Catalase test

General information

Many aerobic bacteria and most of those which are facultatively anaerobic produce the enzyme catalase. The function of this enzyme is to detoxify hydrogenperoxide (H₂O₂), which is formed from the superoxide radical by superoxide dismutase. Many aerotolerant anaerobic bacteria have  peroxidase (which is not the same enzyme as cytochrome c oxidase) instead of catalase.  Obligate anaerobic bacteria lack superoxide dismutase and catalase.

Catalase contains a heme group at the active site and it is catalyzing the following reaction with a very high turnover number:

2 H₂O₂ → 2 H₂O + O₂
 

Method

1. Spread the bacteria on an agar plate and inkubate the plate over night (18-24 h) under appropriate conditions.
2. Collect bacteria from one colony with a sterile inoculating loop (of plastic or platinum) and apply the bacteria on a microscope slide.
3. If the bacteria are collected from a blood agar plate, one has to avoid contamination of agar, because hemoglobin also contains heme groups which can cause a false positive reaction.
4. Add one drop of 3% H₂O₂ to the bacteria and observe the suspension. Be careful with the handling of H₂O₂ which is corrosive!!!

• Positive test resultat: Gas formation (O₂) in the form av bubbles shows that the bacterium has a catalase.
• Negative test resultat: No gas formation.

Applications

The catalse test is primarily used for gram positive bacteria and can for instance be utilized to distinguish Staphylococcus spp. and Micrococcus spp., which are catalase positive from Streptococcus spp. and Enterococcus spp., respectively, which are catalase negative.

Updated: 2017-12-20.

Citrate test

General

Some bacteria can utilize citrate as the only carbon source and the citrate test shows if the actual bacterium has this capability.

Method

1. Inoculate a tube containing citrate medium with a small amount of bacteria. It is also possible to streak or perform a deep inoculation into "Simmons citrate tube".
2. Incubate at 30-37ºC during 24-48 h.

• Positive test result: growth in citrate medium or growth with colour change to blue in Simmon's citrate tube.
• Negative test result: no growth in citrate medium eller growth but no colour change (still green colour) in Simmon's citrate tube.

Use

The citrate test is used to distinguish between, among others Citrobacter freundii and Escherichia coli.

Updated: 2018-06-06.

Coagulase test

General

Method

1. Suspend one colony from the suspected pure culture in 0.5 ml of plasma from horse, rabbit or man.
2. Incubate at 37ºC.
3. Read the test after 4 h. If the result is negative (see below), continue with the incubation.
4. Perform the final read after 24 h.
   

• Positive reaction if the plasma coagulates and the coagulate is stable. It must not be dissolved upon stirring.
• Negative reaction if the plasma does not coagulate or if the coagulate is dissolved again upon stirring.

Use

The coagulase test is used to distinguish between Staphylococcus aureus from coagulase negative Staphylococcus spp. Note, however, that some strains of S. aureus can be coagulase negative, but it is unusual. Some strains of S. hyicus and S. intermedius can br coagulase positve. S. pseudintermedius is coagulase positive, but not until after 24 h.

Updated: 2013-10-01.

DNase test

General

Many bacteria have enzymes that break down nucleic acids. The bacteria can then use the resulting nucleotides to build up their own nucleic acids. DNase is such an enzyme, which thus hydrolyzes DNA. Existence of DNase is characteristic for certain species or strains of bacteria and can be used for typing.

Method

Presence of DNase can be determined by cultivation on an agar plate, which contains DNA. If the bacterium has DNase and if the bacteria are allowed to grow over night, the DNA will be hydrolyzed into the constituting nucleotides. Diluted hydrochloric acid (HCl) is then poured onto the plate and there will be a clear zone close to the colonies or the streak, because individual nucleotides are soluble in diluted HCl, but not DNA, which precipitates in the rest of the plate.

Use

The test is useful to distinguish between:
 

• Serratia spp. and Enterobacter spp.
• Staphylococcus aureus (most strains are coagulase positive) and coagulase negative Staphylococcus spp.
• Moraxella catarrhalis and Neisseria spp.

Updated: 2017-12-08.

Hippurate test

General

Some bacteria can hydrolyze hippurate to the amino acid glycine and benzoate by means of the enzyme hippuricase. Glycine can be detected with ninhydrin (2,2-Dihydroxyindane-1,3-dione), which reacts with free amino groups (-NH₂) and a blue product is formed.

Method

1. Suspend a loop-full of bacteria (for instance Campylobacter sp.) in 0.5 ml of sodium hippurate siolution.
2. Incubate the suspensionen at 37ºC during 2 h in a water bath.
3. Then carefully add 0.2 ml of ninhydrin solution without mixing.
4. Incubete the dube during another 10 min. at 37ºC before reading the result.

• Positive test resultat: Deep blue colour.
• Negative test result: Pale blue colour.

Use

The hippurate test is primarely used to distinguish between Campylobacter jejuni  (hip+) and Campylobacter coli (hip-) and to distinguish between different streptococci (see figure).The test is also used, in combination with other methods, to type Brachyspira spp.

Updated: 2017-03-31.

Hydrogen sulfide production

General

Method

1. Perform a deep inoculation in the H₂S medium with bacteria from one colony.
2. Incubate the tube at 30-37ºC during 24-48 h.
   

• Positive test result: a black precipitate in the medium.
• Negative test result: no precipitate in the medium.
  

Use

The test can be used for differentiation of, among other bacteria, certain Campylobacter spp.

Updated: 2012-12-11.

Indole test

Updated: 2017-12-20.

Lecithinase test

General

Method

1. Apply the bacteria in the form of a streak onto the egg yolk agar.
2. Read the plate after 24 h.

• Positive test result: Precipitation around the streak of bacteria.
• Negative test result: No precipitation.

Use

Can among other things be used to differentiate between certain species within the genus Bacillus.

Updated: 2012-12-11.

Mixed acid fermentation

General

Some bacteria can ferment glucose to a mixture of the following organic acids: formic acid, acetic acid and lactic acid. This is called mixed acid fermentation and it causes highly decreased pH in the medium. Mixed acid fermentation can, therefore, be detected by addition of the pH indicator methyl red (MR). The test method is sometimes called the MR test.

Method

1. Suspend a bacterial colony from a pure culture in the MR/VP medium.
2. Incubate at 30°C during 3 days or at 37°C during 48 h.
3. Add 2-3 drops of a solution of methyl red.

• Positive test result: red colour change
• Negative test result: no colour change.

Use

Some members of the family Enterobacteriaceae have mixed acid fermentation (see the respective bacterial page), which can be used to differentiate these bacteria.

Updated: 2017-12-08.

Oxidase test

General information

Bacteria, which have aerobic respiration, often have cytochrome c and a cytochrome c oxidase. The presence of these components can in combination with other methods be used for typing. A commersial test, which contains an artificial electron acceptor (N, N, N', N'-tetramethyl-p-phenylenediamine, see Fig. 1), is often used. This artificial electron acceptor change colour depending upon redox state. The substance is also referred to as a redox indicator and it can be oxidized by the oxidized form of cytochrome c. Cytochrome c oxidase is the last enzyme of the electron transport chain, where it normally reduces oxygen to water and pump protons to the outside according to the following net reaction:

4 Fe²⁺-cytochrome c_red + 8H⁺_in + O₂ → 4 Fe³⁺-cytochrome c_ox + 2H₂O + 4H⁺_out

Cytochrome c oxidase is a transmembrane protein complex (Complex IV), which is also present in the  cytoplasmic (inner) membrane of mitochondria.

Method

1. Keep the plastic ampoule (which contains a glass ampoule) between your fingers and with the opening upwards and away from your face.  Press the ampouls with your fingers until the glass ampoule breaks.
2. Apply two drops of the oxidase reagent onto a piece of filter paper.
3. Transfer bacteria from one colony with a plastic or platinum loop onto the spot with the oxidase reagent. The colonies should have been incubated at the appropriate temperature for 18-24 h .

• Positive test resultat: Dark blue-purple colour change within 10-30 sec.
• Negative test resultat: No colour change or colour change after more than 30 sec.

Note that the oxidase reagent is not stable after that the ampoule has been opened. It may be used for a couple of hours, but eventually it will be oxidized by the oxygen in the air.

  Applications

The oxidase test is used for identification of gram negative bacteria. For instance to identify members of the family Enterobacteriaceae, which are oxidase negative, except members of the genus Plesiomonas (oxidase positive). Members of the family Pseudomonadaceae, and the genera Aeromonas and Campylobacter are oxidase positive.

Updated: 2017-12-20.

Potassium hydroxide test

General

The purpose of the potassium hydroxide test (KOH test) is to identify gram negative bacteria. KOH dissolves the thin layer of peptidoglycan of the cell walls of gram negative bacteria, but does not affect gram positive cell walls. Disintergration of gram negative cell walls lyses the cell and release its contents, including the DNA. The DNA will make the solution very viscous and the solution will stick to the plastic loop when touched. Gram positive bacteria will not be affected by KOH, because they have thicker peptidoglycan layer in the cell wall. Thus, the cells will not be lysed, the DNA not released and no viscosity will be observed.

Material

• Microscopic slide
• Plastic loop
• 3% KOH

Method

• Apply one drop of 3% KOH on a microscopic slide.
• Use o loop to transfer a generous amount of bacteria (cultivated for 24-48 h) to the drop of KOH
• Stir carefully
• The solution of gram negative bacteria will be viscous and form a mucoid string within 30 sec
• Use positive and negative controls

Positive results: The solution with the bacteria (gram negative) will be viscous

Negative results: The solution with the bacteria (gram positive) will not be viscous

Use

The purpose of the KOH test is to quickly distinguish between gram negative and gram positive bacteria as a complement to Gram staining. The test is not useful for anaerobic bacteria.

Updated: 2017-12-20.

Urease test

General

Some bacteria have the enzyme urease, which in the presence of H₂O converts urea (=carbamide) to NH₃ (ammonia) and CO₂ (carbondioxide), which forms ammonium carbonate in the presence of water. See chemical reaction formula.

Method

By growing the bacteria in urease medium containing a pH indicator, it can be determined if the bacteria express urease. If the bacteria have an urease, urea will be converted to ammonium carbonate and the medium will turn alcaline. Thus, the colour will change to red (cerise).

Urease medium:

Applications

The urease test can be used to distinguish between E. coli, which is urease negative, from Proteus spp., which are urease positive.

Updated: 2017-03-31.

Voges-Proskauer (VP) test

General

2 pyruvate + NADH --> 2CO₂ + 2,3-butanediol.

Method

1. Suspend one colony from the pure culture, which is to be investigated, in VP/MR medium.
   
2. Incubate at 30-37ºC during 24-48 h.
3. Add 0.2 ml of 40% KOH and then 0.6 ml of alpha-naphtol solution.
   

• Positive test result: colour change to pink.
• Negative test result: no colour change.
  

Use

Klebsiella spp. and Enterobacter spp. has the capacity to perform butanediole fermentation in contrast to Escherichia coli, Salmonella spp. and Shigella spp.

Updated: 2012-12-11.