Strains of Staphylococcus aureus that are resistant to multiple antimicrobial compounds, including most available classes of antibiotics and some antiseptics, are a major threat to patient care owing to their stubborn intransigence to chemotherapy and disinfection. This reality has stimulated extensive efforts to understand the genetic nature of the determinants encoding antimicrobial resistance, together with the mechanisms by which these determinants evolve over time and are spread within bacterial populations. Such studies have benefited from the application of molecular genetics and in recent years, the sequencing of over a dozen complete staphylococcal genomes. It is now evident that the evolution of multiresistance is driven by the acquisition of discrete preformed antimicrobial resistance genes that are exchanged between organisms via horizontal gene transfer. Nonetheless, chromosomal mutation is the catalyst of novel resistance determinants and is likely to have an enhanced influence with the ongoing introduction of synthetic antibiotics.

We describe a 4.5 kilobase transposon, Tn4001, which mediates resistance to gentamicin, tobramycin and kanamycin in Staphylococcus aureus. Originally detected in plasmid pSK1, Tn4001 was shown to undergo rec-independent transposition to the chromosome from this plasmid and from an inserted derivative of the plasmid pII147. Heteroduplexes between plasmids with and without Tn4001 demonstrated a characteristic stem and loop structure with inverted repeats of approx. 1.3 kilobases.

Nosocomial infections caused by Staphylococcus aureus strains resistant to methicillin and multiple antibiotics have reached epidemic proportions in Melbourne, Australia, over the past 5 years. Plasmid analysis of representative clinical isolates demonstrated the presence of three classes of plasmid DNA in most strains. Resistance to gentamicin, kanamycin, and tobramycin was usually mediated by an 18-megadalton plasmid but could also be encoded by a related 22-megadalton plasmid. Two distinguishable plasmids of 3 megadaltons each endowed resistance to chloramphenicol, and the third class consisted of small plasmids, each approximately 1 megadalton in size, with no attributable function. An extensive array of resistance determinants, including some which have usually been associated with a plasmid locus, were found to exist on the chromosome. Evidence that resistance to gentamicin, kanamycin, and tobramycin is chromosomally encoded in some clinical isolates suggests that this determinant may have undergone genetic translocation onto the staphylococcal chromosome.

The Staphylococcus aureus plasmid pSKI carries Tn4001, a 4.7-kilobase (kb) transposon which specifies resistance to gentamicin, tobramycin, and kanamycin. In addition, pSKl mediates resistance to trimethoprim and linked resistance to ethidium bromide (Ebr) and to quaternary ammonium compounds (Qar). Restriction endonuclease analysis of pSK1 and a deleted derivative of pSK1 revealed that the gene(s) responsible for Ebr Qar lies within a 5.2-kb Hindlll fragment. This fragment has been cloned into the Escherichia coli plasmid vector pBR322, and transformants of an E. coli K-12 strain exhibited Ebr Qar. Subcloning of the 5.2-kb insert, combined with data from electron microscopic analysis of deleted derivatives of pSK1, located the Ebr Qar determinant(s) on a 2.3-kb segment of pSK1 DNA.

Interactions between Gossypium spp. and the bacterial pathogen Xanthomonas campestris pv. malvacearum are understood in the context of the gene-for-gene concept. Reviewed here are the genetic basis for cotton resistance, with reference to resistance genes, resistance gene analogs, and bacterial avirulence genes, together with the physiological mechanisms involved in the hypersensitive response to the pathogen, including production of signaling hormones, synthesis of antimicrobial molecules and alteration of host cell structures. This host-pathogen interaction represents the most complex resistance gene/avr gene system yet known and is one of the few in which phytoalexins are known to be specifically localized in HR cells at anti-microbial concentrations.