X factor, V factor test - Introduction, Characteristics, Procedure, Result, Interpretation

Introduction to X factor, V factor test

X factor (hemin) and V factor (nicotinamide-adenine-dinucleotide, NAD; nicotinamide adenine dinucleotide phosphate, NADP) tests are done for the identification and/or differentiation of Haemophilus spp., especially the important human pathogen Haemophilus influenzae.

The X factor is used to produce essential respiratory enzymes such as cytochromes, catalases, and peroxidase while the V factor is used as an electron carrier in its oxidation-reduction system.

However, not all Haemophilus spp mark both X factor and V factor as essential and can grow in presence of just one factor. Also, some Haemophilus produce lysin which lyses blood to obtain NAD by themselves.

Characteristics of X factor, V factor test

Some species of Haemophilus require both X factor and V factor or one of the two for growth. They fail to grow on supplemented blood agar, which contains X factor but lacks V factor, or other non-selective general media such as Trypticase Soy Agar (TSA), and Nutrient Agar (NA) which lacks both X and V factor.

Haemophilus spp. easily grows on chocolate agar (CA) because, in CA, the blood cells are already lyzed by heat, releasing NAD into the medium.

Another test similar to the X factor and V factor test with an identical principle is the Satellitism test.

Procedure to X factor, V factor test

The procedure for the X factor, and V factor tests are as follows:

1. Make a suspension of the test organism and match it to 1 McFarland turbidity standard. (Careful measures should be taken not to pick any media particles from the previous culture)

2. Using a sterile swab, lawn culture the test microorganism onto the TSA plate

3. Place each disc at least 4 to 5 cm apart, which has been impregnated with X, V, and XV factors, onto the inoculated plate

4. Inbuate at 35-37°C in a CO² incubator for 18-24 hours.

* Instead of discs, strips of filter paper impregnated with the X factor, V factor, or both can also be used.

Result, Interpretation of X factor, V factor test

The result of X factor, and V factor tests can be interpreted as follows:

Negative

The growth of the test organism remains unaltered by the presence/absence of X, V, or XV discs. Also, the suspected microorganism may be able to grow well in the entire NA and TSA media.

Positive

Depending on the species of Haemophilus, colony growth can be observed near the disc containing the X factor, V factor, or XV factor while in the rest of the media, no growth is observed.

The requirements of Hemin, NAD in Haemophilus are as follows:

* (+) = required for growth

* (-) = not required for growth

Although Haemophilus influenzae and Haemophilus haemolyticus both show their dependence on X and V factors, they can be differentiated as Haemophilus haemolyticus shows β hemolysis on blood agar while Haemophilus influenzae are unable to hemolyze the blood cells.