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SAARS: Family Pasteurellaceae (microbiology)

The Family Pasteurellaceae contains several genera which at least superficially seem unrelated, but all can be found as normal flora of the upper respiratory tract and/or oral mucosa of humans and animals. On Gram stain all are small, pleomorphic Gram-negative rods or coccobacilli (as is the anaerobic genus Bacteroides), and when such organisms are seen on direct Gram stain from clinical specimens, these organisms should be considered. Clinically speaking, the most important members of the family are Haemophilus influenzae and Pasteurella multocida and, to a much lesser extent, H. aegyptius, H. ducreyi, and Actinobacillus actinomycetemcomitans.

Pasteurella spp.

Of the six species of Pasteurella mentioned here, only P. multocida is isolated with any frequency as a human pathogen. All the species can cause endemic diseases among animals including fowl cholera, swine plague, rabbit septicemia, and "shipping fever" and hemorrhagic septicemia in cattle. Sporadic cases of human infection of any of these species usually are associated with animal bites or scratches.

Pasteurella multocida

That most human infections with Pasteurella multocida are subcutaneous wounds which result from the bite or scratch of a dog or cat is not surprising since P. multocida is found in the normal upper respiratory tract (URT) of dogs and cats (most human cases follow cat bites). Specimens which sample animal bites should suggest to the clinical microbiologist that P. multocida may be present.

Gram-stained organisms tend to be small, coccobacillary, Gram-negative rods with bipolar staining suggesting safety pins (see discussion of Yersinia pestis). The organism typically grows as small, smooth, somewhat mucoid non-hemolytic colonies on sheep blood agar after 48 hours of incubation. Colonies may run along the streak line. P. multocida fails to grow on MacConkey agar and is both oxidase- and indole-positive.

P. pneumotropica
P. haemolytica
P. ureae
P. aerogenes
P. gallinarum

Haemophilus spp.

All members of the genus Haemophilus ("blood-loving") are small, nonmotile, pleomorphic Gram-negative rods which require factors present in blood for growth. Some species require exogenous X-factor (probably a group of tetrapyrrole compounds including hemin and hematin) for synthesis of protoporphyrin from delta-aminolevulinic acid (ALA). Some species require V-factor (nicotinamide adenine dinucleotide {NAD, coenzyme I} or NAD phosphate {NADP, coenzyme II}). Both X- and V- factors can be found in sheep blood, which also contains enzymes which inactivate V-factor. However, V-factor-destroying enzymes are denatured when sheep blood agar is heated to produce chocolate agar, so Haemophilus spp. with a requirement for V-factor (including H. influenzae) can grow on chocolate agar but not on blood agar.

Other bacteria (e.g., Staphylococcus aureus) may produce V-factor, so occasionally V-factor-requiring Haemophilus spp. may be observed forming small satellite colonies around these other larger colonies on sheep blood agar.

Haemophilus spp. can be recovered from many mammals and birds. In humans they are known as commensals and pathogens of the upper respiratory tract except for H. ducreyi, which causes the sexually-transmitted disease called chancroid. Generally speaking, the judgement as to the pathogenicity of any isolate of H. influenzae relies heavily on the quality of the specimen (e.g., sputum versus transtrachael aspirate), the direct Gram stain of the specimen (relative numbers of polymorphonuclear white blood cells, epithelial cells, and small Gram-negative rods), the relative numbers of organisms growing on primary isolation media, and clinical feedback from physicians.

All Haemophilus spp. are susceptible to desiccation and cold temperatures, so specimens which have been refrigerated may be compromised.

Haemophilus species other than H. influenzae are uncommonly isolated from human infections except as contaminating normal flora.

Haemophilus influenzae

Haemophilus spp. are uncommonly recovered from blood or wound cultures, and rarely as an agent causing endocarditis (unlike H. aphrophilus, H. paraphrophilus, and H. parainfluenzae). At the present time, in the clinical microbiology laboratory H. influenzae is most frequently recovered from respiratory specimens, not uncommonly mixed with other potential pathogens, and it may be an important agent of pneumonia, especially in older adults with underlying diseases (e.g., alcoholism, COPD, carcinoma of the lung). Pneumonia does not commonly occur in young adults except for those with similar underlying deficiencies.

H. influenzae biotype III (H. aegyptius)

Localized outbreaks of "pink eye" are associated with shared towels or other objects that come in contact with the skin/face/eyes. The diffuse pink discoloration of the sclera with a purulent discharge is practically diagnostic of Haemophilus conjunctivitis.

H. parainfluenzae

H. parainfluenzae is a member of the normal flora of the upper respiratory tract that is seldom implicated in a pathogenic process.

H. haemolyticus

H. haemolyticus is indigenous to the human mouth that is rarely implicated in human disease.

H. parahaemolyticus

H. parahaemolyticus is indigenous to the human mouth that is rarely implicated in human disease.

H. aphrophilus

H. aphrophilus is indigenous to the human mouth. As a pathogen it is most commonly recovered from the blood of patients with endocarditis. Rarely it has been recovered from other wounds, including human bite wounds.

H. paraphrophilus

H. paraphrophilus is indigenous to the human mouth. As a pathogen it is most commonly recovered from the blood of patients with endocarditis.

H. segnis

H. segnis has been recovered from dental plaque and the URT. It may play a role in certain cases of acute appendicitis.

H. ducreyi

The causative agent of chancroid, a highly contagious sexually-transmitted disease characterized by painful genital and perianal ulcers with tender inguinal lymphadenopathy, H. ducreyi is found worldwide and is endemic in South America, Africa, and Asia. Chancroid is associated with poor hygiene and rarely occurs in developed countries. Gram stains made from genital lesion exudate and lymph node drainage may reveal tiny Gram-negative coccobacilli arranged in groups referred to as a "school of fish" appearance.

This organism can be quite fastidious in culture, producing small gray, yellow, or tan colonies which may not closely resemble more familiar Haemophilus colonies. H. ducreyi is catalase-negative and weakly oxidase-positive using the tetra-methyl oxidase reagent.

Actinobacillus spp.

Actinobacillus actinomycetemcomitans

The difficult name of this organism is due to the fact that Actinobacillus actinomycetemcomitans has often been recovered in association with the anaerobic Gram-positive rod Actinomyces israelii, particularly from submucosal or subcutaneous suppurative lesions. Furthermore, A. actinomycetemcomitans has been reported as the sole isolate in several cases of endocarditis. The organism is also an important agent of invasive periodontal disease. Due to cultural and biochemical similarities to H. aphrophilus, the suggestion has been proposed to reassign at least this species to the genus Haemophilus.

These bacteria are small, coccoid Gram-negative organisms, becoming increasingly bacillary with age. Colonies are small, translucent, pulvinate, and non-hemolytic. They tend to be adherent to the agar and are difficult to emulsify. In broth the colonies tend to adhere as a flaky film to the inner surface of the tube.

Act. equuli

Can cause suppurative lesions of the skin and lungs, primarily in sheep.

Act. lignieresii

Causative agent of "woody tongue" in cattle.