Bacteria that give Variable Citrate Test

The citrate test is a chemical analysis which is performed to test whether a micro-organism can  use citrate or its related compounds as its only source of carbon and its only source of energy. This test is also used as a differentiating factor between gram-positive and gram-negative bacillus bacteria. The medium for this experiment is citrate mixed with agar. The medium also contains ammonium salts that are the only source of nitrogen for the medium. The bacteria that can grow in the above-mentioned medium are capable of producing an enzyme called the citrate permease which helps the bacteria to convert the citrate source into pyruvate. This pyruvate then in turn turns into energy by entering the microbe’s metabolic cycle. 

Chemistry Behind The Citrate Test

The citrate is used to identify the ability of a bacteria to utilize citrate as its sole source of energy. If the bacteria is able to utilize citrate, then the colony size of the bacteria increases because growth is indicative of the utilization. These bacteria take up the citrate that is present in the medium and break the ammonium salts down into ammonia. Since ammonia is an alkaline chemical constituent, it increases the overall alkalinity of the medium and hence the bromothymol blue indicator that is present in the medium gives a blue colour. Another scientist called Christensen developed an alternative to the citric acid test in which peptone and cysteine were used instead of a citric acid. Because of these alternate chemical indicators, bacteria that do not use citric acid as its sole source of carbon can also grow on the medium. 

Importance of Variable Citrate Test

The citrate test is performed as a part of the IMVic test series which is used for the differentiation of different strains of various bacterial species. The importance of a variable citrate test includes identification of the members of the Enterobacteriaceae family. The importance of a variable citrate test also includes identifying whether a bacteria can use citrate for the production of energy. It can also be used to differentiate the different E.coli strains by the difference in their citrate consumption.

Bacteria That Give Variable Citrate Test

There is a wide variety of bacteria that can give variable citrate test results when exposed to the citrate test. Some bacteria that use citrate as their only source of carbon give a positive result and the ones who do not give a negative test. There are certain bacteria whose different strains give variable citrate test results. Some examples of these bacteria that give variable citrate tests include Vibrio cholerae, Vibrio parahaemolyticus and Proteus vulgaris. 

Vibrio Cholerae

Vibrio cholerae is a type of gram-negative bacteria. It can be categorized as a facultative anaerobe which means that primarily it respires in the presence of oxygen but in case oxygen is absent it can respire without oxygen as well. It is comma-shaped and they are most commonly found in saltwater. This bacteria can be pathogenic in nature and can attach themselves to the shells of crabs and shrimp and in humans they can cause cholera which is caused by eaten affected raw and undercooked meat. They were first discovered while being attached to a certain protozoa. The bacteria consists of a flagellum and two pairs of circular bacteria. It gives a variable citrate test result.

Vibrio Parahaemolyticus

Vibrio parahaemolyticus is a gram-negative bacteria. It is rod-shaped and slightly curved. This bacteria is mostly found in seawater and can lead to a variety of gastro-intentinal diseases when ingested by humans. This bacteria is motile and consists of a single flagellum. It is facultatively aerobic. Reproduction in this bacteria does not take place with the help of spores. This bacteria can infect humans through their oral and faecal routes. They enter their host through raw or under-cooked seafood. In humans their incubation period can last upto twenty four hours. Some common symptoms of this infection can be severe diarrhoea, nausea and vomiting. It gives a variable citrate test result. 

Proteus Vulgaris

Proteus vulgaris is a gram-negative bacteria that is commonly found in the intestines of humans and other animals. It is a rod-shaped bacteria and it causes the reduction of nitrogen and nitrogen-containing compounds. This bacteria produces hydrogen-sulphide and it is commonly found in soil, water and fecal matter. It is pathogenic in nature. One of the most common diseases caused by this bacteria is the urinary tract infection and it also causes several wound infections that can take very long to recover. It gives a variable citrate test result. 

Conclusion

Hence, it can be concluded that the citrate test is an analysis which is used to identify the citrate consumption of a strain of bacteria. It can also be used for the differentiation between gram positive and gram negative bacteria. Different bacteria can give different results of this test depending on their ability to use the citrate medium as their sole source for the production of carbon which in turn can get converted into energy. There are certain bacteria who give a variable result of the citrate test. This means that certain strains of the bacteria can give a positive test and the others can give a negative test.