Unit 2: Isolation, purification and mass multiplications of entomopathogens - Subjective Questions

Entomopathogenic bacteria are bacteria that infect, weaken, or kill insects through the production of toxins, septicemia, or disruption of normal physiological processes.

Important examples include:

• Bacillus thuringiensis (Bt)
• Bacillus sphaericus (currently classified as Lysinibacillus sphaericus)
• Serratia marcescens
• Pseudomonas entomophila

Importance in pest management:

• They provide an environmentally safer alternative to many synthetic insecticides.
• They are generally biodegradable and leave fewer harmful residues.
• Some species and strains are highly specific to particular insect groups.
• They can be integrated with cultural, mechanical, and biological control practices.
• Bt produces insecticidal crystal proteins that become toxic after ingestion by susceptible larvae.
• Their mass production and formulation make them suitable for commercial biopesticide development.

Thus, entomopathogenic bacteria are important components of integrated pest management (IPM) programs.

The isolation of bacterial biocontrol agents from soil involves the following steps:

1. Sample collection: Collect soil from the rhizosphere, insect habitats, cultivated fields, forest litter, or locations with a history of insect disease. Transfer the samples to sterile containers and label them properly.
2. Sample preparation: Remove stones and plant debris. Suspend a measured quantity of soil, such as 1 g, in sterile distilled water or physiological saline.
3. Serial dilution: Prepare successive dilutions of the soil suspension to reduce the microbial population to countable levels.
4. Selective treatment: For spore-forming bacteria such as Bacillus, heat treatment at approximately for 10 minutes may be used to eliminate many non-spore-forming organisms.
5. Plating: Spread suitable dilutions on nutrient agar or a selective medium under aseptic conditions.
6. Incubation: Incubate the plates at an appropriate temperature, commonly -, until colonies develop.
7. Colony selection: Select colonies on the basis of morphology, pigmentation, texture, and other characteristic features.
8. Purification: Repeatedly streak selected colonies on fresh agar plates to obtain pure cultures.
9. Identification and screening: Examine colony characters, cell morphology, staining reactions, biochemical traits, molecular markers, and insecticidal activity.

The final isolate must be confirmed as pathogenic through a controlled insect bioassay.

Naturally infected insects are useful sources of bacterial biocontrol agents because the microorganisms recovered from them may already be adapted to an insect host.

Isolation procedure:

• Collect dead or diseased insects showing symptoms such as flaccidity, discoloration, foul smell, darkening, or internal tissue breakdown.
• Record the host species, developmental stage, locality, date, and symptoms.
• Keep specimens separately in sterile containers to prevent cross-contamination.
• Surface-sterilize the insect briefly with a suitable disinfectant, followed by rinsing with sterile distilled water.
• Dissect the insect aseptically or crush its internal tissues in sterile saline.
• Streak the tissue suspension on nutrient agar or an appropriate selective medium.
• Incubate the plates under conditions suitable for bacterial growth.
• Select distinct colonies and purify them by repeated streaking.
• Characterize isolates using microscopy, staining, biochemical tests, and molecular methods.
• Conduct a bioassay against healthy insects to verify pathogenicity.

A control group must be included in the bioassay. Re-isolation of the same bacterium from experimentally infected insects helps establish that the isolate caused the observed disease.

In both methods, pure culture preparation, identification, insect bioassays, controls, and re-isolation are required before an isolate can be considered a bacterial biocontrol agent.

Purification aims to separate the desired bacterial strain from other microorganisms present in a mixed culture.

Procedure:

• Observe the mixed culture and identify colonies that possess the expected morphology.
• Pick a well-isolated colony using a sterile inoculating loop or needle.
• Streak it over the surface of a sterile agar plate using the quadrant-streak or dilution-streak method.
• Incubate the plate under suitable conditions.
• Select a single, discrete colony and re-streak it on a fresh plate.
• Repeat the process until all colonies show uniform morphology.
• Examine cells microscopically for uniform shape, size, arrangement, and staining reaction.
• Confirm purity through biochemical tests or molecular analysis when required.
• Prepare an agar slant or broth culture from the verified pure colony.

Precautions:

• Work near a flame or in a laminar airflow cabinet.
• Sterilize instruments before and after inoculation.
• Minimize the time for which culture vessels remain open.
• Label cultures clearly and include uninoculated sterility controls.

Purity must be established before pathogenicity testing, maintenance, or mass multiplication.

Bacterial biocontrol agents must be maintained without contamination, genetic change, or loss of virulence.

Short-term maintenance:

• Pure cultures may be maintained on nutrient agar or another suitable agar slant.
• Slants are stored under refrigeration, commonly around .
• Cultures are transferred periodically to fresh medium using aseptic techniques.

Long-term preservation:

• Cryopreservation: Bacterial suspensions are stored at very low temperatures, often with glycerol as a cryoprotectant.
• Lyophilization: The culture is freeze-dried under vacuum and sealed to protect it from moisture and contamination.
• Spore preservation: Spore-forming bacteria may be stored as stable spore preparations under dry and cool conditions.
• Mineral oil overlay: Agar cultures may be covered with sterile mineral oil to reduce dehydration and metabolic activity, although this is less suitable for some organisms.

Quality checks during maintenance:

• Verify purity by plating and microscopic examination.
• Confirm identity by biochemical or molecular methods.
• Test viability by determining growth or viable count.
• Periodically test insecticidal activity against a susceptible host.
• Maintain a master culture separately from routine working cultures.

Reducing repeated subculturing is important because frequent transfers may cause contamination, mutation, or loss of virulence.

Mass multiplication is the large-scale production of viable bacterial cells, spores, toxins, or a combination of these components.

Major steps:

1. Selection of strain: Choose a pure, virulent, stable, and high-yielding strain.
2. Preparation of inoculum: Revive the master culture and prepare a healthy seed culture through one or more inoculum stages.
3. Medium selection: Use a nutrient medium containing suitable carbon, nitrogen, mineral, and growth-factor sources.
4. Sterilization: Sterilize the medium, fermenter, air supply, pipelines, and accessories.
5. Inoculation: Introduce the seed culture aseptically into the production medium.
6. Fermentation: Maintain suitable temperature, pH, aeration, agitation, dissolved oxygen, and incubation time.
7. Monitoring: Measure growth, contamination, viable count, sporulation, toxin production, and substrate utilization.
8. Harvesting: Collect the biomass, spores, or toxin-containing broth at the optimum production stage.
9. Concentration and formulation: Use centrifugation, filtration, drying, carriers, stabilizers, or protective additives as required.
10. Quality control: Test purity, potency, viable count, moisture, shelf life, and insecticidal activity.
11. Packaging and storage: Pack the product in moisture-resistant, properly labeled containers and store it under recommended conditions.

Successful production requires both high yield and consistent biological efficacy.

Submerged fermentation is widely preferred for bacterial products because bacteria generally grow efficiently in liquid culture and the process permits close control. Solid-state fermentation may be useful when inexpensive substrates support good growth or sporulation.

Physical factors:

• Temperature: Influences growth rate, enzyme activity, sporulation, and toxin production.
• pH: Affects nutrient availability, membrane transport, and metabolic reactions.
• Aeration: Supplies oxygen to aerobic organisms and removes metabolic gases.
• Agitation: Distributes cells, nutrients, heat, and oxygen throughout the culture.
• Dissolved oxygen: Must be maintained above the level that limits growth or product formation.
• Incubation period: Harvesting too early reduces yield, while delayed harvesting may reduce viability or potency.
• Foam: Excessive foam may cause contamination and loss of culture, requiring controlled use of antifoaming agents.

Nutritional factors:

• Carbon sources provide energy and cellular material.
• Nitrogen sources support protein and nucleic acid synthesis.
• Minerals such as magnesium, phosphate, iron, and manganese support enzyme activity and sporulation.
• Vitamins and growth factors may improve biomass or toxin production.
• The carbon-to-nitrogen balance influences vegetative growth, sporulation, and metabolite formation.

Optimum conditions should be determined for each strain because conditions that maximize biomass may not necessarily maximize spores or insecticidal toxins.

Quality control ensures that every production batch is safe, stable, and biologically effective.

Important tests include:

• Identity: Confirm the organism using colony morphology, microscopy, biochemical tests, or molecular markers.
• Purity: Plate the product on suitable media to detect bacterial, fungal, or other contaminants.
• Viable count: Determine the number of living cells or spores, commonly expressed as colony-forming units per unit volume or mass.
• Sporulation: For spore-forming bacteria, determine the proportion and concentration of mature spores.
• Potency: Conduct a standardized bioassay against susceptible target insects and record mortality or growth inhibition.
• Toxin content: Measure relevant insecticidal proteins or metabolites when these constitute the active ingredient.
• Moisture content: Excess moisture can reduce shelf life and encourage contamination in dry formulations.
• Physical properties: Assess particle size, suspension stability, wettability, dispersibility, and packaging integrity.
• Shelf life: Monitor viability and efficacy during storage under recommended conditions.
• Safety: Check for undesirable microorganisms and confirm compliance with applicable product standards.

A batch should be released only when all critical specifications are satisfied.

Entomopathogenic fungi are fungi capable of infecting and causing disease in insects. Important examples include Beauveria bassiana, Metarhizium anisopliae, Lecanicillium lecanii, and Isaria fumosorosea.

General infection process:

1. Attachment: Fungal conidia adhere to the insect cuticle.
2. Germination: Under favorable moisture and temperature, the conidium produces a germ tube.
3. Penetration: The fungus penetrates the cuticle through mechanical pressure and enzymes such as proteases, chitinases, and lipases.
4. Internal multiplication: Hyphae or yeast-like bodies multiply in the haemocoel and obtain nutrients from the host.
5. Host death: Tissue destruction, nutrient depletion, physiological disruption, and fungal metabolites contribute to death.
6. External growth: Under humid conditions, the fungus emerges through the cuticle.
7. Sporulation: New conidia form on the cadaver and may spread to other insects.

Unlike many bacterial agents, entomopathogenic fungi usually do not need to be eaten because they can infect through direct contact with the insect cuticle.

The insect-bait method selectively detects entomopathogenic fungi in soil by exposing susceptible insect larvae to the sample.

Procedure:

1. Collect soil from several locations and place each sample in a clean, labeled container.
2. Remove stones and coarse plant material, and adjust the soil to moderate moisture without waterlogging.
3. Place the soil in sterile or clean containers with ventilated lids.
4. Add healthy, susceptible bait insects, such as appropriate laboratory-reared larvae.
5. Incubate the containers under suitable temperature and humidity and turn them periodically so that the insects contact the soil.
6. Inspect the insects regularly and remove dead individuals promptly.
7. Surface-sterilize the cadavers and place them in a humid chamber.
8. Observe fungal growth and sporulation on the insect surface.
9. Transfer conidia or hyphal material from characteristic growth to a suitable fungal medium.
10. Purify the isolate by single-spore isolation or hyphal-tip culture.
11. Identify the fungus and confirm pathogenicity through a controlled bioassay.

Advantages:

• It enriches for fungi capable of infecting insects.
• It may detect fungal propagules present at low densities.

Limitation:

• Results depend on the susceptibility of the bait insect and the incubation conditions.

In serial-dilution plating, fungal propagules are separated by progressively diluting a soil suspension before plating.

Method:

• Add a known mass of soil to sterile water or a suitable diluent containing a wetting agent when necessary.
• Shake thoroughly to dislodge conidia and fungal fragments from soil particles.
• Prepare a series of decimal dilutions under aseptic conditions.
• Spread measured aliquots of selected dilutions onto a suitable fungal isolation medium.
• Add selective agents when appropriate to suppress rapidly growing bacteria or unwanted fungi.
• Incubate the plates at a temperature suitable for entomopathogenic fungi.
• Examine colonies for characteristic color, texture, growth pattern, and sporulation.
• Transfer selected colonies to fresh medium before they come into contact with neighboring colonies.
• Purify isolates through single-spore or hyphal-tip methods.
• Confirm identity by microscopic and, where available, molecular examination.
• Evaluate insect pathogenicity using standardized bioassays.

This method allows direct estimation and recovery of culturable fungal propagules, but rapidly growing saprophytes may overgrow slower entomopathogenic fungi.

Collection and examination:

• Collect insects showing fungal symptoms such as mummification, external mycelial growth, or colored sporulation.
• Record host, location, date, habitat, and visible symptoms.
• Transport each specimen separately to avoid cross-contamination.

Isolation procedure:

1. Examine the insect under a stereomicroscope.
2. Surface-sterilize the cadaver carefully to remove incidental microorganisms.
3. Rinse it with sterile distilled water and dry it on sterile material.
4. Place the whole cadaver in a humid chamber to encourage fungal emergence, or dissect it aseptically to obtain internal tissue.
5. Transfer conidia, a small piece of mycelium, or infected tissue to a suitable agar medium.
6. Incubate under conditions favorable for fungal growth.
7. Transfer characteristic fungal growth to fresh medium.
8. Obtain a pure culture through single-spore isolation or hyphal-tip culture.
9. Identify the isolate using colony characters, conidial morphology, and molecular methods where available.
10. Inoculate healthy insects and re-isolate the fungus from diseased individuals to confirm pathogenicity.

Surface sterilization must be mild enough to remove contaminants without killing the target fungus within the cadaver.

Using both methods provides broader recovery because each method selects a different portion of the soil fungal community.

Single-spore isolation:

• Prepare a dilute suspension of conidia from the mixed fungal culture.
• Spread the suspension thinly on water agar or another suitable medium.
• Observe the plate microscopically and identify isolated germinating conidia.
• Transfer one germinating conidium to fresh nutrient agar using a sterile fine needle or suitable instrument.
• Incubate and examine the resulting colony for uniformity.

This method produces a culture derived from a single conidium and is particularly useful for sporulating fungi.

Hyphal-tip culture:

• Allow the fungus to grow on a suitable agar medium.
• Locate a clean, actively growing hyphal tip at the outer edge of the colony.
• Excise a very small agar block containing the terminal hypha.
• Transfer it to a fresh sterile medium.
• Repeat the procedure if contaminating organisms remain.

This method is useful for fungi that sporulate poorly or when a clean hyphal region is available.

Difference: Single-spore isolation begins from one conidium, whereas hyphal-tip culture begins from a terminal portion of a growing hypha. Purity should be confirmed by colony uniformity and microscopic examination.

Entomopathogenic fungi must be preserved in a way that maintains viability, purity, sporulation, and virulence.

Short-term maintenance:

• Grow the fungus on a suitable agar slant or plate.
• Store cultures under refrigeration after adequate growth and sporulation.
• Transfer cultures periodically using strict aseptic techniques.

Long-term preservation:

• Cryopreservation: Conidia or mycelial fragments are stored at very low temperatures with a suitable cryoprotectant.
• Lyophilization: Suitable fungal propagules are freeze-dried and stored in sealed containers.
• Storage of dry conidia: Well-dried conidia may be stored in moisture-proof containers at low temperature.
• Storage under sterile oil or water: This may be suitable for certain fungal species but must be validated for viability and stability.
• Preservation on colonized substrate: Sporulated grains or other substrates may be stored under controlled dry and cool conditions for limited periods.

Culture-management practices:

• Maintain separate master, seed, and working cultures.
• Avoid unnecessary subculturing.
• Periodically verify morphology and molecular identity.
• Check germination percentage, sporulation, contamination, and insecticidal activity.
• Record the source, passage history, storage conditions, and test results of each isolate.

Passage through the natural or suitable host may sometimes restore virulence, but it must be standardized to avoid selection of inconsistent traits.

Solid-state fermentation is widely used to produce aerial conidia of entomopathogenic fungi on moist solid substrates.

Production process:

1. Strain selection: Select a pure, virulent strain with high sporulation and good storage stability.
2. Substrate selection: Use grains, cereal bran, or another suitable agricultural substrate.
3. Moisture adjustment: Add the correct amount of water and nutrients to support fungal growth without creating waterlogged conditions.
4. Sterilization: Sterilize the substrate and production containers to eliminate competing microorganisms.
5. Inoculum preparation: Produce a pure liquid or conidial seed culture.
6. Inoculation: Distribute the inoculum uniformly through the cooled substrate under aseptic conditions.
7. Incubation: Maintain suitable temperature, aeration, humidity, and incubation time.
8. Sporulation: Provide conditions that encourage formation of abundant infective conidia.
9. Harvesting: Separate conidia from the substrate by sieving, aspiration, or another suitable recovery process.
10. Drying: Reduce moisture carefully without exposing conidia to damaging heat.
11. Formulation: Blend conidia with carriers, oils, wetting agents, or stabilizers.
12. Quality control: Test purity, conidial concentration, germination, moisture, virulence, and shelf life.

Critical problems include uneven temperature, moisture gradients, poor aeration, and contamination within the substrate bed.

In submerged fermentation, entomopathogenic fungi grow in a sterilized liquid nutrient medium inside a controlled fermenter. Depending on the fungal species and culture conditions, the process may produce mycelial biomass, blastospores, submerged conidia, or metabolites.

Process:

• Prepare and sterilize a liquid medium containing suitable carbon, nitrogen, minerals, and growth factors.
• Prepare a pure seed culture and inoculate the fermenter aseptically.
• Control temperature, pH, aeration, agitation, dissolved oxygen, and foam.
• Monitor biomass, propagule formation, contamination, and nutrient use.
• Harvest fungal propagules or biomass at the optimum stage.
• Separate or concentrate the product by filtration or centrifugation.
• Stabilize and formulate the propagules to preserve viability and infectivity.

Advantages:

• Production conditions can be closely controlled.
• Scale-up and automation are comparatively convenient.
• Production cycles may be shorter than solid-state processes.
• The system can generate large amounts of uniform biomass or blastospores.

Limitations:

• Fermenters and downstream processing are costly.
• Some liquid-produced propagules are sensitive to drying, heat, and storage.
• Excessive agitation can damage fungal structures.
• Liquid culture may produce fewer stable aerial conidia than solid-state fermentation.

The choice of process depends on the desired propagule type and intended formulation.

Factors affecting conidial yield:

• Strain: Fungal isolates differ in growth rate, sporulation capacity, and substrate preference.
• Substrate composition: Carbon, nitrogen, minerals, and particle structure influence biomass and conidium formation.
• Moisture: Insufficient moisture restricts growth, while excess moisture reduces aeration and promotes contamination.
• Temperature: The optimum must be maintained because metabolic heat can accumulate during solid-state fermentation.
• Aeration: Oxygen supports growth and sporulation, while poor gas exchange may reduce yield.
• Inoculum quality: A pure, viable, and uniformly distributed inoculum gives more consistent production.
• Incubation period: Harvesting should occur when conidial yield and viability are highest.
• Light and humidity: These may influence sporulation in certain species.

Factors affecting conidial quality:

• Germination percentage and speed
• Virulence against the target insect
• Tolerance to drying and formulation processes
• Resistance to ultraviolet radiation and temperature stress
• Moisture content during storage
• Freedom from microbial contamination
• Genetic and phenotypic stability

High conidial numbers alone do not guarantee an effective product. Conidia must also remain viable, infective, stable, and suitable for formulation and field application.