Antimicrobial Resistance Mechanisms

Antimicrobial Resistance

• Increasing antimicrobial usage and misuse promotes resistance
• 90% + of Staphylococcus aureus is resistant to penicillin G, originally only 3% were resistant
• Antimicrobial resistance impacts treatment costs, complications, and outcomes
• Understanding resistance mechanisms and spread is required

Mechanisms of Acquired Resistance to Antimicrobials

• Some bacteria develop resistance by inactivating medications via enzymes
• Target molecule alteration
• Decreased medication uptake
• Increased medication elimination

Resistance to Antimicrobial Medications: Mechanisms of Acquired Resistance

• Bacteria produce enzymes that chemically modify drugs to inactivate them
• Penicillinase provides resistance to penicillin
• Extended spectrum β-lactamases confer resistance to many antimicrobials

Alteration in Target Molecule

• Antibiotics interfere with bacterial function by binding to specific molecules
• Minor structural changes in bacteria prevent antibiotic binding
• Modifications to PBPs (β-lactam antibiotics), ribosomal RNA – 50S subunit (macrolides, lincosamides, streptogramins) can cause resistance

Decreased Uptake of Medication

• Changes in porin proteins of outer membrane of Gram-negatives

Increased Elimination of Medication

• Efflux pumps remove compounds from cell
• Increased pump production or structural changes enable faster removal
• Resistance can extend to multiple antimicrobials

Acquisition of Resistance

• Resistance acquired through spontaneous mutation
• Resistance acquired through gene transfer

Spontaneous Mutations

• Mutations occur at low rate during replication, but can have a significant effect
• Aminoglycosides example
• A single base-pair change in a ribosomal protein gene makes bacteria aminoglycoside-resistant
• Spontaneous resistance to antibiotics with several targets or multiple binding sites is less likely
• Noncompliance to antibiotics leads to spontaneous mutations as it goes below the MIC
• Combination therapy of multiple antibiotics is an effective strategy to prevent spontaneous mutations; resistance unlikely
• If resistance develops to one antibiotic, the others will still kill the cell

Gene Transfer

• Genes encoding resistance spread to different strains, species, and even genera
• Original sources of resistance genes include Plasmids
• Plasmids are small circular DNA strands in cytoplasm of bacterium/protozoan
• Resistance genes are commonly transferred through conjugative transfer of R plasmids
• Resistance Genes on R Plasmids Originate from:
• Spontaneous mutations
• Microbes that naturally produce the antibiotic
• Genes coding for enzyme that modifies aminoglycoside originating from Streptomyces species, which produces the antibiotic

Examples of Emerging Resistance

• Enterococci (Group D Streptococci)
• Enterobacteriaceae
• Mycobacterium tuberculosis
• Neisseria gonorrheae
• Staphylococcus aureus
• Streptococcus pneumoniae
• Enterococci are part of normal intestinal microbiota
• Enterococci: Common cause of healthcare-associated infections
• Enterococci intrinsically less susceptible to many antimicrobials
• PBPs in Enterococci's peptidoglycan have low affinity to many β-lactam antibiotics
• Enterococci often have R plasmids
• Some Enterococci code for resistance to vancomycin, create (VRE) and is transferable
• Vancomycin resistance in VRE strains is encoded on a plasmid, so it can be transferred
• Enterobacteriaceae are intrinsically resistant to antimicrobials
• Outer membrane prevents entry of antimicrobials
• Some Enterobacteriaceae developed ability to produce β-lactamases
• Some Enterobacteriaceae developed ability to produce extended-spectrum β-lactamases (ESBLs)
• ESBLs resist cephalosporins and monobactams in addition to penicillins
• Carbapenem-resistant Enterobacteriaceae (CRE) strains have emerged
• CRE resistant to nearly all antibiotics
• CRE was replaced with colistin, but it was considered too toxic for use
• Mycobacterium tuberculosis can become resistant to first-line antibiotics due to spontaneous mutation
• Active infections contain plenty cells, increasing likelihood cell resists antibiotics
• Combination therapy is used because of the resistance
• Treatment lasts 6+ months due to slow growth, so patients may not comply
• Multidrug-resistant tuberculosis (MDR-TB) resist two of the first-line antibiotics : isoniazid and rifampin
• Directly observed therapy (DOT) can prevent emergence
• Extensively drug-resistant tuberculosis (XDR-TB) of greater concern and resists three+ second-line anti-TB medications
• Neisseria gonorrhoeae was once very susceptible to penicillin
• Some strains developed resistance through mutation
• Some acquired a plasmid that encoded production of penicillinase
• Other treatments failed
• Only certain cephalosporins are reliable

Newer Combination Therapy

• Minimizes Resistance
• Uses intramuscular ceftriaxone and oral azithromycin

Staphylococcus aureus

• Common cause of healthcare-associated infections
• Most strains resistant to penicillin, and encode penicillinase
• New strains have PBPs with low affinity for β-lactam antibiotics
• Methicillin-resistant Staphylococcus aureus (MRSA)
• Healthcare-associated (HA-MRSA) resists wide ranges of antibiotics
• Usually treated with vancomycin
• Hospitals reported resistant isolates
• Strict guidelines halted spread of vancomycin-intermediate S. aureus (VISA) and vancomycin-resistantS. aureus (VRSA) strains
• Community acquired (CA-MRSA) currently treatable

Streptococcus pneumoniae

• Was Historically susceptible; some acquired penicillin resistance
• Produces PBPs with lower affinity, likely via DNA-mediated transformation via other Streptococcus species

Antibiogram

• An overall profile of antimicrobial susceptibility testing results of a specific micro-organism to a battery of antimicrobial drugs

Preventing Resistance

• Requires cooperation from everyone globally
• Physicians and Healthcare worker responsibilities include:
• Increasing efforts to identify the cause of the infections
• Only prescribing suitable antimicrobials when necessary

Antimicrobial Stewardship Programs

• Monitor antimicrobial usage
• Could improve the quality and safety of the system

Patient Responsibility

• Carefully follow instructions even if inconvenient
• Essential to maintain adequate blood levels of the antibiotic; missing a dose reduces antibiotic levels
• Allows less-sensitive microbes the chance to grow and spread
• Failure to complete treatment may not kill least-sensitive organisms and allows subsequent spread

Educated Public Importance

• Antibiotics are ineffective against viruses
• Therefore antibiotics cannot cure the common cold
• Misuse selects for antibiotic-resistant bacteria in normal microbiota
• Allows for transfer of R plasmids to be transferred to pathogens

Preventing Resistance

• Overuse is a worldwide concern due to the worldwide boundaries
• Improper use can be caused by Antimicrobial antibiotics available without a prescription
• Low-level use of Antimicrobial antibiotics in animal feeds to enhance growth selects for antibiotic-resistant microbes
• Resistant Salmonella strains are linked to animals

Antiviral Medications: Mechanisms of Action

• Viruses are difficult to target for selective toxicity
• Virus must rely of host cell’s metabolic machinery
• Viruses lack cell walls, and ribosomes
• Many viruses encode polymerases
• Potential target for antiviral medications (antivirals)
• Only effective against specific virus type
• Greatest Variety directed towards HIV
• Antivirals targeting SARS-CoV-2 cause COVID-19
• Antivirals are only effective against replicating viruses
• Herpes and HIV infections are latent in cells, so cannot be cured
• Combination therapy used where viruses evolve rapidly ( for viruses e.g HIV and HCV [hepatitis C virus] ) to develop against single medications

Prevent Viral Entry

• Some HIV medications prevent virus from entering host cell either by preventing attachment or preventing fusion of viral envelope with the host cell membrane
  • Attachment inhibitors— binds to HIV surface; can't attach to CD4 on host -Post-attachment inhibitors--monoclonal antibody (mAb)
• Binds to HIV receptor CD4 and prevents HIV particle from undergoing a change required for the virus to bind to a co-receptor
  • CCR5 antagonist blocks the HIV co-receptor CCR5 -Fusion inhibitor protein that promotes fusion of viral envelope

Interfere with Viral Uncoating

• Uncoating--nucleic acid of viral particle releases from a protein coat -Amantadine and rimantadine -medications interfering with influenza A
• Capsid Inhibitors
• Lenacapavir (LEN) binds to the protein making up the subunits of the viral capsid -Useful for patients with multi-drug-resistant infection

Interfere with Nucleic Acid Synthesis

• Most antivirals target virally-encoded enzymes used during replication of viral nucleic acid -Limited to herpesviruses, HBV, HCV, and HIV -Target for COVID-19 medications
• Nucleoside and nucleotide analogs -Chemicals structurally similar to building blocks of DNA/RNA Incorporated into nucleotide chain where analogs act as chain terminators

Selective Toxicity

-More damage done to rapidly replicating viral genome -Acyclovir attacks herpesviruses (chickenpox,cold sores)

• Sofosbuvir interferes specifically with HCV's replicase Highly effective against hepatitis C
• Nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) treat HBV, HIV -Often used in combination with anti-retroviral meds to minimize development resistance Examples include Zidovudine(anti-HIV) and Tenofovir(anti-HBV) -Some are reserved for severe infections and lead to significant side effects -Example-Ganciclovir treats life-threatening Cytomegalovirus(CMV) infections-immunocompromised
• Non-nucleoside polymerase inhibitors Inhibit viral polymerases by binding to site - nucleotide-binding site Dasabuvir used for fixed treatment with HCV
• Foscarnet used to treat resistant herpesviruses
• Non-nucleoside reverse transcriptase inhibitors(NNRTIs) inhibit reverse transcriptase by binding to different sites
• NS5A inhibitors- relatively new for HCV

Viral Particle Prevention Strategies

• Inhibit viral NS5A and HCV-encoded protein required for viral replication
• Integrase inhibitors interfere with the HIV encoded enzymes and integrase
• -Prevnts Virus from inseeting DNA copy of genome into hosts'

Protease Inhibitors

• Proteins are translated as a polyprotein in some viruses
• Must be cleaved by protease
• Virus-specific Includes atazanavir (anti-HIV) and grazoprevir (anti-HCV)
• Neuraminidase inhibitors
• Enzyme encoded by influenza viruses needed for release and ingested/ inhaled

Antifungal Medications

• Eukaryotic pathogens hard to target
• More closely resemble human cells
• --Fewer targets for antifungals Acquired resistance is a significant concern
• Antifungal Medcations interfere with fungal cytoplasmic membrane, cell wall systhesis, cell divisioin and nueclid acid synthesis

Cytoplasmic Membrane Synthesis and Fnctions

• antifungal chemicals target ergosterol which humans lack
• -Azoles - membrane leaks are inhibited Three families Imidazoles, Traizoles and Tetrazoles
• --Newer less toxic Triazoles are used to systemmic infections and nail infections
• Polyenes- produced by sterptomyces by forming a complex
• Toxic to Humans ( amphotericin B, Nystatin)
• Allyamines and Butenfine
• -Inhibit enzymes in ergosterol that apply to the skin

Interfere with cell wall Synthesis

Fungal cell walls have components that animals lack

• -Echinocandinas interference with synthesis that cuases cells to burst
• --treatments for Candia Strains
• --Causpophungiaso use to treat Invasive Aspengillosis
• --Titerpenpods: treats candidiais for b-synthesis

Interfere in cell division

Targets cell division by inferfing in tublulin

• -Treats nail skinninfeftions
• --Fingi need Kerartin to grow

Interfre with Nuclei Acis

• Common in eukaryotes
• --Poor target
• ---Cytosine Taken up by yeast cells will inhibit nurcluic actds
• Signicant side effcts

Protein Synhesis

• New antifungal infites protetein
• ---Amino acids
• -------use to treat nal

Antiprotozoan and Antibelminthic Medication

• --Inteftes with biosyntethis of neuornmusclar
• --diseasws will spread
• ---used totreat apicomex

Plamonsai: spweicied will result

• ------antibacterial drugs