Testing the susceptibility of Staphylococcus aureus to antibiotics by the Kirby-Bauer disk diffusion method. Antibiotics diffuse out from antibiotic-containing disks and inhibit growth of S. aureus resulting in a zone of inhibition. Photo: CDC, Don Stalons.
The repeated emergence of antibiotic-resistant bacterial strains is a problem that has long plagued public health. Bacteria have always possessed the ability to protect themselves from naturally occurring antibiotics by acquiring resistance through the exchange of genetic material with other bacteria. In the last two decades, however, the problem has escalated as the prevalence of antibiotic-resistant bacteria has increased and multi-drug-resistant strains have emerged in many species that cause disease in humans.
The prognosis is grim. There are no treatments available for infections caused by many of the antibiotic-resistant bacteria, and resistance to commonly used antibiotics is steadily increasing. In fact, no class of drugs with a novel mode of action has been developed since the introduction of nalidixic acid in 1962. Alternative methods to combat antibiotic-resistant bacteria are needed and scientists have begun to search for antimicrobial drugs in vertebrates, invertebrates, and even bacteria and fungi in Earth’s most extreme environments—from Yellowstone National Park’s hot springs to the 120,000-year-old glaciers in Greenland.
Historical timeline of antibiotics
Antibiotics have proven to be a major asset in the fight against infectious bacteria.
Louis Pasteur unknowingly described the first antibiotic in 1877 when he observed that certain bacteria release substances that kill other bacteria.
In 1909, Paul Ehrlich discovered arsphenamine (Salvarsan), an arsenic compound that kills Treponema palladium, the bacterium causing the sexually transmitted disease, syphilis.
In 1928 Alexander Fleming discovered that a mold inhibited the growth of staphylococcal bacteria and named the substance it produced “penicillin” (possibly Pasteur’s unknown substance).
It was not until 1940 that Howard Florey and Ernst Chain isolated the active ingredient in Fleming’s mold.
With wide-scale production of penicillin, the use of antibiotics increased, leading to an average eight-year increase in human life span between 1944 and 1972. Unfortunately, many bacterial species continued to survive penicillin treatment due to their resistance mechanisms.
There is an alarming rise in the occurrence of antimicrobial resistance. For example:
- Staphylococcus aureus is a prevalent bacterium carried by humans that can cause a number of problems, from mild skin infections to serious diseases including food poisoning, wound infections, pneumonia, and toxic shock syndrome. The World Health Organization (WHO) recently reported that more than 95% of S. aureus worldwide is resistant to penicillin, and 60% to its derivative methicillin.
- Today in the U.S. more than 20% of all enterococcal infections, that is, infections caused by intestinal colonizing bacteria in the genus Enterococcus, are resistant to vancomycin, once considered the antibiotic of last resort.
Antibiotics are the third largest selling class of drugs, with an annual market between $7 billion and $22 billion. Current estimates suggest that of this expenditure $4 billion to $5 billion results from antibiotic-resistant bacteria. Although the resistance problem continues to mount, pharmaceutical companies have made little progress in the development of new bactericidal drugs. Consequently, surveillance programs for early detection of multi-drug-resistant bacteria, such as Sentry, have been implemented. Supported by the University of Iowa and private donations, Sentry conducts microbial surveys in 33 nations on 5 continents, gathering over 50,000 samples of various infectious bacteria. Other programs include England’s Alexander Project and programs directed by the Centers for Disease Control and Prevention (CDC) and WHO. Their goal is to explore short- and long-term strategies to combat antibiotic resistance.
Causes of antibiotic resistance
For many years it was believed that antibiotic resistance was only caused by the failure of prescribed drug regimens. It is now accepted that human errors also contribute to the development of antibiotic-resistant bacteria.
- Misuse of antibiotics occurs in medicine, agriculture, and household products. Common examples include erroneous antibiotic prescriptions for nonbacterial infections and the addition of antibiotics to livestock feed and cleaning agents, which have helped create a reservoir of antibiotic-resistant bacteria.
Anomalous combinations have perpetuated drug-resistant microbes. For example, one study on Rhesus monkeys reports that mercury in dental amalgam fillings fostered a 61% increase in antibiotic-resistant bacteria. Upon removal of the amalgam fillings, drug-resistant bacteria dropped 58%. In another example, S. aureus was shown to acquire vancomycin resistance genes through cohabitation with the vancomycin-resistant bacteria, Enterococcus faecalis, in the wound of a hospitalized patient. Through mechanisms of genetic exchange between bacterial species, the mere coexistence of these two particular bacteria helped to bring about drug resistance in S. aureus.
Enhanced transmission of resistance factors, or the increased efficiency with which resistance genes are exchanged, is another important way that antibiotic resistance is perpetuated. Factors that contribute to enhanced transmission include the survival of patients with chronic disease, an increased number of immunosuppressed individuals, substandard hospital hygiene, more international travel, and budget cuts in health care administration.
The reservoir hypothesis suggests that antibiotic-resistant bacteria have evolved because of the selective pressures applied by antibiotic drugs; moreover, the hypothesis states that each antibiotic has a threshold level that is required to induce and maintain antibiotic resistance. After a decline in the populations of susceptible bacteria from antibiotic treatment, naturally resistant bacteria begin to thrive, creating a reservoir of antibiotic-resistant bacteria.
New era of antimicrobial therapeutics
It is a fact that selection of multi-drug-resistant bacteria has occurred throughout history. Unfortunately, however, drug-resistant bacteria have been met with antibiotics that are nothing more than recapitulations of earlier drugs. There has been an urgent need for new avenues of therapeutic treatment, and a new era of prophalytic (preventative) treatment has begun. Here the most plausible approaches are described:
- bacterial interference
- bacteriophage therapy
- bacterial vaccines
- cationic peptides
- cyclic D,L-a-peptides
Bacterial interference, also known as bacteriotherapy, is the practice of deliberately inoculating hosts with nonpathogenic (commensal) bacteria to prevent infection by pathogenic strains. To establish an infection and propagate disease, pathogenic bacteria must find nutrients and attachment sites (adhesion receptors). Infection by pathogenic bacteria is prevented by commensal bacteria, which compete with pathogenic bacteria for nutrients and adhesion receptors or spur attack through secretion of antimicrobial compounds.
This treatment has had promising results in infections of the gut, urogenital tract, and wound sites. The major advantage of using bacteria in a positive way to benefit health, known as “probiotic” usage, is that infection is avoided without stimulating the host’s immune system and decreases selection for antibiotic resistance. Understanding how bacterial species compete, an essential criterion for research, has been known for at least 20 years but its practical application has yet to be realized.
Bacteriophages (commonly called “phages”) are viruses that infect bacteria and were recognized as early as 1896 as natural killers of bacteria. Bacteriophages take over the host’s protein-making machinery, directing the host bacteria to make viral proteins of their own. Therapeutically, bacteriophages were used as a prophylaxis against cholera, typhoid fever, and dysentery from the 1920s to the early 1940s. The practice was abruptly stopped when synthetic antibiotics were introduced after World War II. Now that there is a plethora of multi-drug-resistant bacteria, bacteriophage therapy once again has become of keen interest.
Bacteriophage therapy is quite attractive for the following reasons:
- phage particles are narrow spectrum agents, which means they posses an inherent mechanism to not only infect bacteria but specific strains
- other pathogens may be targeted through manipulation of phage DNA
- exponential growth and natural mutational ability make bacteriophages great candidates for thwarting bacterial resistance
Development of bacterial vaccines has become an increasingly popular idea with the advent of complete genomic sequencing and the understanding of virulence regulatory mechanisms.
Bacterial genomics allows scientists to scan an entire bacterial genome for specific sequences that may be used to stimulate a protective immune response against specific bacterial strains. This approach expedites the drug discovery process and, more importantly, provides a more rational, target-based approach.
The best targets are essential bacterial genes that are common to many species of bacteria, which code for proteins with the ability to gain accesses through lipid membranes, and possess no homology to human genes.
Regulatory genes that control virulence protein production are excellent vaccine candidates for priming the human immune system or inhibiting virulence production.
Bacterial genomics can also detect conserved sequences from bacterial species and strains worldwide. This technology will inevitably yield superior clinical vaccine candidates.
These diverse peptides are natural compounds that posses both hydrophobic and hydrophilic characteristics, which means portions of the molecule are water avoiding or water loving. Cationic peptides are found throughout nature in the immune systems of bacteria, plants, invertebrates, and vertebrates.
These peptides are not the usual synthetic drugs encountered in pharmaceutical drug design; however, they do exhibit antibacterial effects. Cationic peptides have several mechanisms of action, all of which involve interaction with the bacterial cell membrane leading to cell death. From a therapeutic standpoint, these proteins have great promise, as they have coevolved with commensal bacteria yet have maintained the ability to target pathogenic bacteria.
Unlike cationic peptides, cyclic D,L-a-peptides are synthetic and amphipathic (molecules having both water loving and water hating characteristics) cell membrane disruptors. As the name implies these peptides are cyclic in nature and are composed of alternating D and L amino acids. Cyclic D,L-a-peptides are engineered to target gram-positive and negative membranes (not mammalian cell membranes). In contrast to any other known class of peptides, these peptides can self-assemble into flat ring shaped conformations forming structures known as nanotubes, which specifically target and puncture bacterial cell membranes resulting in rapid cell death.
Antibiotic resistance is a continually evolving and dangerous problem that requires immediate attention as well as future planning to impede a global health crisis. Is it not time too seriously consider other methods for which current antibiotic therapies are ineffective and therefore prolong sickness, treatment, an even sometimes result in mortality? Many feel these new alternatives, such as those discussed in this article, are not mainstream. I would agree, but since the efficacy of current therapies is waning and conventional antibiotics are a temporary fix to bacterial multi-drug resistance, society must look elsewhere. If the reservoir hypothesis is true, as most scientists agree, then curbing drug usage to prevent resistant bacteria should be key. Although this viewpoint is highly debated, it holds some merit. Bacteria thrive on mutations and removal of selective pressures should slow mutational rates. Indeed, the alternative methods mentioned have begun to target the pathogen and not the organism.
In addition to current research efforts, the world’s health organizations, such as WHO, CDC, and the Food and Drug Administration (FDA) are building better monitoring systems to detect rising numbers of multi-drug-resistant bacteria. It is not enough, however, physicians and patients must do their part by understanding the ease with which bacteria develop resistance and the consequences of antibiotic misuse.
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