MLS-resistance determinants in Staphylococcus aureus and their molecular evolution.

This paper describes the genetic phenomenology of resistance to macrolide-lincosamide-streptogramin B antibiotics (MLSr) in Staphylococcus aureus and attempts to place this phenomenology in a broad evolutionary context. As antibiotic resistance in general and MLS resistance in particular are typical variable traits in bacteria of clinical interest, we shall begin by introducing the concept of variable genetic traits, as outlined in Figure 1. Variable traits are those that are expressed by some strains of a given species but not by others--in comparison to constant traits which are always present as part of the standard genetic make-up of the species and have constant chromosomal locations. Variable traits are often associated with variable and mobile genetic elements and it is suggested that, in general, they are not likely to have evolved as such in the species in which they are found. Rather, they will most probably have evolved as constant (chromosomal) traits in other species and acquired genetic mobility much later as a rare occurrence in that species. These rarely occurring mobile variants would then spread horizontally within a range of new species. The MLSr determinants in Gram-positive bacteria would appear to represent a classic example of this process. Their remarkable variability will be described as the extant end-point of the process and a probable evolutionary pathway will be traced back to the streptomycetes which are a likely primary source.