Porphyrin synthesis test explained

The porphyrin synthesis test is used to identify haemin producing Haemophilus species. It avoids the problems of red cell carry over associated with tests for X and V dependence. The porphyrin test is considered to be the definitive method for the differentiation of Haemophilus species.

Haemophilus species are not readily distinguishable by their colonial morphology or gram stain appearance. Oral flora grown from routine sputum cultures often contains organisms which resemble Haemophilus species. H. influenzae, which is the principal human pathogen, can be distinguished from other Haemophilus species and oral flora by determining the need for essential factors for growth, specifically Haemin (X factor) and Nicotinamide-adenine dinucleotide (NAD/V factor). H. influenzae requires both factors for growth where as some of the other species require only one. The requirement for one or both of the growth factors nicotinamide adenine dinucleotide (NAD or V factor) and haemin (X factor) is used to characterise Haemophilus species.

Strains which produce their own haemin possess the enzyme porphobilinogen synthase which can convert d-aminolaevulinic acid (ALA) to protoporphyrin and ultimately haemin.

This test demonstrates the ability of a bacterium supplied with d-aminolaevulinic acid to synthesise and excrete porphobilinogen and other porphyrins, indicating that they are not X dependent.

In relation to the porphyrin synthesis test, Public Health England has issued a technical report, including safety information. The report can be accessed here.

Posted by Tim Sandle