Drug Resistance

Mechanisms of Resistance

Development of microbial resistance to antimicrobial agents requires alterations in the microbe’s cell physiology and structure.

Learning Objectives

Describe the mechanisms bacteria use to develop antimicrobial resistance and the factors that can lead to it

Key Takeaways

Key Points

  • Antimicrobial resistance can be mediated by the environment or the microorganism itself.
  • Environmentally-mediated antimicrobial resistance results from physical or chemical characteristics of the environment that can affect the antimicrobial agent or the microorganism.
  • Microorganism-mediated antimicrobial resistance can be intrinsic or acquired.

Key Terms

  • intrinsic: innate, inherent, inseparable from the thing itself, essential.

Development of microbial resistance to antimicrobial agents requires alterations in the microbe ‘s cell physiology and structure. Antimicrobial resistance is defined as the loss of susceptibility to an extent that the drug is no longer effective for clinical use against an organism. Resistance can be mediated by the environment or the microorganism itself.

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Resistant bacterial strain: Methicillin-resistant Staphylococcus aureus.

Environmentally-mediated antimicrobial resistance is affected by the environment’s chemical and physical properties such as pH, anaerobic conditions, cation concentrations (calcium, magnesium), and thymine-thymidine content (available metabolites and nutrients).

Microorganism-mediated antimicrobial resistance is due to genetically-encoded traits of the microorganism and can be divided into intrinsic or acquired. Intrinsic resistance is considered to be a natural and inherited property with high predictability. Once the identity of the organism is known, the aspects of its anti-microbial resistance are also recognized. On the other hand, acquired resistance results from a change in the organism’s genetic makeup. This trait is associated with only some strains of an organism’s group but not the others. It is also an unpredictable trait and necessitates the development of laboratory methods to detect it. Microorganism-mediated antimicrobial resistance is acquired by gene change or exchange such as genetic mutations, acquisition of genes from other organisms via gene transfer mechanisms, or a combination of mutational and gene transfer events. Some common pathways bacteria use to effect antimicrobial resistance include: enzymatic degradation or modification of the antimicrobial agent, decreased uptake or accumulation of the antimicrobial agent, altered antimicrobial target, circumvention of consequences of antimicrobial actions, uncoupling of antimicrobial agent-target interaction, or any combination of these mechanisms.

Antibiotic Misuse

Antibiotic misuse is one factor responsible for the emergence of antimicrobial resistant bacterial strains.

Learning Objectives

Explain the effects of antibiotic misuse

Key Takeaways

Key Points

  • Antimicrobial resistance is a major public health concern.
  • Antimicrobial resistance is brought about by antibiotic misuse, such as overuse, misuse, or interrupted treatment.
  • Food industries, physicians, and patients play a role in minimizing the spread of resistance by adhering to good antibiotic practice.

Key Terms

  • course of antibiotics: a period of continuous treatment with a drug.
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Antibiotic misuse: Antibiotics are not effective against viral infections. Misusing them leads to resistant bacterial strains.

With the introduction of antibiotics into medical practice, clinically-relevant bacteria have had to adopt resistance mechanisms as part of their survival strategy. Antibiotic resistance occurs when antibiotics no longer work against disease-causing bacteria. These infections are difficult to treat and can mean longer-lasting illnesses, more doctor visits or extended hospital stays, and the need for more expensive and toxic medications. Some resistant infections can even cause death. Developing new antibiotics and other treatments to keep pace with antibiotic-resistant strains of bacteria is necessary. However, using antibiotics wisely is equally important for preventing the spread of resistant strains.

Antibiotic misuse has contributed largely to the emergence of new resistant strains. It is caused by taking an antibiotic too often for a condition it cannot treat such as viral infections and the common cold or in the wrong doses. It can also be manifested by not finishing a course of antibiotics as prescribed (stopping the antibiotic before the infection is fully cleared from the body). Overuse of antibiotics affects the body’s normal flora and disrupts the balance between beneficial bacteria that help digestion for example, and harmful bacteria. Excessive use of antibiotics in intensive farming units, particularly pig and poultry farms, is also seen as a growing threat. Scientists say antimicrobial resistance may be passing between animals and humans through food consumption, making the need to cut unnecessary use of antibiotics in farming even more urgent. Responsible antibiotic use in industry, and good practice for patients and physicians, are essential to keep resistant bacterial strains curable, and antibiotic treatment affordable to patients.

Cost and Prevention of Resistance

Antimicrobial resistance is a major public health and economic burden on patients, affected communities, and healthcare providers.

Learning Objectives

Examine the causes and effects of multidrug-resistant organisms on healthcare

Key Takeaways

Key Points

  • Antimicrobial resistance to available drugs requires the development of new drugs to effectively treat resistant strains and reduce mortality from bacterial infections.
  • Antimicrobial resistance can be prevented by practicing good hygiene, and being responsible with antibiotic use.
  • Treating antibiotic-resistant bacterial strains is expensive for both the patient and the healthcare provider. The treatment requires extended hospital stay and costly medications.

Key Terms

  • multidrug resistance: A condition enabling a disease-causing organism to resist distinct drugs or chemicals of a wide variety of structure and function targeted at eradicating the organism.

Prevention and control of microbial-resistant organisms is one of the most complex management issues that health care professionals face. The clinical and financial burden to patients and health care providers is staggering. Patients who are infected with bacterial strains resistant to more than one type or class of drugs (multidrug-resistant organisms, MDRO) often have an increased risk of prolonged illness, extended hospital stay, and mortality.

The cost of care for these patients can be more than double compared to those without an MDRO infection. The alternative medication they are prescribed to overcome the infection is often substantially more costly. Multidrug resistance forces healthcare providers to use antibiotics that are more expensive or more toxic to the patient.

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Antibiotics: Antibiotic misuse is a major cause of the staggering healthcare costs for the treatment of resistant bacterial strains.

When no antibiotic is effective, healthcare providers may be limited to providing supportive care rather than directly treating an infection. In a 2008 study of attributable medical costs for antibiotic resistant infections, it was estimated that infections in 188 patients from a single healthcare institution cost between $13.35 and $18.75 million dollars.

Research and development of new drugs effective against resistant bacterial strains also comes at a cost. To prevent antimicrobial resistance, the patient and the healthcare provider should discuss the appropriate medicine for the illness. Patients should follow prescription directions and should not share or take medicine that was prescribed for someone else; these virtues should be strictly practiced. Healthy lifestyle habits, including proper diet, exercise, and sleeping patterns, as well as good hygiene such as frequent hand washing, can help prevent illness. These practices, therefore, also help prevent the overuse or misuse of antibiotics and the emergence of problematic resistant strains.

Biofilms, Persisters, and Antibiotic Tolerance

Biofilms and persisters are bacterial communities responsible for chronic diseases and antibiotic tolerance.

Learning Objectives

Explain the role of biofilms and persisters in multidrug tolerance, distinguishing this from multidrug resistance

Key Takeaways

Key Points

  • Biofilms are aggregates of microbial cells that form to avoid antimicrobial agents or attack by the immune system.
  • Persisters are slow-growing, dormant microbial cells that can tolerate antibiotic treatment.
  • Biofilms and persisters are responsible for chronic bacterial infections and recurrent disease.
  • Antibiotic tolerance is different from antibiotic resistance but equally important as a public health burden for eradication of serious bacterial diseases.

Key Terms

  • gingivitis: inflammation of the gums or gingivae
  • extracellular matrix: All the connective tissues and fibers that are not part of a cell, but rather provide support.

Biofilms are bacteria that have formed a gated community. Biofilms are composed of an aggregate of bacterial cells and are essentially considered a multi-cellular organism. They are characterized by structural heterogeneity, genetic diversity, complex community interactions, and an extracellular matrix of polymeric substances. They live on solid surfaces (e.g., catheters, ) and the extracellular material they produce protects them from external threats, such as attacks by the body’s immune cells. The property of biofilms constitute a penetration barrier for most antibiotics therefore preventing the drug from reaching the microbes. It is being widely recognized that bacterial biofilms are responsible for several chronic diseases that are difficult to treat, hence hard to eradicate (e.g., cystitis, endocarditis, urinary tract infections, gingivitis, dental plaque, and other yet to be identified conditions). They differ from free-floating or planktonic bacteria that cause acute infections and are managed by antimicrobial drugs.

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Staphylococcus aureus biofilm: Staphylococcus aureus forming a biofilm on a catheter.

Persisters are multidrug tolerant cells present in all bacterial populations. Bacterial populations that produce persister cells that neither grow nor die in the presence of microbicidal antibiotics are largely responsible for high levels of biofilm tolerance to antimicrobials. Persisters are not mutants, but rather phenotypic variants of the wild-type that upon inoculation produce a culture with similar levels of tolerance. Elimination of persisters remains an obstacle for the eradication of some tenacious and highly recurrent bacterial infections. Biofilms and persisters are the cause of multidrug tolerance. Multidrug tolerance differs from multidrug resistance in that it is not caused by mutant microbes but rather by microbial cells that exist in a transient, dormant state. These non-dividing cells often survive antibiotic exposure targeted to kill highly proliferating bacteria.

Finding New Antimicrobial Drugs

Antimicrobial resistance has created a public health crisis in the treatment of infectious diseases and necessitates the discovery of new drugs.

Learning Objectives

Explain the reasons for low production of new antibiotics and discuss the proposed mechanisms to evade antimicrobial resistance

Key Takeaways

Key Points

  • Finding new antimicrobial drugs requires researchers, pharmaceutical, and biotech companies to invest in new technology and discover new sources for antibiotic development.
  • Proposed mechanisms to circumvent antimicrobial resistance range from exploring the list of resistance genes to antibody-based therapy and vaccines.
  • Finding new candidates to target is essential but it needs to be accompanied by awareness on antibiotic misuse with the prospect to eliminate the root of the problem.

Key Terms

  • mimetic: A substance with similar pharmacological effects to another substance.

Antimicrobial resistance: the problem

Antibiotics, more than any other medicines, have improved the life expectancy of mankind, however, multi-drug resistance has become common in pathogenic bacteria and multiple drugs are losing efficacy. Recent reports on the occurrence of panresistant gram-negative strains, i.e. strains resistant to every registered antibacterial drug, indicate that we are on the verge to lose the battle, taking us back to the pre-antibiotic era. There is world-wide consensus that the medical need for novel antiinfective drugs is enormous and that we are running out of time. Many achievements of modern medicine, not only treatment of infectious diseases, depend on the availability of efficacious antibiotics, still, the antibacterial development pipeline is slow and the number of new drugs reaching the market is alarmingly low. There are many reasons for this at all levels of the discovery and development process. Investments into antibiotic research and technologies is minimal; socioeconomic considerations together with regulatory hurdles have prompted pharmaceutical companies to exit the field and innovative biotech companies were confronted with problems beyond their control. Answers are needed as to where and how we can find new lead compounds with unprecedented activities?

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Bacterial infections of the human body: most bacterial species listed in this figure have developed resistance to available antibiotics necessitating new drug discovery.

Finding new antimicrobial drugs: the solution

Research on new antimicrobial compounds is geared towards innovative targets to circumvent resistance. Some of the proposed areas to investigate include: collecting and examining the list of antimicrobial resistance genes (e.g. exploring the resistome), targeting teichoic acid biosynthesis as a new method to compromise the bacterial wall integrity, producing ribosomal inhibitors to target protein synthesis, targeting outer-membrane transporters with protein epitope mimetics (e.g. mimetics of the cationic antimicrobial peptides that form part of the immune response to microbes), and developing antibody-based strategies and vaccines. The initiative to develop new antimicrobial agents is urgently needed but is a long process from invention, to development, to actual clinical application. It is also necessary to initiate a worldwide awareness on antibiotic misuse and overuse as a mean to address the root of the problem for antimicrobial resistance.

Antimicrobial Peptides

Antimicrobial peptides exhibit cytotoxic activity against all microbes.

Learning Objectives

Discuss the structure, mechanism, and targets of antimicrobial peptides

Key Takeaways

Key Points

  • Antimicrobial peptides (AMPs) are a unique and assorted group of molecules produced by living organisms of all types, considered to be part of the innate immunity of a host.
  • These peptides demonstrate potent antimicrobial activity and are rapidly mobilized to neutralize a broad range of microbes, such as viruses, bacteria, protozoa, and fungi.
  • The ability of these natural molecules to kill multidrug-resistant microorganisms has gained them considerable attention and clinical interest.

Key Terms

  • neutropenia: A hematological disorder characterized by an abnormally low neutrophil count.
  • atopic dermatitis: An atopic, hereditary, and non-contagious skin disease characterized by chronic inflammation of the skin.

A first line of defense against pathogenic insult is called the innate immune system, which is followed by acquired immune responses associated with the activation of T and B cells aimed against specific antigens. In contrast to the clonal, acquired adaptive immunity, endogenous peptide antibiotics or antimicrobial peptides provide a fast and energy-effective mechanism as front-line defense.

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Various AMPs: These are various antimicrobial peptide structures.

Antimicrobial peptides (AMPs) are small molecular weight proteins with broad spectrum antimicrobial activity against bacteria, viruses, and fungi. They are classified on the basis of their structure and amino acid motifs. Peptides of the defensin, cathelicidin, and histatin classes are found in humans. These evolutionarily conserved peptides are usually positively charged and have both a hydrophobic and hydrophilic side that enables the molecule to be soluble in aqueous environments yet also enter lipid-rich membranes. Once in a target microbial membrane, the peptide kills target cells through diverse mechanisms. AMPs secrete lytic enzymes, nutrient-binding proteins or contain sites that target specific microbial macromolecules.

Cathelicidins and defensins are major groups of epidermal AMPs. Decreased levels of these peptides have been noted for patients with atopic dermatitis and Kostmann’s syndrome, a congenital neutropenia. AMPs have proven effective against multidrug-resistant microbes. In addition to important antimicrobial properties, growing evidence indicates that AMPs alter the host immune response through receptor-dependent interactions. AMPs have been shown to be important in such diverse functions as angiogenesis, wound healing, cytokine release, chemotaxis, and regulation of the adaptive immune system. These peptides qualify as innovative drugs that might be used as antibiotics, anti-lipopolysaccharide drugs, or modifiers of inflammation reactions.

Antisense Agents

Antisense agents are short oligonucleotides that bind to target messenger RNA and inhibit protein synthesis.

Learning Objectives

Discuss the mechanism of antisense agents and the advantages and disadvantages of antisense therapy

Key Takeaways

Key Points

  • Antisense agents have broad applications in several diseases. Their use for treating microbial infections is promising.
  • They are synthetic oligonucleotides that can be manufactured quickly and their biological effect is long-lasting.
  • Their use for the treatment of antibiotic resistant bacterial infections is possible but limited by their poor uptake by the bacterial cell. Studies are being developed to improve their penetration into the cell.

Key Terms

  • messenger RNA: RNA that encodes and carries information from DNA during transcription to sites of protein synthesis to undergo translation in order to yield a protein
  • nuclease: Any of several enzymes capable of cleaving the phosphodiester bonds between the nucleotide subunits of nucleic acids.
  • hybridize: To combine complementary subunits of multiple biological macromolecules.
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DNA to Protein: Role of messenger RNA in protein synthesis.

Antisense agents are synthetic, single-stranded short sequences of DNA bases designed to hybridize to specific sequences of messenger RNA (mRNA) forming a duplex. This DNA-RNA coupling attracts an endogenous nuclease, RNase H that destroys the bound RNA and frees the DNA antisense to rehybridize with another copy of mRNA. In this way, the effect is not only highly specific but prolonged because of the recycling of the antisense DNA sequence. When this agent binds to the pathogen DNA or messenger RNA, the biosynthesis of target proteins is disrupted. Therefore, there are at least two ways in which antisense agents act to effectively reduce the amount of pathogenic protein being synthesized – RNase H based degradation of RNA and prevention of ribosomal assembly and translation. This approach has a great advantage. It prevents a pathogenic protein from being produced, rather than trying to selectively neutralize it once it is made.

Antisense agents can be specifically targeted to genes that control expression of antibiotic resistance mechanisms, thereby potentially restoring an antibiotic-sensitive phenotype to the cell. A limiting factor in their potential application as therapeutic agents for bacterial infections is their poor uptake by bacterial cells. These agents have been successfully developed for the treatment of viral infections such as cytomegalovirus, hepatitis C, and HIV infections. The advantage of antisense therapy is that they can be manufactured fairly fast, they produce a lasting clinical effect, and they are highly specific to the target. Antisense agents also exhibit efficacy in broader clinical applications such as cancer therapy.