Unit 2: Microbiological Quality Assessment of Organic Manures

I. Orientation: Microbiological Quality of Organic Manures

Microbiological quality assessment determines the abundance, activity, and sanitary significance of microorganisms present in organic manures. Organic manures such as farmyard manure, compost, vermicompost, poultry manure, and green-waste compost contain diverse bacteria, fungi, actinomycetes, and other organisms. Their microbial status reflects the nature of the raw material, degree of decomposition, storage conditions, and handling practices.

• Governing principle: Viable microorganisms in a manure suspension can multiply on a suitable solid culture medium and form visible colonies under defined incubation conditions.

• Total viable count: The total viable count, or TVC, estimates the number of living, culturable microorganisms in a sample and is usually expressed as colony-forming units per gram of manure (CFU/g).

• Colony-forming unit: One CFU represents a viable microbial cell or an aggregate of cells capable of producing one visible colony; it does not necessarily correspond to one individual cell.

• Microbial diversity: Organic manures may contain:

  • Decomposer bacteria such as species of Bacillus, Pseudomonas, and related genera.
  • Fungi involved in cellulose and lignin degradation.
  • Actinomycetes associated with the later stages of compost maturation.
  • Beneficial functional organisms, including nitrogen fixers and phosphate solubilizers.
  • Undesirable organisms introduced through feces, diseased plant residues, water, soil, or poor handling.

• Assessment objective: TVC is used to compare microbial abundance among samples, evaluate processing or storage effects, and detect major changes in the microbiological condition of manure.

• Culture dependence: TVC measures only organisms that grow on the selected medium under the specified temperature, atmosphere, and incubation period. It is therefore an operational count rather than the absolute number of living cells.

• Representative sampling: Because microorganisms are distributed unevenly in solid manure, laboratory results depend on collecting several portions from different locations and combining them into a representative sample.

• Aseptic convention: Sterile containers, diluents, pipettes, media, and instruments are required to prevent external microorganisms from increasing the observed count.

• Result convention: Counts are reported together with the medium and incubation conditions because these variables determine which organisms are recovered.

II. Total Viable Count of Organic Manures — Enumeration by Serial Dilution and Plating

The total viable count method separates microorganisms through serial dilution, distributes a measured volume on or within a nutrient medium, and counts the colonies that develop. The procedure normally includes sampling, preparation of the initial suspension, decimal dilution, inoculation, incubation, colony counting, calculation, and interpretation.

A. Study of the microbial status (total viable count) of organic manures

The microbial status of an organic manure is studied by enumerating its culturable microorganisms under standardized laboratory conditions.

• Sample collection: A representative composite sample should be prepared from multiple portions of the manure mass.

  • Portions are collected from the surface, center, and deeper layers using sterile tools.
  • Wet, dry, compacted, and visibly decomposed areas are included in proportion to their occurrence.
  • The sample is mixed in a sterile container without exposing it unnecessarily to air, dust, or contaminated surfaces.
  • Analysis should begin promptly; prolonged storage can cause microbial growth, death, or changes in community composition.

• Sample description: Important observations recorded before analysis include manure type, age, treatment, odor, color, texture, temperature, moisture condition, and storage history. A recently prepared poultry manure and a mature compost should not be interpreted as microbiologically equivalent materials.

• Initial suspension: A known mass of manure is dispersed in a measured volume of sterile diluent. For example, 10 g of manure added to 90 mL of sterile physiological saline or buffered diluent produces a nominal 10^-1 suspension when the sample volume is treated as part of the total mixture.

• Homogenization: Shaking, blending, or stomaching releases microorganisms attached to manure particles.

  • Insufficient mixing gives a falsely low and poorly reproducible count.
  • Excessively harsh treatment may injure cells.
  • A consistent mixing time, such as several minutes on a mechanical shaker, improves comparison among samples.

• Serial dilution: The initial suspension is diluted stepwise, commonly by transferring 1 mL into 9 mL of sterile diluent.

10^-1 -> 10^-2 -> 10^-3 -> 10^-4 -> 10^-5 -> 10^-6

• Each transfer produces a tenfold dilution.
• A fresh sterile pipette or tip is used at every step.
• Each tube is mixed before the next transfer because cells and manure particles settle rapidly.
• Several dilutions are plated so that at least one plate yields a countable number of colonies.

• Culture medium: A general-purpose medium such as plate count agar or nutrient agar is commonly used for aerobic heterotrophic bacteria. Fungal counts require an appropriate fungal medium and incubation conditions, so bacterial TVC and fungal count should not be treated as interchangeable results.

• Plating methods: The diluted suspension may be inoculated by either of two standard approaches.

1. Spread-plate method: A measured volume, often 0.1 mL, is placed on the surface of solidified agar and spread evenly with a sterile spreader.

   • Colonies develop on the agar surface and are generally easy to observe.
   • The small inoculum volume adds an extra tenfold factor to the calculation.
   • The plate surface should be dry enough to prevent the inoculum from pooling.

2. Pour-plate method: A measured volume, commonly 1 mL, is placed in a sterile Petri dish and mixed with molten agar cooled to approximately 44-47°C.

   • Colonies develop both within and on the surface of the medium.
   • Warm agar may injure heat-sensitive microorganisms.
   • Embedded colonies may be smaller and more difficult to distinguish than surface colonies.

• Replicate plating: Duplicate or triplicate plates reduce the influence of pipetting errors, uneven spreading, and random colony distribution. Replicate results should be reasonably consistent before their mean is used.

• Incubation conditions: Plates are incubated at a defined temperature, duration, and atmospheric condition.

  • Mesophilic aerobic bacteria are commonly assessed near 30°C or 35-37°C, depending on the method’s purpose.
  • Plates are generally incubated in an inverted position to prevent condensation from spreading colonies.
  • The chosen conditions must be reported because a change in temperature or time can produce a different count.

• Colony selection: A countable plate contains enough colonies for reasonable precision but not so many that colonies overlap. A commonly applied working range is approximately 30-300 colonies for bacterial plate counts, although the accepted range may vary with the prescribed method and medium.

• Counting rules: Each distinct colony is counted as one CFU.

  • Spreading colonies should be recorded because they can obscure neighboring colonies.
  • Plates with confluent growth are reported as too numerous to count rather than assigned an estimated exact value.
  • Plates with very few colonies have greater relative sampling error.
  • Colonies differing in size, color, margin, elevation, or texture may be described, but appearance alone does not establish microbial identity.

• Calculation: For a single countable plate, the viable count is calculated from the colony number, dilution, and volume plated.

TVC (CFU/g) = C / (D x V)

C = number of colonies counted
D = dilution plated
V = volume plated in millilitres

• Worked example: Suppose 145 colonies develop after spreading 0.1 mL of the 10^-5 dilution.

TVC = 145 / (10^-5 x 0.1)
    = 145 / 10^-6
    = 1.45 x 10^8 CFU/g

The result is reported as 1.45 x 10^8 CFU/g under the stated medium and incubation conditions.

• Expression on a dry-weight basis: Moisture content can strongly affect comparisons because 1 g of wet manure contains less solid material than 1 g of dry manure. Where required, the wet-weight count can be converted using the dry-matter fraction.

CFU/g dry matter = CFU/g wet sample / dry-matter fraction

If the manure contains 60% dry matter, the dry-matter fraction is 0.60.

• Logarithmic reporting: Large counts may be expressed as base-10 logarithms. A result of 1.45 x 10^8 CFU/g is approximately 8.16 log10 CFU/g. Log transformation facilitates comparison of samples whose counts differ by several orders of magnitude.

B. Interpretation, Quality Control, and Limitations

TVC results are meaningful only when analytical controls, manure characteristics, and the limitations of culture-based enumeration are considered together.

• Microbial activity indicator: A high TVC generally indicates a large population of culturable organisms, often associated with abundant biodegradable organic matter. It does not by itself prove that the manure is mature, beneficial, or free from pathogens.

• Processing effects: Thermophilic composting may initially increase decomposer activity and later reduce mesophilic counts during heating. Subsequent cooling can permit recolonization, so composting history must accompany the numerical result.

• Quality controls: A sterile diluent or uninoculated medium plate serves as a negative control; colony growth on it indicates contamination. Known cultures, where appropriate, can verify that the medium and incubation system support microbial growth.

• Major error sources: Poor sampling, cell clumping, inaccurate pipetting, failure to mix dilution tubes, unsuitable media, incorrect incubation, and counting overcrowded plates can alter the final CFU value by one or more orders of magnitude.

• Culturability limitation: Dormant, injured, slow-growing, anaerobic, or nutritionally demanding organisms may remain undetected. Different organisms may also merge into one colony when attached to the same manure particle.

• Sanitary limitation: General TVC cannot identify fecal indicators or specific pathogens such as Salmonella or pathogenic Escherichia coli. These require selective enrichment, differential media, biochemical identification, immunological methods, or molecular assays.

• Comparative significance: Reliable comparisons require the same sampling basis, diluent, dilution procedure, medium, plating volume, incubation temperature, incubation time, and reporting basis. Without such standardization, a difference in TVC may reflect methodology rather than manure quality.