The genus Haemophilus consists of gram-negative, pleomorphic coccobacilli or rods that may vary microscopically from small coccobacilli in direct smears of clinical material to long filaments occasionally seen in stained smears of colony growth. They are nonmotile and facultatively anaerobic, ferment carbohydrates, are generally oxidase and catalase positive, reduce nitrates to nitrites, and are obligate parasites on the mucous membranes of humans and animals.
According to Bergey’s Manual of Systematic Bacteriology10species of Haemophilus are associated with humans:
H. influenzae, H. parainfluenzae, H. haemolyticus, H. parahaemolyticus, H. aphrophilus, H. paraphrophilus, H. paraphrohaemolyticus, H. aegyptius, H. segnis, and H. ducreyi. Most members of the genus are nonpathogenic or produce opportunistic infections. The emphasis of this section is on the major pathogenic species: H. influenzae, H. aegyptius, and H. ducreyi.
The genus name Haemophilus is derived from the Greek words meaning “blood-lover.” As the name implies, Haemophilus organisms require preformed growth factors present in blood: X factor (hemin, hematin) and/or V factor (nicotinamide-adenine dinucleotide [NADD. Traditionally, a small, gram-negative bacillus (coccobacillus) is assigned to this genus based on its requirements for X and/or V factor. Species of Haemophilus with the prefix para only require V factor for growth. The production of hemolysis on 5% horse or rabbit blood agar is an important differential characteristic.
Although certain species are also hemolytic on sheep blood agar (SBA), the organisms will not grow in pure culture on this medium.
Both X and V factors are found within red blood cells. Most laboratories use SBAprepared by commercial sources. Only X factor is directly available in this medium. Haemophilus species that are V factor dependent do not grow because the red blood cells are still intact and the sheep red blood cells contain enzymes (NADases) that hydrolyze V factor. To alleviate this problem, most clinical laboratories use chocolate agar (CHOC) to facilitate the recovery of Haemophilus spp.
from clinical specimens. The lysing of the red blood cells by heat in the preparation of CHOC releases both the X and the V factors and inactivates NADases.
A phenomenon that helps in the recognition of Haemophilus spp. that require V factor is satellitism.
Satellitism occurs when an organism such as Staphylococcus aureus, Streptococcus pneumoniae, or Neisseria spp. produces V factor as a by-product of its metabolism.
The Haemophilus isolate obtains X factor from the SBA and V factor from one of these organisms mentioned previously. On SBA plates, tiny colonies of Haemophilus may be seen growing or engaging in satellitism around the V factor-producing organism.
Figure 19-2illustrates H. influenzae satellitism around colonies of S. aureus. Except for H. aphrophilus and H. ducreyi, all clinically significant Haemophilus spp.
require V factor for growth and display this unusual growth pattern.
The indigenous flora of the healthy upper respiratory tract consists of many different genera and species of organisms (see Chapters 2 and 32). Approximately 10% of this normal bacterial flora in adults consists of
Haemophilus spp., with the majority of the organisms being H. parainf/uenzae and nonencapsulated H. inf/uenzae. Of the two, H. parainf/uenzae is thepredominant species. Colonization begins in infancy with encapsulated strains and average 2% to 6% throughout childhood. In selected populations, such as children who attend daycare centers, colonization may reach as high as 60%. Nonencapsulated strains of H. influenzae in healthy children average 2% of the normal