Unit 1: Introduction to laboratory techniques and media preparations - Subjective Questions

Biopesticides are pest-management agents derived from living organisms, their products, or naturally occurring substances. They suppress insect pests through pathogenicity, toxicity, repellence, growth regulation, or behavioral modification.

Major characteristics:

• They are generally target-specific and cause less damage to natural enemies, pollinators, and other non-target organisms.
• Many are biodegradable and leave fewer harmful residues in food, soil, and water.
• Microbial biopesticides include entomopathogenic bacteria, fungi, viruses, nematodes, and protozoa.
• Biochemical biopesticides include pheromones, attractants, repellents, and insect growth regulators of natural origin.
• Their performance may be influenced by temperature, humidity, sunlight, and pest developmental stage.
• They are important components of integrated pest management, where they are combined with cultural, mechanical, biological, and need-based chemical methods.

A biorational pesticide is a pest-control agent that is effective against a target pest but has relatively low adverse effects on humans, beneficial organisms, and the environment when used correctly.

Distinction:

• Biopesticides:
  • Derived specifically from microorganisms, plants, animals, or naturally occurring biological substances.
  • Examples include Bacillus thuringiensis, Beauveria bassiana, nucleopolyhedroviruses, neem products, and entomopathogenic nematodes.
• Biorational pesticides:
  • A broader functional category based on selectivity, low non-target toxicity, and environmental compatibility.
  • It includes many biopesticides as well as certain selective synthetic compounds, insect growth regulators, pheromones, soaps, and oils.

Thus, many biopesticides are biorational, but not every biorational pesticide is necessarily of biological origin.

A well-designed biopesticide laboratory should contain separate areas to minimize contamination and ensure safe workflow.

Essential sections and facilities include:

• Media-preparation room: Balances, pH meter, hot plate, magnetic stirrer, water purification system, and glassware.
• Sterilization area: Autoclave, hot-air oven, disinfectants, and waste-treatment facilities.
• Aseptic transfer room: Laminar airflow cabinet or biosafety cabinet, ultraviolet lamp, spirit lamp or micro-incinerator, and sterile tools.
• Culture room: Incubators, biological oxygen demand incubators, shaker incubators, and controlled-temperature storage.
• Microscopy section: Compound microscope, stereomicroscope, counting chamber, and staining equipment.
• Insect-rearing facility: Environmental chambers, cages, artificial diets, and separate rooms for healthy and infected insects.
• Cold-storage facility: Refrigerators, deep freezers, and cryopreservation equipment where required.
• Quality-control area: Instruments for checking purity, viability, spore or cell concentration, virulence, moisture, and contamination.
• Documentation and safety facilities: Laboratory records, labels, personal protective equipment, emergency equipment, and restricted-access storage.

Laboratory rules and biosafety precautions:

• Wear a laboratory coat, gloves, closed footwear, and eye protection whenever required.
• Do not eat, drink, smoke, or mouth-pipette in the laboratory.
• Label all cultures with organism name, strain, date, medium, and operator details.
• Disinfect benches before and after work using an appropriate disinfectant such as ethanol.
• Perform culture transfer in a laminar airflow cabinet or other suitable containment system.
• Minimize aerosol production while vortexing, pipetting, or opening culture vessels.
• Keep pure cultures separate from field samples and contaminated materials.
• Autoclave used media, cultures, disposable plates, and infected insect material before disposal.
• Wash hands after handling cultures and before leaving the laboratory.
• Report spills immediately and disinfect them according to the laboratory spill-response procedure.
• Maintain inventory, access, and incident records.

These practices protect the worker, preserve culture purity, and prevent accidental release of microorganisms.

A culture medium is a nutrient preparation used to isolate, cultivate, identify, maintain, or multiply microorganisms under controlled laboratory conditions.

Classification by physical state:

• Liquid or broth medium: Contains no solidifying agent and is used for biomass production or enrichment.
• Semisolid medium: Contains a low concentration of agar and may be used for motility studies or specialized cultivation.
• Solid medium: Usually contains about – agar and supports colony development and isolation.

Classification by function:

• Basal or general-purpose media: Support non-fastidious organisms.
• Enriched media: Basal media supplemented with blood, yeast extract, or other nutrients.
• Enrichment media: Usually liquid media that favor multiplication of a desired organism.
• Selective media: Contain conditions or inhibitors that suppress unwanted microbes.
• Differential media: Reveal visible biochemical differences among organisms.
• Maintenance or preservation media: Help retain viability and characteristics during storage.
• Production media: Designed to obtain a high yield of cells, spores, toxins, or other active components.

Major medium ingredients and their functions:

• Water: Acts as the solvent and must be clean and free from inhibitory contaminants.
• Carbon source: Sugars such as glucose or dextrose supply energy and cellular carbon.
• Nitrogen source: Peptone, tryptone, yeast extract, or inorganic nitrogen supports protein and nucleic-acid synthesis.
• Mineral salts: Supply phosphorus, sulfur, magnesium, potassium, iron, and other essential elements.
• Growth factors: Vitamins, amino acids, blood components, or yeast extract support organisms with complex requirements.
• Buffers: Phosphate or other buffering systems resist undesirable pH changes.
• Sodium chloride: Helps maintain osmotic balance in many bacterial media.
• Agar: A relatively inert solidifying agent that is generally not degraded by most microorganisms.
• Selective agents: Antibiotics, dyes, salts, or specific chemicals suppress competing microorganisms.
• Indicators: pH or biochemical indicators help differentiate microbial reactions.
• Antifoaming agents: May be added during liquid fermentation to control excessive foam.

Procedure:

1. Read the formulation and calculate the quantities required for the desired final volume.
2. Clean the workspace and assemble chemicals, distilled water, balance, flask, pH meter, agar, and dispensing vessels.
3. Weigh the ingredients accurately and dissolve soluble components in approximately – of the final water volume.
4. Add agar and heat carefully with mixing until it dissolves completely.
5. Adjust the pH to the required value and then make up the final volume with distilled water.
6. Dispense the medium into suitable flasks or tubes, leaving adequate headspace.
7. Close the vessels with caps or plugs and label them appropriately.
8. Sterilize, commonly by autoclaving at and approximately psi gauge pressure for a validated holding time, often about 15 minutes for small volumes.
9. Cool molten agar to approximately –.
10. Add separately sterilized heat-sensitive supplements aseptically, if required.
11. Pour the medium into sterile Petri dishes under aseptic conditions and allow it to solidify.
12. Check representative uninoculated plates for sterility, label the batch, and store under specified conditions.

Potato dextrose agar (PDA) is a carbohydrate-rich medium widely used for cultivating fungi such as Beauveria, Metarhizium, Lecanicillium, and Isaria/Cordyceps species.

Typical composition per litre:

• Potato infusion prepared from approximately g peeled potatoes
• Dextrose: g
• Agar: – g
• Distilled water: to L

Preparation:

1. Cut and boil the potatoes in water, then filter the infusion.
2. Dissolve dextrose and agar in the filtrate with heating.
3. Make the volume up to L and adjust the pH, commonly to about when appropriate.
4. Dispense and sterilize using a validated autoclave cycle.
5. Cool and pour into sterile plates aseptically.
6. If bacterial suppression is required, add a suitable filter-sterilized antibacterial supplement only after cooling and according to laboratory protocols.

Uses:

• Primary cultivation and maintenance of entomopathogenic fungi
• Observation of colony morphology and pigmentation
• Isolation from infected insects after surface disinfection
• Production and assessment of fungal conidia

Potato dextrose agar (PDA):

• Contains potato infusion, dextrose, and agar.
• Supports broad fungal growth and is useful for observing colony morphology and pigmentation.
• Its natural potato infusion may show some batch-to-batch variation.

Sabouraud dextrose agar (SDA):

• Commonly contains peptone, a relatively high concentration of dextrose, and agar.
• Its acidic pH and high carbohydrate content favor fungi over many bacteria.
• It is widely used for routine fungal isolation and cultivation.

Sabouraud maltose agar with yeast extract (SMAY):

• Contains peptone, maltose, agar, and yeast extract.
• Yeast extract supplies vitamins and additional nitrogenous nutrients.
• It often supports good vegetative growth and sporulation of several entomopathogenic fungi.

Comparison:

• PDA is a useful general fungal medium.
• SDA is nutritionally defined more consistently and favors many fungi.
• SMAY is nutritionally richer and may improve growth or conidial yield, although the best medium depends on the fungal species and strain.
• Medium selection should be validated using colony growth, sporulation, viability, purity, and virulence.

Nutrient broth is a general-purpose liquid medium, while nutrient agar is the corresponding solid medium containing agar.

Typical components:

• Peptone as a nitrogen and amino-acid source
• Beef extract or yeast extract as a source of vitamins and growth factors
• Sodium chloride for osmotic balance
• Agar in nutrient agar for solidification
• Distilled water

Preparation:

1. Dissolve the measured ingredients in distilled water.
2. Adjust the pH to the value required by the organism, commonly near neutrality for many bacteria.
3. Add and dissolve agar if a solid medium is required.
4. Dispense into flasks or tubes and sterilize using a validated autoclave cycle.
5. Cool and pour nutrient agar aseptically, or use the sterile broth directly for inoculation.

Uses:

• Nutrient agar supports colony isolation, purity checking, and morphological observation.
• Nutrient broth supports rapid multiplication and preparation of bacterial inoculum.
• Both may be used for preliminary cultivation of entomopathogenic bacteria, although specialized production or sporulation media may be required for organisms such as Bacillus thuringiensis.

Bacillus thuringiensis is an aerobic, spore-forming entomopathogenic bacterium that produces insecticidal crystal proteins during sporulation.

Media:

• General media such as nutrient agar or Luria–Bertani agar can be used for recovery and routine cultivation.
• Sporulation media should provide balanced carbon, nitrogen, and mineral salts to support both spore and crystal formation.
• Acetate-based selective media may aid isolation by exploiting differences in spore germination, but presumptive colonies must be confirmed.

Isolation considerations:

• Soil, insect cadavers, stored products, and plant surfaces may be sampled.
• A heat-treatment or selective enrichment step can reduce non-spore-forming bacteria.
• Serial dilution and plating are used to obtain discrete colonies.
• Pure isolates are examined by microscopy for vegetative cells, spores, and parasporal crystals.

Cultivation and confirmation:

• Maintain adequate aeration in broth cultures because oxygen availability affects growth and sporulation.
• Evaluate colony purity, sporulation, crystal production, viable count, and insecticidal activity.
• Identification should not rely only on colony appearance; biochemical, microscopic, molecular, or bioassay evidence may be required.

Entomopathogenic viruses are obligate intracellular parasites. They lack the complete metabolic and protein-synthesis machinery required for independent multiplication. Therefore, nutrient agar, fungal media, and ordinary cell-free broths cannot support viral replication.

Appropriate multiplication methods:

• In vivo production: Susceptible host larvae are infected orally, by injection, or by another appropriate route. Diseased larvae are collected, homogenized, filtered, and purified to recover viral occlusion bodies.
• In vitro production: Suitable insect cell lines are grown in sterile cell-culture media and infected under controlled conditions.

Important considerations:

• A susceptible host species, developmental stage, and correct virus strain are essential.
• Strict asepsis is needed to prevent bacterial and fungal contamination.
• Viral identity and concentration may be assessed using microscopy, molecular tests, or infectivity bioassays.
• In vivo production is often simpler, whereas cell culture offers greater process control but requires specialized facilities and is more expensive.

Aseptic technique consists of practices that prevent unwanted microorganisms from entering cultures, samples, media, or the working environment.

Key practices:

• Disinfect the work surface before and after handling cultures.
• Use sterile media, pipette tips, loops, forceps, Petri dishes, and culture vessels.
• Perform transfers in a laminar airflow cabinet or suitable biosafety cabinet.
• Open plates and tubes only for the shortest possible time.
• Avoid talking, coughing, or making rapid movements over exposed cultures.
• Sterilize inoculating tools before and after use and allow them to cool before touching the culture.
• Use separate sterile instruments for different isolates.
• Keep field samples, pure cultures, and waste physically separated.
• Include uninoculated sterility controls where appropriate.

Importance:
Aseptic technique prevents false identification, competition by contaminants, altered sporulation, unreliable bioassays, loss of pure cultures, and accidental environmental release.

• Sterilization: Complete destruction or removal of all forms of microbial life, including resistant spores. An example is autoclaving culture media and contaminated glassware using a validated cycle.
• Disinfection: Elimination of many or all pathogenic microorganisms on inanimate objects, but not necessarily all bacterial spores. An example is treating a workbench with an appropriate disinfectant.
• Antisepsis: Application of an antimicrobial chemical to living tissue. An example is cleaning the skin with alcohol before an injection.
• Sanitization: Reduction of microbial numbers to a level considered acceptable for public-health or operational purposes. An example is routine cleaning and sanitizing of laboratory floors and washable surfaces.

The terms are not interchangeable. Culture media must normally be sterilized, whereas benches are generally disinfected and hands or skin are treated with antiseptics.

Autoclaving:

• Uses saturated steam under pressure.
• Suitable for culture media, aqueous solutions, dressings, and heat-resistant contaminated waste.
• It is unsuitable for moisture-sensitive materials and may damage heat-labile compounds.

Hot-air sterilization:

• Uses dry heat at a higher temperature and for a longer time than steam sterilization.
• Suitable for dry glassware, metal instruments, powders, and certain oils.
• It is not appropriate for most prepared culture media.

Membrane filtration:

• Physically removes microorganisms from heat-sensitive liquids.
• Commonly used for antibiotic solutions, vitamins, and other thermolabile supplements.
• It does not necessarily remove all viruses, toxins, or very small particles unless an appropriate validated filter is used.

Ultraviolet treatment:

• Reduces surface and airborne microbial contamination in exposed areas.
• Has poor penetration and is affected by distance, dust, shadows, and lamp condition.
• It is an adjunct to cleaning, not a reliable substitute for sterilizing culture media or instruments.

The pH of a culture medium influences nutrient solubility, membrane transport, enzyme activity, microbial growth, sporulation, toxin production, and the suppression of contaminants. Fungi often tolerate mildly acidic media, whereas many entomopathogenic bacteria grow well near neutral pH.

The relationship is expressed as:

Adjustment and verification:

1. Calibrate the pH meter with appropriate standard buffers.
2. Measure the medium at the temperature specified in the procedure because pH readings vary with temperature.
3. Add dilute acid or alkali gradually while stirring.
4. Recheck the pH after thorough mixing.
5. Make up the final volume only after the ingredients have dissolved.
6. Where important, verify the pH after sterilization because heating can alter it.
7. Record the target pH, measured value, temperature, and any adjustment made.

Excessively concentrated acid or alkali should be avoided because local extremes can damage nutrients or cause precipitation.

The scaling factor is:

Each ingredient is multiplied by .

• Dextrose:
  

• Agar:
  

• Peptone:
  

Therefore, L of medium requires 50 g dextrose, 37.5 g agar, and 25 g peptone. The ingredients should first be dissolved in less than the final volume of water. After dissolution and pH adjustment, distilled water is added to obtain a final volume of exactly L.

In general:

where is the original ingredient mass, is the required mass, and and are the original and desired volumes.

Isolation procedure:

1. Select a cadaver showing signs suggestive of fungal infection and record its host, location, date, and symptoms.
2. Transport it in a clean, labeled container while avoiding excessive moisture and cross-contamination.
3. Examine the specimen under a stereomicroscope.
4. Surface-disinfect the cadaver using a validated sequence appropriate to the insect and fungus, followed by sterile-water rinses.
5. Blot the cadaver dry with sterile material.
6. Under aseptic conditions, transfer internal tissue or a small amount of characteristic sporulation to a suitable fungal medium such as PDA, SDA, or SMAY.
7. Include a sterility control, such as plating the final rinse when appropriate.
8. Incubate under temperature, light, and humidity conditions suitable for the suspected fungus.
9. Observe plates regularly and transfer a hyphal tip or a well-separated colony to fresh medium.
10. Obtain a pure culture through repeated subculture, single-spore isolation, or another suitable purification method.
11. Identify the isolate using colony features, microscopic structures, and, when necessary, molecular methods.
12. Confirm entomopathogenicity by a controlled bioassay and re-isolation, consistent with Koch's postulates.

Common causes:

• Incorrect sterilization temperature, time, pressure, or load arrangement
• Use of contaminated water, chemicals, glassware, or supplements
• Prolonged exposure of sterile media during pouring
• Poor hand hygiene or improper aseptic transfer
• Unclean airflow cabinets, incubators, pipettes, or work surfaces
• Addition of non-sterile heat-sensitive supplements
• Damaged filters, loose closures, or cracked culture vessels
• Cross-contamination caused by aerosols or shared instruments
• Condensation dripping onto agar surfaces

Corrective and preventive measures:

• Quarantine and discard contaminated batches safely after decontamination.
• Verify autoclave performance with physical, chemical, and biological indicators as scheduled.
• Clean and disinfect equipment according to standard operating procedures.
• Filter-sterilize heat-sensitive supplements and add them aseptically after cooling.
• Reduce plate exposure time and use sterile tools for every culture.
• Include uninoculated medium controls.
• Inspect water quality, reagent condition, filters, and container integrity.
• Train personnel and document recurring contamination patterns.
• Investigate the likely source before preparing a replacement batch.

Quality-control tests include:

• Appearance: Check color, clarity, gel consistency, depth, precipitation, dehydration, cracks, and excessive surface moisture.
• pH: Verify that the final pH lies within the specified range, preferably after sterilization when required.
• Sterility test: Incubate representative uninoculated containers and observe them for microbial growth.
• Growth-promotion test: Inoculate the medium with a known reference or working culture and confirm expected growth.
• Selectivity test: For selective media, verify that the target organism grows while specified non-target organisms are inhibited appropriately.
• Productivity and morphology: Confirm acceptable colony size, pigmentation, sporulation, or other expected characteristics.
• Gel strength: Ensure solid media have an appropriate agar concentration and remain firm under incubation conditions.
• Batch documentation: Record the formulation, lot numbers, preparation date, pH, sterilization cycle, test results, storage conditions, expiry date, and preparer's identity.

A medium should be released for use only after it meets predetermined acceptance criteria.