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Unknown bacteria “F” Lab Report


Laboratory testing plays a significant role in identifying different types of organisms. In order to find a helpful treatment of the diseases occurring in human beings, identifying these microorganisms is important. The biochemical reactions involve in identifying a microorganism helps in providing vital information related to the microorganism. Laboratory testing is also important for understanding the positive and negative aspects of the microorganism.


The purpose of this lab report is to identify the unknown bacteria “F” by incorporating different biochemical tests used in the microbiology lab. These biochemical tests will not only identify but also provide characteristics of this unknown bacteria “F”.

Observation/ Result

Different biochemical tests were performed on unknown “F” to find out the characteristics of this unknown bacteria. The culture of the unknown bacteria “F” was streaked on the EMB (Eosin methylene blue agar), MacConkey, MSA (Mannitol Salt Agar), and SS (Salmonella Shigella agar) plates. On the SS Agar plate, the culture became Colorless, transparent, with a black center.

The following table explains these tests, the purpose of the test, reagents, and observation/ result of the unknown bacteria “F”.

Test Purpose Reagents Observation/Result
Gram Stain To define the chemical structure of the cell wall of the organism by gram reaction
  • Crystal violet
  • Iodine
  • Decolorizer
  • Afranin
  • Gram- Negative
  • Rod Shaped
Mannitol Salt Agar To view if the respective bacteria ferments the mannitol Phenol red (pH indicator)
  • The bacteria did not ferment mannitol
  • colonies are deep red
MacConkey Agar To differentiate lactose fermenting bacteria from lactose non-fermenting gram negative bacteria by isolation Lactose
  • Non lactose fermenting
  • Smooth colonies
Eosin methylene blue agar  (EMB) To distinguish coliforms and fecal coliforms responsible for water contamination Eosin Y

Methylene blue

  • Growth, colorless colonies
Salmonella Shigella (SS) Agar  highly selective medium for isolation of Salmonella species Agar


Ferric Citrate

Sodium Citrate

  • Smooth and opaque or colorless


Gram Reaction

A gram reaction is a chemical method used to differentiate two large groups of bacteria segregated on the basis of their cell wall structure. The cell wall of a bacteria can be stained positive or negative. When the sample of bacteria is combined with gram stain (purple stain), the bacteria will stay purple (gram-positive) or become pink or red (gram-negative).

As the unknown bacteria “F” was smeared on the SS agar plate, the gram stain reaction was performed on the unknown “F” culture from the LB broth tube. The test result of unknown “F” came out to be gram-negative.


After a series of different biochemical tests of this gram-negative bacteria, it is deduced that this unknown bacteria “F” is Salmonella Typhi. The gram stain result showed this bacteria as gram-negative rod-shaped bacteria. When the culture of this bacteria was smeared on the SS Agar plate, the culture became colorless, transparent, with a black center. This observation along with the other biological tests helped to discover this bacteria. All the biochemical tests were carried out well.

Salmonella Typhi is gram-negative bacteria that is responsible for causing typhoid fever. This bacteria affects the intestinal tract and the blood. This bacterial type produces a wide range of infections. It can cause gastroenteritis, septicemia, and life-threatening typhoid fever. Almost all the strains of Salmonella Typhi are pathogenic. The most effective treatment of the infections caused by these bacteria is antibiotics. The most commonly used antibiotics are ciprofloxacin and ceftriaxone (Typhoid).

Works Cited

Typhoid. Accessed 11 Nov. 2021.



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