Bacterial resistance is, beyond the burden it imposes to public health, a superb example of biological evolution. It is also the consequence of a wide variety of biological, but mostly of non-biological factors: marketing, economics, legislation, and education. Is the acquisition of resistance the only difference between pre-antibiotic era microbes and present-day ones? Several other traits might have been selected during these 60 years. This is not only making our current antibiotic arsenal useless but could be potentially undermining future efforts to control infections through antimicrobial agents. Also, a wide variety of non-antibiotic agents are known to select, co-select, or induce resistance phenotypes. Instead of being a "hospital phenomenon", resistance is perhaps being initially selected by antibiotic use, but then maintained by environmental pressures that we might not even suspect. Furthermore, antibiotic abuse is certainly obeying non-biological conditions. There is an intense economic pressure favoring it, and pharmaceutical companies seem to have lost the interest in the research and development of new antimicrobial drugs. Very few approaches are at hand to deal with infectious diseases in the near future.
Gathering of Resistance Genes in Gram-Negative Bacteria: An Overview
Carlos F. Amábile-Cuevas
In clinically-relevant gram-negative bacteria, antibiotic resistance genes have been maintained from early gram-negative antibiotic-producing bacteria, or horizontally transferred from other types of antibiotic-producing organisms; also, mutations can generate resistance phenotypes. Resistance genes, most likely originating in chromosomes, gained increased mobility by their translocation to plasmids, that can be conjugatively transferred between a wide variety of organisms. The mechanisms enabling the inter-cellular and inter-molecular mobilization of resistance genes also gave rise to the accumulation of resistance determinants, first within a single cell, then within a single genetic element. Along with the direct clinical consequences of this accumulation, multi-resistance plasmids are now maintained by a variety of selective pressures, including some non-antibiotics agents, due to linkage and co-selection. As we released copious amounts of antibiotics into the environment, we induced a shift in the evolution of bacteria and their genetic elements that is likely to make it much more diffucult to cope with the negative effects of bacterial growth in the future.
Evolution of Antimicrobial Multi-Resistance in Gram-Positive Bacteria
The treatment of bacterial infection is increasingly being complicated by the emergence of bacterial strains resistant to a range of agents commonly used to combat them. Among the most problematic in this regard are Gram-positive pathogens such as staphylococci and enterococci. It is now abundantly clear that such organisms possess a toolkit that has allowed them to generate the genetic variation needed to meet the evolutionary challenge that antimicrobial chemotherapy represents. Specifically, through the concerted activities of mobile genetic elements, such as plasmids and transposable elements, and mechanisms of horizontal genetic exchange, these bacteria have assembled arsenals of resistance mechanisms from an extended pool of determinants. Features of specific elements that have played prominent roles in the process will be described, and synergistic interactions between elements will be discussed with due consideration to the significance of selection.
Multiple Resistance Mediated by Individual Genetic Loci.
Bruce Demple and Carlos F. Amábile-Cuevas
In addition to the clustering of single-drug-resistance genes, and to the decreased susceptibility to antibiotics displayed by biofilms, multi-resistance can arise as the consequence of the activation of stress responses, or mutations in regulatory genes governing those responses. Two regulons originally characterized in E. coli but known to be present in other gram-negative bacteria, confer multi-resistance when activated, usually by non-antibiotic agents: marRAB and soxRS. The resistance phenotype is caused by both, decreased permeability of the outer membrane, and by the expression of efflux pumps. Regulatory proteins of both systems share great homology, and the regulons overlap to some extent. Mutations affecting these regulatory genes, resulting in constitutive expression of the regulons, can also confer multi-resistance. Such mutants have been isolated from antibiotic-resistant infections, although the epidemiological relevance of such organisms remains to be established. Other proteins, homologous to MarA or SoxS, when overexpressed, are capable of diminishing the susceptibility to antibiotics; plasmids bearing mar, sox, or homologous genes, are now only laboratory constructs, but could be naturally selected for by several environmental pressures, including but not limited to antibiotics.
Biofilms and Bacterial Multi-Resistance
Peter Gilbert and Alexander H. Rickard
Microbial biofilm has become inexorably linked with man's failure to control them by treatment regimes that are effective against suspended bacteria. This failure has been related to a localised concentration of bacteria and their extracellular products (exopolymers and extracellular enzymes), that moderates the access of treatment agents and starves the more deeply placed cells. Biofilms, therefore present gradients of physiology, and of concentration for the imposed treatment agent, where small sub-populations sometimes survive inimical treatments, and death is generally delayed for the least susceptible cells. Such cells must either possess innate insensitivity to a wide variety of treatment agents or they must adopt resistant phenotypes during the sub-lethal phases of treatment. Sub-lethal exposure to chemical anti-microbial agents has been shown to induce expression of multi-drug efflux pumps and to favour efflux mutants within populations. Since not all anti-microbial agents are substrates for energetic efflux, then this cannot provide a singular explanation of biofilm-resistance. Indeed it is the diversity of action mechanisms within those agents towards which biofilms are resistant that makes singular explanations of resistance phenomenon difficult. Recently a number of concepts have been introduced that impinge greatly upon this area of research. The first of these introduces the concept that sub-lethally damaged micro-organisms undergo apoptosis (suicide) before the levels of damage achieve critical dimension, and that a small proportion of cells within any population is mutated such that they do not. This provides a common mechanism of death for a wide-variety of treatment agents and thereby the possibility of common resistance mechanisms that may be of particular advantage within biofilm communities. The second recognises that materials lost from damaged cells may act as signals, alarmones that induce a less susceptible phenotype in the vicinity of the inimical stress. These and the more classical explanations of the resistance of microbial biofilms will be presented and discussed in the light of up-to-date literature.
Vancomycin Resistant Enterococci and Methicillin Resistant Staphylococcus Aureus
Henry S. Fraimow
Vancomycin-Resistant Enterococci (VRE) and Methicillin-Resistant Staphylococcus aureus (MRSA) have emerged as major nosocomial pathogens with enormous public health significance. VRE initially appeared in the late 1980's. In Europe, emergence of VRE may be due to glycopeptide use in animal feeds and VRE can be found in healthy outpatients, but spread in hospitals is more limited than in the United States. In the U.S., emergence of VRE in hospitals is closely associated with increases in vancomycin use, especially for treatment of MRSA infection, as well as overuse of other antimicrobials. VRE now comprise 10% of hospital enterococcal isolates, with rates up to 25% of isolates from ICU infections. Up to 80-90% of strains of E. faecium may be vancomycin resistant. Risks for colonization with VRE include exposure to vancomycin and other antimicrobials, ICU stay, gastrointestinal manipulations and renal failure. Once patients are colonized with VRE, organisms are shed from the gastrointestinal tract for prolonged periods. This has complicated control efforts and has contributed to VRE becoming endemic in chronic care settings. Fortunately, transmission of vancomycin resistance genes from enterococci to other pathogenic organisms is extremely rare. Treatment of VRE is limited by the intrinsic resistance of enterococci and the development of acquired multi-drug resistance, especially in E. faecium. Several new antimicrobials, including quinupristin/dalfopristin and linezolid, have been developed for treatment of VRE infections. MRSA first emerged as a significant public health problem in the 1980s and are now a global issue, although there is geographic variability in the prevalence of MRSA. Up to 70% of S.aureus in large U.S. teaching hospitals are methicillin resistant, far above the threshold requiring clinicians to empirically use vancomycin for the treatment of suspected staphylococcal infections. These high prevalence rates have also caused a reappraisal of traditional infection control practices designed to limit the spread of MRSA. MRSA are also seen in community acquired infections in intravenous drug users and patients with indirect hospital contacts, but are also now reported in children and adults with no known risk factors. In hospital settings, antibiotic usage remains the major risk for acquisition of MRSA. Treatment options for MRSA infections are limited by frequent multi-resistance. Vancomycin remains the primary treatment option but is associated with a high clinical failure rate and strains with decreased susceptibility to glycopeptides have also been described. New agents such as quinupristin/dalfopristin and linezolid may be useful for treatment of MRSA infections.
Gene transfer between organisms is usually studied from the view of how the transferred genes benefit the organism to which they have transferred. Indeed, many studies of horizontal (lateral) gene transfer are really descriptions of organisms that have survived because of, or despite, past gene transfers. Other studies focus on the biochemical mechanisms of gene transfer. Few studies are about how horizontal gene transfer evolved and is evolving. The conclusions of those studies are emphasized here. Antibiotic use in medicine and agriculture has made an important contribution to our understanding of gene transfer. We will discuss how understanding the evolution of horizontal gene transfer could help in the search for a new generation of agents to treat infectious diseases.