The Role of efflux pumps in the nosocomial pathogens Staphylococcus aureus and Acinetobacter baumannii

Infections caused by opportunistic nosocomial pathogens can complicate the treatment of patients admitted to intensive care units. Most nosocomial bacterial pathogens possess an extended collection of resistance strategies to circumvent the effects of continuous exposure to the antimicrobial stresses present in these environments. Of these, active efflux has proven to be a successful detoxification mechanism employed by both Gram-positive and Gram-negative nosocomial pathogens, such as Staphylococcus aureus and Acinetobacter baumannii, respectively. Bioinformatic analyses of the genomes of these organisms determined that they each encode a large number of putative drug efflux systems including representatives from each of the five main families of efflux systems. Nonetheless, the repertoires of putative efflux systems encoded by these bacteria differ, primarily with respect to the dominant families of protective efflux systems; a trend that is likely to extend to most Gram-negative and Gram-positive bacteria. Various S. aureus transporters were among the first drug efflux systems to be described. Consequently, these proteins have been extensively characterized in work that has significantly advanced our fundamental knowledge of the structure and function of these complex proteins. In contrast, studies of A. baumannii efflux systems have been largely genetic, predominantly focused on explaining the high level of multidrug resistance observed in this bacterium without examining the biochemical or structural nature of the transporters themselves. Regulatory control of the genes encoding drug transporters is of major importance for multidrug resistance in both S. aureus and A. baumannii, since the overproduction of these proteins is typically detrimental to cell growth under non-selective conditions. Accordingly, it is not unusual for these systems to be controlled at a number of levels, both globally and locally by a range of regulatory factors. Here, we compare and contrast the efflux capabilities of these two bacterial pathogens.