The Genus Cardiobacterium

Abstract

The genus Cardiobacterium consists of one species, Cardiobacterium hominis, originally designated “group II D organisms” until further described and named by Slotnick and Dougherty (1964). This organism is a fastidious, facultatively anaerobic, nonmotile, pleomorphic Gram-negative rod with a fermentative type of metabolism.

The first known report of these organisms was by Tucker et al. (1962). This paper described four cases of endocarditis caused by a Pasteurella-like organism. These isolates and two subsequent isolates, also from cases of endocarditis, were designated as “group II D” by the Centers for Disease Control, Atlanta, GA, until the name C. hominis was proposed by Slotnick and Daugherty (1964). The current assumption is that infectious endocarditis is the only disease caused by C. hominis in humans. This assumption may be valid or it may be based on a lack of more complete knowledge of the biology of the organism. Although past isolation of C. hominis has been almost exclusively limited to blood cultures, there have been at least two isolations from cerebral spinal fluid (Slotnick, 1968; Francioli et al., 1983).

Reference is not an exact match Slotnick (1964, 1968), in an attempt to extend our knowledge of the human occurrence of this organism, used fluorescent antibody smears and culture techniques to sample other sites. He and his colleagues isolated C. hominis from nose and throat specimens of 68 of 100 persons sampled, from cervical and vaginal cultures of 2 among 159 studied, and obtained positive fluorescent smears from stool specimens of 14 of 20 individuals. No positive stool cultures were obtained because of a lack of a selective medium and the overgrowth of C. hominis by the enteric bacilli. No isolations or positive smears were obtained from any urine specimens collected (no sample size was given for the urine specimens). These investigators concluded that C. hominis is part of the indigenous commensal respiratory flora and supported this conclusion with the following observations: 1) None of the individuals harboring the organism had any signs or symptoms of disease. 2) Injection of viable suspensions of representative strains into laboratory animals elicited no visible sign of disease. 3) In relation to the high proportion of individuals who harbor the organism, few clinical cases occur. With the exception of these studies, there is a paucity of information concerning the ecology and pathogenicity of these organisms. All recently published information is limited to descriptive case reports of isolation of C. hominis from bacterial endocarditis, with the exception of one case report of C. hominis meningitis (Francioli et al., 1983). There appear to be no published reports on pathogenicity or virulence mechanisms.

Even though C. hominis was originally called a Pasteurella-like organism (Tucker et al., 1962), it was soon discovered that it was antigenically unrelated to members of the genus Pasteurella as well as to the genera Brucella, Bordetella, Moraxella, Hemophilus, Streptobacillus, Corynebacterium, Bacteroides, Neisseria, Escherichia, Aerobacter (Enterobacter) and Lactobacillus (Slotnick et al., 1964). It was compared to both Gram-negative and Gram-positive organisms because of the Gram-variable characteristics noted by early investigators. Because of this anomalous Gram-stain reaction, the fine structure of C. hominis was studied by Reyn et al. (1971). These investigators found that the cell wall was of the Gram-negative type, but an unusual feature of all strains examined was a 20–40-nm thick polar cap. This material was strictly limited to the terminal portion of the cell, adhering to the outside of the cell wall. Profile sections of this cap indicated that it was formed by radial arrangements of tufts of material but subsequent studies employing freeze-etching and negative-staining techniques were unable to further elucidate the nature of this cap material. The other notable feature of the C. hominis cell wall was an unusually dense outer layer. This layer was composed of a repeating structure which consisted of units exhibiting tetragonal or rectangular packing. The periodicity of these arrays measured 5.5 nm; the average diameter of the units was 3.4 nm and the space between them was 2 nm. Surface arrays are more typical of Gram-positive bacteria than of Gram-negative bacteria but are not unknown in the latter (Reyn et al., 1971).

Limited genetic studies have demonstrated that the GC content of the DNA for C. hominis is quite different from the other Gram-negative rod-shaped organisms which have similar physiological characteristics. C. hominis GC is 59–60%, Haemophilus aphrophilus is 42%, Kingella sp. is 47.3–54.8%, Actinobacillus actinomycetemcomitans is 42.7%, Pasteurella sp. is 40–45%, Eikenella corrodens is 56.2–58.2%, and Capnocytophaga sp. is 33–41% (Weaver, 1984).