Question

what is the identity and the name of this organism?

what are the colony characteristics and how are they identified based on the results?

5 Мас KIA SIM Urease Gram stain Streak on BHI PR Lactose Triptone 10 um

Answer 1

1. Gram Staining: The organism is a gram negative rod (seen larger due to higher magnification) as depicted by the Gram staining. They have lower peptidoglycan content, which allows the decolorizer to remove crystal violet stain. Thus, safranin can stain the cells pink.

BHI test: Brain heart infusion agar is a nutrient medium that allows the growth of many organisms due to presence of brain heart infusion, peptone and glucose. There is no sheep blood added in this BHI medium (lack of red color in the medium). The organism can grow on BHI medium.

MacConkey’s Test: MacConkey’s agar inhibits the growth of gram positive bacteria due to presence of crystal violet and bile salts. It differentiates between lactose fermenting and non-fermenting enteric bacteria based on ability to ferment lactose. Lactose fermenters produce acid which makes the neutral red gain a red color. Hence, lactose fermenters will give pink colony while non-fermenters are colorless. The bacterium shown is lactose fermenting enteric bacteria due to pink colonies seen on MacConkey’s agar.

KIA test: Liger’s Iron Agar tests the ability of the bacterium to ferment glucose and lactose to form acid and gas. It also identifies sulfate reducing bacteria by presence of black precipitate. As the KIA tube shows yellow color with no breaks in agar and blackening, it is a lactose fermenter that does not produce H2S or Gas. If H2S was produced, a black precipitate would have been seen. If the organism cannot utilize lactose, it will use amino acids after glucose is exhausted to produce ammonia. This makes the medium alkaline and thus, medium turns red. However, this organism is lactose fermenter (acid turns the indicator yellow) with no gas or H2S.

SIM test: SIM (Sulphates, indole motility test) detects the H2S production, motility and ability to produce indole. Black coloration in medium detects the presence of H2S production. Indole production from tryptophan is detected by presence of red ring coloration upon addition of Kodak’s reagent. Motility is detected by growth all over the tube.

There is no red coloration or black precipitate observed. However, there is dispersed growth all over the tube. Hence, this organism is motile with no indole or H2S production seen.

Urease test: Urease is an enzyme that breaks down urea to ammonia and CO2. This makes the medium alkaline, turning the phenol red to pink. As there is no pink coloration observed, this organism is urease negative enteric organism.

Try tone test: The medium contain 1% try tone. Tryptophan’s enzyme in bacteria can be break down try tone to pyruvate ammonia and indole. Indole reacts with Kodak’s reagent to give red ring. As there is no red ring formed, this organism lacks tryptophan’s enzyme.

PR lactose: PR Lactose medium is lactose medium that has phenol red indicator. Lactose fermenters can break down lactose to acid, which turns phenol red yellow. This organism is a lactose fermenter as the medium turn yellow. There is no gas seen in Durham’s tube.

The organism is Enterbacter aerogenes.

E. aerogenes is gram negative motile rod which belongs to Enterobactericiae family. It is lactose fermenting enteric bacterium as detected by MacConkey’s test and KIA test. It is urease and indole negative with H2S production. Although E. aerogenes is usually gas positive, many times there is no gas produced. This organism is not E.coli as it is indole negative. E.coli is urease negative. Klebsiella pneumonia, another lactose fermenting enteric bacteria is non-motile and urease positive. Serratia marcescens are lactose non-fermenters. Citrobacter are slow lactose fermenters but have H2S production.

2. Colony characteristics are the morphology of the colonies on growth medium. They include, shape, color, elevation, texture, height edge. Each colony represents one bacterium. The colony characteristic s of E. aerogenes can be identified by growth of individual colonies in BHI and MacConkey’s agar plate. E. aerogenes forms smooth, irregular round mucoid colonies. They are rod shaped bacilli as per the gram staining from the colonies. Their size is 0.6-1 microns.