The nucleotide sequence of the aadB gene which confers resistance to kanamycin, gentamicin, and tobramycin has been determined. The size of the longest reading frame is 747 bases encoding a protein of predicted size 27,992 daltons. A segment of the aadB gene sequence (including the promoter region) was found upstream of the aadA gene in R538-1 and of the dhfrII gene in R388 and the proposed promoters for these genes coincide with the aadB promoter region. The sequence homology extends upstream to the end of the sequenced regions of R388 and R538-1. Almost perfect homology was also found between the sequences 3'- to the aadB gene and 3'- to the aadA genes of R538-1 and pSa. This segment includes a 59 base element previously found flanking the Tn7 aadA gene. A model is presented for the evolution of this region of the plasmid genomes in which the 59- base element functions as an insertional "hot spot" and the possibility that this region is analogous to the aadA/aadB region of the Tn21- like transposon family is considered.

ObjectiveTo review the epidemiology, risk factors for acquisition, clinical features and outcomes of Listeria monocytogenes infection in Sydney. DesignA retrospective study over the period 1983‐1992 at four university teaching hospitals in Sydney. Cases were identified from microbiology laboratory records of the isolation of L. monocytogenes from sterile sites. ResultsEighty‐four cases were reviewed, with 72 patients (86%) having a predisposing underlying condition, including 13 perinatal patients (15%). Septicaemia (56%) and central nervous system disease (41 %) were the major clinical presentations. Nineteen patients (23%) had hospital‐associated infection. A mortality of 21% (18 patients) was directly attributable to L. monocytogenes infection, with another 10% (nine patients) dying of their underlying disease during admission. The 84 cases represented 80% of all L. monocytogenes cases occurring in Sydney during the study period. ConclusionsListeriosis is predominantly a disease of the elderly or of immunosuppressed individuals, pregnant women and neonates. The presentation and outcome in these groups are similar to those reported in other Western countries. A significant feature of this study was the number of cases occurring in already hospitalised patients, suggesting that L. monocytogenes may be an important hospital‐associated pathogen in immunocompromised patients.

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