Gram negative, small, medium sized coccobacilli or rods, often markedly pleomorphic, sometimes filamentous, non-motile, non-spore forming and non-acid fast.
Capsules can be produced. H.inflenzae, H.parasuis and H.paragallinarum require one or both of two accessory growth factors X and V.
They are aerobic and facultatively anaerobic. The opt.temp.is 370C.
They are nutritionally fastidious, will not grow on nutrient agar and MacConkey agar.
The X and V factors must be supplied for all the Haemophilus species except H.somnus.
The X factor is a heat labile iron-porphyrin-haematin or other haemins.
It is necessary for the synthesis of catalase and other enzymes involved in aerobic respiration.
The V factor is a heat labile factor, present in RBC’s and in many other animal and plant cells.
It is synthesized by fungi and some bacteria ( eg: Staphylococcus aureus).
The V factor can be supplied as co-enzyme I, NAD or NADP+.
It appears to act as a hydrogen acceptor in the metabolism of the cell.
X and V factors requirement of Haemophilus species
Species
X factor
V factor
H. influenzae
+
+
H. aegyptius
+
+
H.parasuis
-
+
H.paragallinarum
-
+
H.parinfluenzae
-
+
H.somnus
-
-
In addition to X and V factors, the growth of the many of the Haemophilus species is enhanced by 10% C02.
Haemophilus grows on blood agar, but growth is scanty, as the V factor is present mainly intra cellularly in red cells.
The chocolate agar is the most suitable medium for isolation of Haemophilus.
In chocolate agar, the V factor is released from the red cell, and the heat stable X factor is still present.
On chocolate agar, H.paragallinarum produces typical dewdrop like colonies and H.somnus shows characteristic yellow tinge colonies.
Sattelitism
When Staphylococcus aureus is streaked across a plate of blood agar on which a specimen containing Haemophilus has been inoculated, after 18-24hrs incubation at 370C under 5-10% Co2, the colonies of the Haemophilus will be large, well developed alongside the streak of Staphylococcus, and smaller farther away.
This phenonmenon is called sattelitism and demonstrate the dependence of Haemophilus on V factor, which is available in high concentration near the staphylococcal growth and only in smaller quantities away from it.
Media supplemented with yeast extract, Levinthals medium (clear transparent media may be prepared by boiling and filtering a mixture of blood and nutrient broth) or Filde’s agar (by adding a peptic digest of blood to nutrient agar) are also suitable for the primary isolation of Haemophilus.
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