Antibiotic Resistance: A Review on Different Microorganisms, Varying Perspectives, and Control
By Adam Currie, Laila Larachi, and Kenny Schonberger
Presented on February 18th, 2009
Introduction:
Microorganism’s resistance has increased around the world. Some bacterial organisms that cause infections in the healthcare setting are currently resistant to all antimicrobial agents. Bacterial resistance in community setting is also crucial in countries with limited resources. Resistance among non-bacterial organisms is of concern, as reflected in the widespread occurrence of primary human immunodeficiency virus (HIV) resistance2, extensively drug-resistant tuberculosis (XDR-TB), and drug-resistant Plasmodium falciparum malaria. These increases in the dissemination of resistance are both a medical and public health concern since they threaten both optimal cares of patients with infection as well as the viability of current healthcare systems2.As a result, Microorganisms resistance is a major threat to public health and medicine. First, from the public health perspective, drug resistance has clear effects on the patient mortality, morbidity, and treatment cost since patients infected with resistant organisms have higher healthcare-related costs than patients infected with non-resistant organisms due to the use of a different antimicrobial agent which is costly and might be toxic. From the medicine perspective, drug resistance has a clear economic impact for the medical healthcare systems because of the increase costs associated with the increases on the prevalence of multidrug-resistant (MDR). Human migration and “global trade” will also be an important factor on the spread of the antimicrobial resistance across any geographic or political boundary2. For instance, Resistant Salmonella spp.isolates was introduced to Denmark through the importation of boar’s meat from Canada2. Examples in the United States include the emergence of multidrug resistance in Mycobacterium tuberculosis7, penicillin resistance in pneumocci7. The roots of the resistance are multifactorial and are abundant while the resources to control the resistance are very limited.
Major factors that contribute to the emergence of resistance are7:
· Changes in human demographics
· Advances in technologies and industry
· Economic development
· International travel and commerce
· Microbial adaptation and change
· Deterioration in the public health infrastructure at the federal, state and local levels in the U.S.
As a consequence, the control of the resistance will demand efforts from multiple health and industry sectors at multinational and international level.
Mechanisms for Antimicrobial Resistance
Resistance to antimicrobial treatment has emerged as a significant barrier to the elimination of infections since antibiotic treatment became a common treatment method in the 1930s. There are a variety of antimicrobial mechanisms that disable or eliminate the target microbe. For many of these mechanisms, changes in the genotype or phenotype in multiple microbial infections has reduced or eliminated the effectiveness of the various treatment methods.
For example, the popular antibiotic compound, penicillin, disrupts cell division by binding to penicillin-binding proteins (PBPs). Mutations within microbes have rendered this treatment less effective by a rate of 0 to 30%. This change occurs in two distinct modes17. First, mutations to the PBP reduces the ability of penicillin to bind and disrupt cell division. This change is referred to as a “tolerance” to the penicillin. In addition changes to the membrane permeability reduce the ability of penicillin to enter the cell. This change in the cell structure confers “resistance” to the treatment.
Another common treatment of microbial infection is the use of the nitroimidazoles, such as metronidazole and tinidazole. This class of antimicrobial treatment utilizes the reduction f the drug by nitroreductase and leads to damage to the microbe’s DNA. Resistance rates for this drug are now between 20 to 95%. The mechanism of resistance is an absence of the reduction of an intermediate compound, imidazole. This occurs because of a reduction or abolished activit of electron transport proteins17.
A third class of microbial resistance targets the macrolides class of microbial treatment. Macrolides act to disable cell protein synthesis by binding to 235 rRNA. Resistance rates for this treatment are now between 0 and 50%. The resistance mechanism for macrolides are point mutations in the 235 rRNA genes, resulting in a reduced ability of the drug to bind to the RNA17.
Many other classes of microbial treatments also exist, with varying mechanisms of action including damage to DNA; inhibition of transcription, protein, ATP, and cell membrane synthesis; reduction of proton motive force of the bacterium; and disruption of DNA replication processes. In each of these cases, many microbes have developed ways to avoid the disruption enough to continue multiplying. In many cases, the mechanisms for resistance to these treatments is still unknown. These changes in the structure and function of infectious and non-infectious microbes present significant challenges in the treatment of infection.
Mechanisms for Antimicrobial Resistance
Several different modes of spreading resistance to antimicrobial treatments exist. The most simple and intuitive of spreading resistance is termed “vertical transport” through selection. Vertical transport refers to the passing of genes from one resistant microbe through cell division and increased viability due to the benefit of resistance. While important, vertical transport of resistance to antimicrobial treatment is not the primary method of the spread of resistance. Rather, horizontal transfer of microbial resistance allows microbes that have developed resistance to particular treatments to spread the genes faster and to a more broad spectrum of microbes.
“Horizontal transfer” of resistance refers to intercellular sharing of the genes that confer resistance to a particular antimicrobial drug. There are three main types of self-transmissible DNA elements that can be passed from one microbe to another. These are Plasmids, transposons, and bacteriophages18. Transposons are mobile strands of DNA that attach to certain common strings of base pairings within a strand of DNA. These can also attach to mobile elements such as plasmids to be transferred to other cells. Bacteriophages are frequently in the form of viruses that infect the bacteria cell and transpose foreign DNA into the host cell’s genome. The new material occasionally causes changes that give the cell resistance to treatments. The remainder of this section will focus on plasmid transfer of antimicrobial resistance.
Plasmids are self contained loops of DNA that are able to copy between cells. Most antimicrobial resistance that is transferred through horizontal means occurs by utilizing plasmids. It has been shown in several circumstances that plasmids frequently code for multiple drug resistant (MDR) genes18,19,20,21. The existence of MDR genes encoded on a single plasmid can be shown to exist by comparing the transfer efficiencies of different forms of antimicrobial resistance. In one study, the transfer efficiency for resistance to streptomycin, chloraphenicol, tetracycline, and kanamycin were all identical, suggesting that resistance to all four treatments were carried on one conjugative plasmid21.
A sub-mechanism for the transfer of resistance to antimicrobial treatment is through integrons18,21. An integron is “a site specific recombination system that recognizes and captures mobile gene cassettes that normally encode for antibiotic resistance. 18” Integrons can exist both on the cell’s genome, and/or on plasmids within the cell. When the integron is located on a plasmid, it is able to transfer to other cells. Integrons are frequently identified in cells through polymerase chain reaction (PCR) and pulse field gel electrophoresis21. Additional information on integron regulated resistance and transfer mechanisms is discussed in following sections.
Antimicrobial Resistance in Escheria Coli
Resistance to antimicrobial treatment has become common in several strains of the escheria coli (e. coli) bacteria. Several studies have shown similar and startling results on the spread of antimicrobial resistance in e. coli. One such study focuses on the transfer of resistant genes between shiga toxin-producing e. coli (STEC) and a separate species of e. coli18. The STEC were tested and positive for resistance to streptomycin and suflisoxazole, which were coded on a class 1 integron, and trimethroprim and streptothricin, which were coded on a class 2 integron.
The ability to transfer between e. coli species was tested in vitro. The results show that resistance to antimicrobial treatment that was coded for on the class 1 integron had a steady transfer rate18. In addition, resistance to tetracycline and oxytetracycline were cotransfered with other antimicrobial resistance. It is most likely that the carrier plasmid for the class 1 integron also coded for these resistances, and was transferred to the separate species of e. coli at the same rate. The class 2 integron appears to have had no transfer between e. coli species, as none of the antimicrobial resistance appeared in the receiving e. coli.
A separate study aimed to show that e. coli resistance genes could transfer from food borne pathogens to endogenous bacterial flora in vivo20. Previous studies had been unable to show stable transfer of resistance between food borne pathogens and the host’s own flora. Temporary transfer of resistance had been shown, but loss of acquired resistance followed rapidly. The results of the current study showed that transfer of resistance to antimicrobial treatment can occur in vivo, but failed to show that transfer took place between the food borne pathogen and the local flora. Results indicated that the transfer took place between the host’s own e. coli and a recipient cell, as the genetic similarities of the transferred resistance gene.
Antimicrobial Resistance in Salmonella
Antimicrobial resistance in various salmonella strains is also common and increasingly problematic. Studies range from various different strains and cover multiple types of antimicrobial treatment. One study done in china focuses on the increase in resistance within salmonella enterica serovar Pullorum (SESP) over the period between 1962 to 200719. Results from the study showed that:
I. High levels of resistance to ampicillin, carbenicillin, streptocycin, tetracycline, trimethoprim, and sulfafurazole were consistant in SESP.
II. Increased resistance rates to several of these and other antimicrobial treatments were observed over time.
III. MDR occurance rates showed high levels of increasing trends over time.
These results are consistent with the presumption that resistance to antimicrobial treatment follow the introduction of new antimicrobial elements over time19.
A 2008 study focused on salmonella Heidelberg and resistance to microbial treatment in humans, swine, and turkeys21. The analysis identified that resistance genes were carried and transferred by both class 1 and 2 integrons. In swine, the majority of salmonella Heidelberg sampled (73%) showed resistance to three different forms of treatment. The same salmonella strain in human cultures was pan-susceptible in 80% of the samples. The difference in the resistance that was present in swine and human strains of the salmonella can be partially explained by the difference in the way antimicrobial treatments are administered. Additional discussion on antimicrobial treatment to animals is covered in following sections.
Antimicrobial Resistance in Infected Livestock and Animals
Frequently, antimicrobial resistance is prominent in microbes that colonize livestock and other animals that receive antimicrobial treatment. Large amounts of antibiotics are frequently used in modern food production in an attempt to reduce or eliminate food-borne pathogens. This over-dosing of antimicrobial treatment creates an ideal environment for the spread an persistence of resistance to antimicrobial treatment22. This can be of especial importance since resistance to antimicrobial treatment may be transmitted to humans via the food chain19.
Other animals, such as domesticated animals and non-food based product livestock may have increased antimicrobial resistance as well. A 2008 study focusing on the prudent use of antimicrobials for mink showed an increasing trend of antimicrobial resistance among S. intermedius, e. coli, and other microbial infections23. It was also shown that antimicrobial resistance rates were generally lower in mink than for those of domesticated dogs. This can be explained by the less frequent use of antimicrobial compounds on mink.
A separate study focusing on healthy dogs assayed the prevalence of antimicrobial resistance in e. coli and enterococcus spp24. The results of the investigation showed significant association between recent antimicrobial treatment in dogs and resistance in both microbes. The study also reported for the first time the appearance of ampicillin resistant Enterococcus faecium in dogs. This discovery provides additional evidence that antimicrobial resistance continues to emerge as new treatments are introduced. The first contact with resistance of this kind is important to both animal and human health, and the researchers recommend further investigation on the prevalence of AREF in dogs.
Antibiotic Resistance in Different Parts of the World
Global Perspective
Antibiotic resistance is a major issue when it comes to global health8. Infectious diseases in all parts of the world have shown signs of antimicrobial resistance. For example, strains of Streptococcus pneumoniae (S.s pneumoniae) have been found all over the world (see figure KX.1 below). Part of the global problem with antimicrobial resistance is increased population growth, trade, and travel have increased and caused the spread of many strains of resistance8. Trade and travel has allowed for must faster spread of antimicrobial resistant pathogens through the transportation of infected people and food which has been genetically modified or contains antibiotic resistant microorganisms between countries.15 Global travel has become such an issue that “visitors to developing countries acquire antibiotic-resistant E. coli [(Escherichia coli)] as part of their normal flora.”14 Travelers then serve as vectors and can spread antibiotic-resistant E. coli when they return to their home countries. Obviously, population growth has led to an increase in the number of people who become sick and need treatment. The use of antibiotics on such people has helped lead to the increase of antimicrobial resistance strains of pathogens.
Figure KX.1. Percentage of S. pneumoniae resistant to multiple antiobiotics9
Cooperation between countries is key to preventing the global spread of antimicrobial resistant strains8. The World Health Organization (WHO) plays a major role in uniting countries internationally around the world to help improve global health, including dealing with antibiotic resistant microorganisms. In 2001, WHO published a report on Global Strategy for Containment of Antimicrobial Resistance. In the report, recommendations are given , such as “improving access to and use of antimicrobials, strengthening surveillance capabilities and other aspects of healthcare systems, enforcing regulations, and developing new preventative and therapeutic medications”8. Although many of these recommendations aren’t an issue in developed countries, they tend to be major issues in developing countries.