The periodontal pathogen Actinobacillus actinomycetemcomitans produces a leukotoxin that is considered a primary virulence factor in localized juvenile periodontitis (LJP). Select strains of the bacterium contain a 530‐bp deletion in the promoter region of the leukotoxin gene operon which results in enhanced transcription of the leukotoxin. DNA hybridization and polymerase chain reaction (PCR) were used to examine genetic variants of A. actinomycetemcomitans in 24 LJP‐susceptible children from 21 families having a history of the disease and 34 control children from non‐LJP families. A significant association was found between the detection of variants that had a deletion in the leukotoxin promoter region, indicative of a high level expression leukotoxin genotype, and conversion from a healthy periodontal status to disease. Subjects harboring A. actinomycetemcomitans of this genotype were more likely to convert to LJP than those subjects who had variants containing the full length leukotoxin promoter region (odds ratio = 22.50, 95% C.I.). These findings support the concept that highly virulent strains or clonai types of periodontal pathogens play a major role in the initiation of periodontal disease in susceptible hosts. J Periodontol 1998;69:998–1007.

A genetic locus for a cytolethal distending toxin (CDT) was identified in a polymorphic region of the chromosome ofActinobacillus actinomycetemcomitans, a predominant oral pathogen. The locus was comprised of three open reading frames (ORFs) that had significant amino acid sequence similarity and more than 90% sequence identity to the cdtABC genes of some pathogenicEscherichia coli strains and Haemophilus ducreyi, respectively. Sonic extracts from recombinant E. coli, containing the A. actinomycetemcomitans ORFs, caused the distension and killing of Chinese hamster ovary cells characteristic of a CDT. Monoclonal antibodies made reactive with the CdtA, CdtB, and CdtC proteins of H. ducreyi recognized the corresponding gene products from the recombinant strain. CDT-like activities were no longer expressed by the recombinant strain when an ΩKan-2 interposon was inserted into the cdtA andcdtB genes. Expression of the CDT-like activities inA. actinomycetemcomitans was strain specific. Naturally occurring expression-negative strains had large deletions within the region of the cdt locus. The cdtABC genes were flanked by an ORF (virulence plasmid protein), a partial ORF (integrase), and DNA sequences (bacteriophage integration site) characteristic of virulence-associated regions. These results provide evidence for a functional CDT in a human oral pathogen.

Highly morphogenic callus cultures were isolated from stamens of a wild peanut species, Arachis paraguariensis. These cultures were initiated on modified N6 medium containing 0.2 mg 1-4amino-3,5,6-trichloro-picolinic acid (picloram) and 0.5mgl -l 6-benzylaminopurine (BAP) and were maintained on modified N6 medium with 0.008 mg 1-' picloram and 0.25 mg 1-i BAP. Buds formed on the calli growing on the maintenance medium developed into shoots when they were transferred to a MS salts based medium with no hormones. The cultures could also be maintained as a suspension culture in N6 liquid medium. When cell clumps larger tham 840/zm were collected from the suspension culture and transferred to MS medium without hormones, they formed shoots in liquid culture. Root formation rarely occurred in agar or liquid cultures. Therefore, grafting to stems of rooted seedlings was used to obtain plants from regenerated shoots. Eight out of 50 field grown plants produced viable seed.

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