Answer to Question #101900 in Microbiology for Ayla

Question #101900
Assume you have just completed three experiments to determine the composition of a substance you recently discovered in the back of your refrigerator (age unknown). The tests you conducted and results were :
Expert's answer

Food spoilage refers to a process in which food items deteriorate to the point in which it is not safe for human consumption.

Some of the causes of food spoilage include bacteria and fungi.

Spoilt food is characterized by a change in food colour, odour and texture implying decomposition has taken place

You cannot tell whether food is contaminated with organisms by just looking at the food. Rather, by carrying out the following tests:

1.Gram staining to identify whether the bacteria is gram-positive or negative rods or cocci.

2.Culture: This entails growing microorganisms in culture media containing specific nutrients and other growth factors in their right concentrations. This enhances isolation and identification of these spoilage organisms based on their colonial appearance in terms of shape, margin, size and texture. Some of the culture media used during the study are; blood agar, chocolate blood agar, MacCkonkey (for bacteria) and savoured agar(for fungi).

The substance was inoculated in the media mentioned above and incubated at 37ºC for 24 hours.

The media were then observed for growth and the organisms grouped based on their morphological characteristics.

Secondary Gram stain was then performed to confirm the identity of the isolated organism based on Gram’s reaction.

3.Biochemical tests: These procedures help to identify food spoilage organisms based on their ability to produce enzymes that catalyze chemical reactions and also able to metabolize given substrates.

These tests include:

1.Kliger’s Iron Agar (KIA) for pseudomonas species.

2.Spirit Blue agar for Bacillus subtilis which can produce lipase enzyme that hydrolyzes triglycerides to glycerol.

3. Starch hydrolysis test to differentiate the species from the genera Clostridium and Bacillus. Bacillus subtilis is positive for starch hydrolysis.


The organisms isolated are pseudomonas fragi, Pseudomonas putida, Bacillus subtilis and Shewanella species

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