeks abundant, viable and infective conidia, blastospores or other propagules suitable for formulation and field application.

• Solid-state fermentation: Sterilized rice, wheat, sorghum or another grain supports aerial conidium production by Beauveria and Metarhizium.
  • Grain moisture, aeration, temperature and incubation time determine yield.
  • Excess moisture promotes clumping, anaerobic zones and contamination.
• Submerged fermentation: Liquid media produce mycelial biomass, blastospores or microsclerotia in stirred tanks.
  • Carbon-to-nitrogen balance, oxygen transfer and pH influence propagule type.
  • Blastospores often grow rapidly but may be less storage-tolerant than dry aerial conidia.
• Biphasic production: Liquid fermentation first generates inoculum, which is then distributed over a sterilized solid substrate for conidiation.
• Harvesting: Mature conidia are separated by sieving, aspiration or mechanical agitation and dried under controlled, non-damaging conditions.
• Formulation: Spores may be mixed with talc, clay, oils, surfactants, ultraviolet protectants or desiccants to improve handling and survival.
• Quality control: Conidial concentration, germination percentage, moisture, contaminant load, particle size, storage stability and insecticidal efficacy are measured.
• Biological constraint: Fungal efficacy depends strongly on humidity, temperature and ultraviolet exposure; high laboratory virulence may not translate directly into field performance.