Microbial Culture

Given / Concept: Microorganisms, like all living organisms, need specific nutrients to carry out metabolic activities, grow, and reproduce.

Nutritional Requirements:

1. Carbon source: Microorganisms require carbon as the backbone of all organic molecules. Autotrophs use CO₂; heterotrophs use organic compounds (glucose, starch, etc.).

2. Nitrogen source: Required for synthesis of proteins, nucleic acids, and other nitrogenous compounds. Sources include NH₄⁺ salts, NO₃⁻, amino acids, or atmospheric N₂ (for nitrogen-fixing bacteria).

3. Energy source:
- *Phototrophs* use light energy.
- *Chemotrophs* obtain energy by oxidising chemical compounds (organic or inorganic).

4. Oxygen: Aerobes require O₂ for respiration; anaerobes grow in its absence; facultative anaerobes can grow with or without O₂.

5. Minerals and micronutrients:
- *Macroelements:* Phosphorus (P), Sulphur (S), Potassium (K), Magnesium (Mg), Calcium (Ca), Iron (Fe) — needed in relatively large amounts for structural and enzymatic functions.
- *Micronutrients (trace elements):* Zinc (Zn), Manganese (Mn), Cobalt (Co), Copper (Cu), Molybdenum (Mo) — needed in very small amounts as cofactors for enzymes.

6. Water: Acts as a solvent for all biochemical reactions and is essential for cell turgor and transport.

7. Growth factors: Some microorganisms (auxotrophs) cannot synthesise certain organic compounds (vitamins, amino acids, purines, pyrimidines) and must obtain them from the medium.

Conclusion: The combination of a carbon source, nitrogen source, energy source, minerals, water, and growth factors constitutes the complete nutritional requirement of microorganisms.

Definition of Culture Media:
A culture medium (plural: culture media) is a nutrient preparation — liquid, semi-solid, or solid — that provides all the essential nutrients required for the growth and multiplication of microorganisms under laboratory conditions.

Classification of Culture Media:

A. Based on Chemical Composition:

| Type | Description | Example |
|---|---|---|
| Synthetic (Defined) Medium | All components are chemically known and precisely defined. | Minimal salt medium |
| Complex (Non-defined) Medium | Contains ingredients of unknown or variable chemical composition (e.g., yeast extract, peptone). | Nutrient broth, LB broth |

B. Based on Physical Consistency (State):

1. Liquid (Broth) Medium: No solidifying agent; used for large-scale growth and biochemical studies. *Example:* Nutrient broth.
2. Solid Medium: Contains 1.5–2% agar as a solidifying agent; used for colony isolation. *Example:* Nutrient agar.
3. Semi-solid Medium: Contains 0.5% agar; used to study motility of bacteria. *Example:* SIM medium.

C. Based on Application and Function:

1. Selective Medium: Allows growth of specific microorganisms while inhibiting others. *Example:* MacConkey agar (selects Gram-negative bacteria); medium with Ampicillin (selects resistant strains).
2. Differential Medium: Allows different types of microorganisms to be distinguished by their appearance or reactions. *Example:* Blood agar (differentiates haemolytic from non-haemolytic bacteria).
3. Enrichment Medium: Enriches the proportion of a desired microorganism from a mixed population. *Example:* Selenite broth for *Salmonella*.

Conclusion: The choice of culture medium depends on the type of microorganism, the purpose of the study, and prior knowledge of the organism's habitat.

Concept: When a microorganism is inoculated into a fresh liquid medium and incubated under optimal conditions, its growth follows a characteristic pattern. Plotting the logarithm of the number of viable cells against incubation time gives a bacterial growth curve with four distinct phases.

$$\text{Growth Curve: } \log N \text{ vs. Time}$$

Phase 1 — Lag Phase:
- Immediately after inoculation, there is little or no increase in cell number.
- Cells are metabolically active: synthesising enzymes, RNA, and adapting to the new environment.
- Duration depends on the age of the inoculum and the composition of the medium.

Phase 2 — Exponential (Log) Phase:
- Cells divide at a constant, maximum rate; cell number doubles at regular intervals.
- Growth is logarithmic (exponential).
- Generation time ($t_d$) is shortest and constant during this phase.
- Mathematically: $$N_t = N_0 \times 2^n$$ where $n$ = number of generations.
- Specific growth rate ($\mu$) is calculated as: $$\mu = \frac{2.303(\log N_t - \log N_0)}{t}$$
- Cells are most uniform and physiologically active — ideal for biochemical studies.

Phase 3 — Stationary Phase:
- The rate of cell division equals the rate of cell death; net growth = zero.
- Nutrients become limiting, toxic metabolic waste products accumulate, and space becomes restricted.
- Total viable cell count remains constant.

Phase 4 — Death (Decline) Phase:
- Death rate exceeds growth rate; viable cell count decreases exponentially.
- Accumulation of toxic products (e.g., lactic acid produced by *Streptococci* makes the medium acidic) and exhaustion of nutrients cause irreversible cell damage.
- Some cells may form endospores to survive.

Significance: Understanding the growth curve helps in optimising fermentation processes, determining the best time to harvest cells or products, and studying antibiotic effects.

Definition: A pure culture contains only one type (species) of microorganism, derived from a single cell or colony.

Method 1 — Streak Plate Method:

*Principle:* Diluting a mixed culture progressively across the surface of a solid agar plate so that individual cells are deposited separately and each gives rise to a distinct, isolated colony.

*Procedure:*
1. A loopful of the mixed culture is streaked across one sector (Zone 1) of a sterile agar plate.
2. The loop is flamed and cooled, then streaked from the end of Zone 1 into Zone 2.
3. The process is repeated for Zone 3 and Zone 4.
4. With each successive streak, the number of cells decreases.
5. After incubation, well-separated individual colonies appear in the later zones.
6. A single colony is picked and sub-cultured to obtain a pure culture.

*Advantage:* Simple, quick, and widely used.

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Method 2 — Pour Plate Method:

*Principle:* Serial dilutions of the mixed culture are mixed with molten agar (cooled to ~45°C) and poured into Petri dishes. Upon solidification and incubation, individual cells form isolated colonies both on the surface and within the agar.

*Procedure:*
1. Prepare serial dilutions ($10^{-1}, 10^{-2}, \ldots, 10^{-6}$) of the mixed culture.
2. Add 1 mL of each dilution to a sterile Petri dish.
3. Pour molten agar (~45°C) into each dish and mix gently by rotating.
4. Allow to solidify and incubate.
5. Isolated colonies are picked and sub-cultured.

*Advantage:* Gives a count of viable cells; useful for quantitative analysis.

Conclusion: Both methods rely on physical separation of cells so that each colony originates from a single cell, yielding a pure culture.

1. Sterilisation:
Sterilisation is the process by which all living microorganisms, including highly resistant bacterial endospores, are completely killed or removed from a material or surface.
- It is an absolute process — the material is rendered completely free of all viable microbes.
- Methods include autoclaving, dry heat, radiation, and filtration.
- *Example:* Autoclaving culture media at 121°C, 15 psi for 15 minutes.

2. Disinfection:
Disinfection is the process of eliminating most pathogenic (disease-causing) microorganisms (but not necessarily all spores) from inanimate surfaces or objects using chemical or physical agents called disinfectants.
- It does not guarantee complete sterility.
- *Example:* Treating surfaces with 70% ethanol, bleach (sodium hypochlorite), or phenol.
- Disinfectants are generally too harsh to be used on living tissue.

3. Sanitisation:
Sanitisation is the process of reducing the microbial load on surfaces or objects to a level considered safe by public health standards, without necessarily eliminating all microorganisms.
- It is commonly used in food processing, catering, and public health settings.
- *Example:* Washing utensils with hot water and detergent; using sanitisers on hands.

Summary Table:

| Term | Kills all microbes? | Kills spores? | Used on |
|---|---|---|---|
| Sterilisation | Yes | Yes | Inanimate objects/media |
| Disinfection | Most pathogens | Not always | Inanimate surfaces |
| Sanitisation | Reduces to safe level | No | Food contact surfaces, skin |

Definition: Sterilisation is the complete destruction or removal of all living microorganisms, including spores, from a material.

Methods of Sterilisation:

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A. Physical Methods

I. Heat Sterilisation

(a) Moist Heat (Autoclaving):
- Uses saturated steam under pressure.
- Standard conditions: 121°C, 15 psi (103 kPa), 15–20 minutes.
- Kills all vegetative cells and spores by denaturing proteins and disrupting membranes.
- Used for: culture media, glassware, surgical instruments, biohazardous waste.
- *Tyndallisation (Fractional Sterilisation):* Heating at 100°C for 30 min on three consecutive days; used for heat-sensitive media.

(b) Dry Heat:
- Hot air oven: 160–180°C for 1–2 hours.
- Kills by oxidation of cellular components.
- Used for: glassware, metal instruments, oils, powders (materials that cannot be moistened).

(c) Pasteurisation:
- Not true sterilisation; kills pathogens but not all spores.
- HTST (High Temperature Short Time): 72°C for 15 seconds.
- UHT (Ultra High Temperature): 135°C for 1–2 seconds.
- Used for: milk, fruit juices, beverages.

(d) Incineration:
- Burning at very high temperatures.
- Used for: inoculation loops (flaming in Bunsen burner), biohazardous waste disposal.

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II. Radiation Sterilisation

(a) UV Radiation:
- Wavelength ~260 nm; causes thymine dimer formation in DNA, inhibiting replication.
- Limited penetrating power; used for surface sterilisation of laminar flow hoods, operation theatres.

(b) Ionising Radiation (Gamma rays, X-rays):
- High penetrating power; causes DNA strand breaks and free radical formation.
- Used for: sterilisation of disposable plastic ware, sutures, pharmaceutical products.

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III. Filtration
- Removes microorganisms by physical exclusion through membrane filters (pore size 0.22 µm or 0.45 µm).
- Does not kill microbes; removes them.
- Used for: heat-sensitive solutions such as serum, antibiotics (e.g., Ampicillin), vitamins, enzyme solutions.
- Types: membrane filters, depth filters, HEPA filters (for air sterilisation in laminar flow cabinets).

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B. Chemical Methods

- Chemical agents (sterilants/disinfectants) denature proteins, disrupt membranes, or oxidise cellular components.
- Examples:
- Alcohols (70% ethanol, isopropanol): denature proteins; used for skin and surface disinfection.
- Aldehydes (formaldehyde, glutaraldehyde): cross-link proteins and nucleic acids; used for cold sterilisation of instruments.
- Halogens (chlorine, iodine): oxidising agents; used for water treatment and wound disinfection.
- Ethylene oxide (EtO) gas: alkylating agent; used for sterilising heat-sensitive medical devices.
- Phenolics: disrupt cell membranes; used as laboratory disinfectants.

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Conclusion: The choice of sterilisation method depends on the nature of the material (heat-sensitive or not), the type of microorganism to be eliminated, and the intended use of the sterilised material.

Correct Answer: (b) Prototroph

Justification:
Prototrophs are wild-type microorganisms that can synthesise all the organic compounds they need from simple inorganic sources; they do not require any additional organic supplements (growth factors) in their medium. In contrast, auxotrophs have lost the ability to synthesise one or more essential organic molecules and therefore require those as supplements.

Correct Answer: (b) Robert Koch

Justification:
Robert Koch pioneered the use of solid media (using gelatin and later agar, suggested by Fannie Hesse) for isolating pure cultures by the streak plate method. He was the first to develop the technique of colony purification on solid media, which became fundamental to microbiology and helped establish the germ theory of disease.

Correct Answer: (a) Pasteurisation

Justification:
HTST (High Temperature Short Time — 72°C for 15 seconds) and UHT (Ultra High Temperature — 135°C for 1–2 seconds) are both variants of pasteurisation. They are used to kill pathogenic microorganisms in liquids like milk and fruit juices without complete sterilisation.

Correct Answer: (b) Lazzaro Spallanzani

Justification:
The theory of spontaneous generation (abiogenesis) — the idea that living organisms can arise spontaneously from non-living matter — was strongly advocated and experimentally tested (though not correctly) by Lazzaro Spallanzani. It was later definitively disproved by Louis Pasteur through his swan-neck flask experiments. (Note: Francesco Redi earlier challenged spontaneous generation for maggots, but Spallanzani is specifically associated with the microbial spontaneous generation debate.)

Correct Answer: (d) Louis Pasteur

Justification:
Louis Pasteur proposed the Germ Theory of Disease, which states that specific microorganisms (germs) are the cause of specific diseases. Robert Koch later provided experimental proof through his famous Koch's Postulates, but the original theory was proposed by Pasteur.

Given:
- Initial cell count: $N_0 = 10^4$ cells/mL
- Final cell count: $N_t = 10^7$ cells/mL
- Time of exponential growth: $t = 4$ hours

Formulae Used:

$$\mu = \frac{2.303 \times (\log N_t - \log N_0)}{t}$$

$$t_d = \frac{\ln 2}{\mu} = \frac{0.693}{\mu}$$

Step 1: Calculate Specific Growth Rate ($\mu$)

$$\log N_t = \log(10^7) = 7$$
$$\log N_0 = \log(10^4) = 4$$

$$\mu = \frac{2.303 \times (7 - 4)}{4}$$

$$\mu = \frac{2.303 \times 3}{4} = \frac{6.909}{4}$$

$$\boxed{\mu = 1.727 \approx 1.72 \ \text{hr}^{-1}}$$

Step 2: Calculate Generation (Doubling) Time ($t_d$)

$$t_d = \frac{0.693}{\mu} = \frac{0.693}{1.727}$$

$$\boxed{t_d = 0.401 \approx 0.4 \ \text{h} = 0.4 \times 60 = 24 \ \text{minutes}}$$

Result:
- Specific growth rate: $\mu = 1.72 \ \text{hr}^{-1}$
- Generation time: $t_d = 0.4 \ \text{h}$ or 24 minutes

Correct Answer: (a) Both assertion and reason are true and the reason is the correct explanation of the assertion.

Justification:
In a batch culture of *Saccharomyces cerevisiae*, alcohol (ethanol) is produced as a metabolic end-product of fermentation. As ethanol accumulates in the medium, it reaches a concentration of approximately 13%, which is toxic to the yeast cells themselves. This toxicity inhibits further yeast growth and metabolic activity, causing alcohol production to decline — even when physical conditions like temperature remain optimal. Thus, the reason correctly and completely explains the assertion.

Correct Answer: (a) Both assertion and reason are true and reason is the correct explanation of the assertion.

Justification:
Ampicillin is a heat-sensitive antibiotic. When the medium containing ampicillin is autoclaved (121°C, 15 psi), the high temperature degrades and inactivates the ampicillin. As a result, the medium loses its selective property and both ampicillin-sensitive (amp$^s$) and ampicillin-resistant (amp$^R$) microbes can grow on it. The correct procedure is to sterilise the medium by autoclaving first (without ampicillin), allow it to cool to ~50°C, and then add ampicillin that has been separately sterilised by membrane filtration (0.22 µm filter). This preserves the antibiotic's activity and maintains the selective nature of the medium. Hence, the reason correctly explains the assertion.

Correct Answer: (b) Both assertion and reason are true but reason is not the correct explanation of the assertion.

Justification:
The assertion is true: microorganisms collectively grow over an extremely wide temperature range — from near-freezing temperatures (psychrophiles grow at 0–15°C) to boiling and beyond (extreme thermophiles/hyperthermophiles grow above 80–100°C).

The reason is also true: extreme thermophiles (hyperthermophiles), such as *Pyrolobus fumarii*, can indeed tolerate and grow at temperatures above 100°C (up to ~121°C) due to heat-stable enzymes and membranes.

However, the reason is not the correct explanation of the assertion. The assertion refers to the wide temperature range across *all* microorganisms as a group (from psychrophiles to thermophiles), whereas the reason only mentions one extreme end (hyperthermophiles above 100°C). The ability of extreme thermophiles to survive above 100°C is just one example of this wide range, not the explanation for it. Therefore, option (b) is correct.