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Isolation and screening of amylase producing bacteria from sewage water and soil receiving kitchen waste

This study conducted at Debre Berhan University, Ethiopia, successfully isolated and screened amylase-producing bacteria from sewage water and kitchen waste-contaminated soil, revealing that 50% of the ten bacterial isolates were amylase producers, with a higher prevalence (60%) found in soil samples compared to sewage water (40%).

Original authors: Shumate Kebede Hirpa

Published 2026-07-03

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Shumate Kebede Hirpa

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine you are a treasure hunter, but instead of gold, you are looking for tiny, invisible factories inside the ground and water that can make a very specific tool: an enzyme called amylase.

Think of amylase as a pair of microscopic scissors. Its only job is to cut up starch (like the stuff in bread, potatoes, and kitchen leftovers) into smaller, usable pieces. While these scissors exist in plants and animals, the paper explains that bacteria are the best "scissor-makers" because they are cheap to grow, work fast, and are tough enough to handle heat.

The Hunt: Where did the hunters look?

The researchers, led by Shumate Kebede Hirpa from Debre Dawa University in Ethiopia, decided to look for these bacterial "scissor-makers" in two very specific, messy places:

Soil that has been fed kitchen waste: Imagine a patch of dirt where people have been tossing their vegetable peels and food scraps. This soil is full of hungry bacteria trying to digest that food.
Sewage water: The wastewater flowing out of the university, which carries all sorts of organic gunk.

They collected samples from these two spots in March and June 2017.

The Method: How did they find them?

The scientists used a technique called the "Serial Dilution Agar Plate Method."

The Soup: They took a spoonful of soil or water and mixed it into a jar of sterile water, then took a drop of that and mixed it into another jar, and so on. This is like diluting a very strong cup of coffee until it's just a hint of flavor, just to make sure they don't get too many bacteria in one spot.
The Petri Dish: They poured this diluted "soup" onto special plates (agar) that contained starch (like a giant piece of bread).
The Wait: They let the bacteria grow for 24 hours at body temperature (37°C).
The Magic Test: After the bacteria grew, they poured a blue liquid (iodine) over the plates.
- If the bacteria didn't make scissors (amylase), the blue liquid would turn the whole plate dark blue (like a blueberry muffin).
- If the bacteria did make scissors, they would have cut up the starch around them. When the blue liquid hit that spot, there was no starch left to turn blue, leaving a clear circle (a "halo") around the bacteria. This clear circle was the "X marks the spot" for the treasure.

The Results: Who won the treasure hunt?

The researchers found 10 different bacterial colonies in total.

5 came from the kitchen-waste soil.
5 came from the sewage water.

When they tested them with the blue liquid:

50% of the total bacteria (5 out of 10) were successful "scissor-makers."
The Soil Winners: 60% of the bacteria found in the kitchen-waste soil could make amylase. (3 out of 5).
The Water Winners: 40% of the bacteria found in the sewage water could make amylase. (2 out of 5).

The Conclusion

The paper concludes that both the soil eating kitchen scraps and the sewage water are excellent places to find bacteria that can produce amylase. In fact, half of the bacteria they found were capable of this job.

The authors suggest that these specific bacteria could be used as a source for making industrial amylase. However, they stop short of saying exactly how to use them yet. They simply recommend that the next step is to run more tests to identify exactly what kind of bacteria these are (looking at their shape, how they react to chemicals, and how they live) before they can be fully utilized.

In short: The researchers proved that if you look in the dirt where food waste rots or in the university's dirty water, you can find a 50/50 chance of finding bacteria that are naturally equipped to cut up starch, making them potential candidates for industrial use.

Technical Summary: Isolation and Screening of Amylase-Producing Bacteria from Sewage Water and Soil Receiving Kitchen Waste

Problem Statement and Context
Industrial enzymes, particularly hydrolytic enzymes, constitute approximately 75% of global industrial enzyme production, with amylases accounting for roughly 30% of this volume. While amylases are ubiquitous in animals, plants, bacteria, and fungi, bacterial and fungal sources are preferred in industrial applications due to their shorter production times, cost-effectiveness, consistency, and thermo-stability. Specifically, bacterial amylases are favored for their ease of process optimization and high production rates compared to other organisms.

Despite the global market for industrial enzymes being valued at $68 billion in 2020 with a projected growth rate of 3.3%, Ethiopia relies heavily on imports for industrial needs, such as brewing, which consumes significant quantities of malt annually. This study addresses the need to minimize reliance on imported enzymes by identifying local sources of amylase-producing bacteria. The research focuses on utilizing waste-rich environments—specifically soil receiving kitchen waste and sewage water—as potential reservoirs for these microorganisms.

Methodology
An experimental study was conducted at Debre Berhan University, Ethiopia, between March and June 2017. The research employed a serial dilution agar plate method to isolate and screen bacteria from two specific sample types:

Soil Samples: Collected purposively from soil receiving kitchen waste at a depth of 5 cm.
Sewage Water Samples: Collected from sewage water near a wastewater treatment plant at the university.

Isolation and Purification:

Soil: 10g of soil was dissolved in 90ml of sterile distilled water and serially diluted from $10^{-1}$ to $10^{-6}$ . A pour plate method was utilized, where 0.1ml of dilutions was spread on nutrient agar plates and incubated at 37°C for 24 hours. Five well-grown colonies from the $10^{-4}$ dilution were purified via sub-culturing.
Sewage Water: 1ml of water was serially diluted from $10^{-1}$ to $10^{-7}$ . Samples from $10^{-5}$ to $10^{-7}$ were spread on nutrient agar plates and incubated at 37°C for 24 hours. Five colonies from the $10^{-6}$ dilution were purified.

Screening for Amylase Activity:
All purified isolates were screened using a starch hydrolysis test. Isolates were streaked onto nutrient agar plates containing 1% starch and incubated at 37°C for 24 hours. Post-incubation, plates were flooded with iodine solution. A clear zone surrounding the bacterial growth (indicating the absence of the blue starch-iodine complex) confirmed amylase production, while the absence of a clear zone indicated non-producers.

Key Results
From a total of 10 bacterial isolates (5 from soil and 5 from sewage water), 50% (5 isolates) were identified as amylase producers. The distribution of these producers was as follows:

Soil Receiving Kitchen Waste: 3 out of 5 isolates (60%) were amylase producers.
Sewage Water: 2 out of 5 isolates (40%) were amylase producers.

The specific isolates identified as producers were s3, s4, s5 from the soil samples and w3, w5 from the sewage water samples. These findings align with some previous studies (e.g., Parmar & Pandya, 2012) while differing in yield from others (e.g., Tole et al., 2016), suggesting variability in isolation success based on source and methodology.

Significance and Claims
The study concludes that sewage water and soil receiving kitchen waste are viable sources for isolating amylase-producing bacteria. The authors claim that these waste-derived environments can serve as effective reservoirs for the production of amylase enzymes, potentially contributing to local enzyme production and reducing import dependency.

The paper maintains a modest scope regarding its findings. It does not claim to have fully characterized the specific bacterial species or optimized production conditions. Instead, it explicitly recommends further characterization studies, including morphological, biochemical, and physiological tests, to fully identify and validate the potential of these isolates for industrial application.

The Hunt: Where did the hunters look?

The Method: How did they find them?

The Results: Who won the treasure hunt?

The Conclusion

Technical Summary: Isolation and Screening of Amylase-Producing Bacteria from Sewage Water and Soil Receiving Kitchen Waste

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