One of the most commonly encountered types of bacteria is the Gram negative rod-shaped bacillus. These bacteria exhibit a typical Gram negative cell wall structure. Many are known for their pathogenicity - we hear of food poisoning outbreaks caused by E. coli or Salmonella, or epidemics of cholera or typhoid. However, there are numerous harmless species as well, microbes more familiar with leaves and soil and water than with human infections.
How can a researcher tell the difference between scores of species of bacteria that look similar under the microscope? We must turn to the metabolic abilities of the unknown being studied to find its identity. Over the years, scientists have studied the metabolism of countless strains of bacteria and found that an unknown can be identified to species based on the similarity of its metabolic abilities with those of previously studied isolates. They do a battery of biochemical tests on their unknown and compare results with those for known species. If the tests match E. coli, for example, then the identity of the bacterium is E. coli. So choosing appropriate tests for identification and accurately reading and interpreting the results can provide the key information to identify an unknown organism.
In this section, we will walk through the processes used to identify Gram negative enteric bacilli.