Comprehensive Guide to Antibiotic Mechanisms and Resistance Strategies and Resistance

Targets of Antibiotics: Phospholipids and the Cell Membrane

• One primary target for antibiotics is the phospholipids within the cell membrane.
• The cell membrane structure consists of a double layer of phospholipids.
• It is critical to distinguish the cell membrane from the cell wall:
  • The bacterial cell wall is primarily composed of peptidoglycan.
  • Penicillin is the antibiotic that targets this cell wall structure, specifically utilizing its beta leptin ring.
  • The cell membrane, however, is targeted by a drug referred to as polymyosin (or polynyasin).
• Mechanism of Action for Polymyosin:
  • It dissolves the phospholipids, essentially creating holes within the cell membrane.
  • These holes cause the bacterial cell to leak its internal content.
  • Contents such as the cytoplasm and genetic material leak out.
  • As a result, the bacterial cell shrivels up and eventually dies.

Spectrum of Activity and Bacterial Classes

• Narrow Spectrum Antibiotics: These target specific classes of bacteria.
  • Penicillin: Typically more effective against gram positive bacteria. This is because gram positive bacteria possess a thicker cell wall with a higher concentration of capsidoglycan (peptidoglycan), which is the specific target of penicillin.
  • Polymyosin: Typically more effective against gram negative bacteria. These bacteria have an additional outer cell membrane, making them a primary target for drugs that attack phospholipids.
• Broad Spectrum Antibiotics: Generally, antibiotics that target protein synthesis are considered broad spectrum.
  • Because all bacteria use ribosomes to produce proteins, these drugs can target a wide range of bacteria, including both gram positive and gram negative strains.
  • Effectiveness vs. Risk: While effective, broad spectrum antibiotics have the downside of destroying normal biota (‘good’ bacteria). This can lead to complications known as super infections, such as C. Diff (Clostridioides difficile) and yeast infections.

Antibiotics Inhibiting Protein Synthesis

• General Mode of Action: These antibiotics target the bacterial ribosome, preventing the synthesis of proteins.
• Consequence of Inhibition: Bacterial cells that cannot make proteins cannot sustain life. They are unable to replicate, complete binary fission cycles, or produce energy.
• Standard Ribosomal Subunits: Bacteria have $30\,S$ and $50\,S$ subunits.
• Antibiotics Targeting the $30\,S$ Ribosomal Subunit (Small Subunit):
  • Streptomycin
  • Tetracycline
• Antibiotics Targeting the $50\,S$ Ribosomal Subunit (Large Subunit):
  • Erythromycin
  • Form of the phenol (also referred to as Thorn Clinical or cornflinocle in the transcript).
• Safety in Humans: Human cells are not affected because human ribosomes are larger, consisting of $60\,S$ and $40\,S$ subunits (totaling $80\,S$). The antibiotics are specific to the $30\,S$ and $50\,S$ sizes found in bacteria and do not harm healthy immune cells.

Antibiotics Inhibiting Nucleic Acid Synthesis

• Importance of Nucleic Acids: If either DNA or RNA is destroyed, the bacterial cell cannot survive.
  • DNA destruction prevents replication and the production of new cells via binary fission.
  • RNA destruction prevents translation and subsequent protein production.
• Specific Medications:
  • Ripopen: This antibiotic stops RNA synthesis by specifically inhibiting transcription. It inhibits the enzyme RNA polymerase, which is responsible for turning DNA into RNA. Without RNA polymerase, transcription fails, leading to a lack of proteins and cell death.
  • Cipro: This antibiotic stops DNA synthesis/replication.
• The Mechanism of Cipro and DNA Gyrase:
  • Prior to enzymes like helicase breaking hydrogen bonds, DNA exists in a tightly coiled helical format (similar to a slinky).
  • The DNA gyrase enzyme is required to uncoil and straighten the DNA so that helicase and other enzymes (DNA polymerase three, DNA polymerase one) can access the strands.
  • Cipro inhibits the DNA gyrase enzyme from working. If the DNA cannot uncoil, replication cannot proceed, binary fission stops, and the bacterial cell dies.

Metabolic Pathways: Folic Acid Inhibition

• Trimethoprim (also associated with Bactrim): This antibiotic inhibits the production of folic acid in bacterial cells.
• Importance of Folic Acid: Bacteria must produce folic acid as it is required for both DNA replication and RNA production.
• Medical Caveat for Pregnancy: Trimethoprim can be dangerous for pregnant women, particularly in the first trimester.
  • Folic acid is a primary ingredient in prenatal vitamins because it is essential for the embryological development of tissues and organs.
  • Taking trimethoprim can drop folic acid levels, potentially harming fetal development. It should generally never be prescribed during pregnancy due to the risk to fetal embryology.

Mechanisms of Antibiotic Resistance

• Definition: Drug resistance is an adaptive, learned response where bacterial cells learn to survive in the presence of an antibiotic to which they were once susceptible.
• Spontaneous Mutations: These are random errors in the genetic code made by DNA polymerase three during replication. While many mutations have no effect, some can teach a cell to become resistant. Frequent replication increases the likelihood of acquiring such mutations.
• Laboratory Safety Note: Bacteria used in labs are often fresh cultures ($24$ to $48\,hours$ old) and generally nonpathogenic. If left to divide for many days, they could potentially mutate to become pathogenic, resistant, or produce toxins. This is why cultures are sterilized in an autoclave after use.
• Horizontal Gene Transfer (HGT): This is the exchange of genetic information between different organisms. There are three types:
  1. Transformation: Bacterial cells pick up ‘naked DNA’ (DNA released into the environment following cell lysis/explosion). The cell incorporates this DNA into its own chromosome, transforming its traits (e.g., gaining resistance or the ability to produce toxins).
  2. Transduction: Mediated by a bacteriophage (a virus that infects bacteria). The phage injects DNA into the bacterium. If this DNA integrates into the bacterial chromosome and provides resistance information, it is a form of lysogenic conversion called transduction.
  3. Conjugation: Information exchange between living bacterial cells through a pili (a long, hollow tube).

The Griffith Pneumococcus Mouse Experiment

• This study first demonstrated transformation using pneumococcus bacteria in mice.
• Bacteria Strains:
  • Pneumococcus with a capsule: Highly dangerous; capsules prevent phagocytosis by immune cells like macrophages and neutrophils, leading to deadly pneumonia.
  • Pneumococcus without a capsule: Mild; easily cleared by the immune system.
• Experimental Groups:
  • Mouse $1$: Injected with living pneumococcus without a capsule. Result: Lived (immune system cleared it).
  • Mouse $2$: Injected with living pneumococcus with a capsule. Result: Died of severe pneumonia.
  • Mouse $3$: Injected with a dead version of the capsulated bacteria. Result: Lived.
  • Mouse $4$: Injected with a mixture of dead capsulated bacteria and living non-capsulated bacteria. Result: Died.
• Conclusion: The living non-capsulated bacteria picked up naked DNA from the dead capsulated bacteria. This DNA contained the ‘recipe’ for building a capsule. The living cells transformed into the dangerous strain.

Dynamics of Resistant Populations

• Survival of the Fittest: In a typical bacterial population, most are susceptible (yellow) and a few are resistant (red).
• If an antibiotic is used continuously, it wipes out the susceptible population.
• This removes competition for space and nutrients, allowing the resistant strain to grow out of control.
• Bystander Effect: Normal biota in the gut can become resistant via exposure. While resistance in ‘good’ bacteria seems harmless, they can use conjugation to share resistance plasmids (R plasmids) with dangerous pathogens like Salmonella, making infections more difficult to treat.

Causes and Contributing Factors to Antibiotic Resistance

• Overprescribing: Antibiotics are often requested by patients and prescribed by clinicians for viral infections (e.g., respiratory viruses). Antibiotics are completely ineffective against viruses because viruses lack the bacterial targets like cell walls and ribosomes.
• Lack of Diagnostic Testing: In an ideal scenario, a culture should be performed to determine if an infection is bacterial or viral before treating. Often, patients are simply given a broad spectrum antibiotic out of convenience.
• Patient Non-compliance: Patients often stop taking antibiotics after approximately $3\,days$ because they start to feel better as the bacterial load decreases. However, the bacteria remaining are those that require later doses to be killed. Stopping early allows these bacteria to grow back, potentially with newly acquired resistance.
• Food Industry: The use of antibiotics in animal agriculture (e.g., poultry) to prevent sickness continuously exposes environmental bacteria to drugs, facilitating the selection of resistant strains.
• Hospital Infection Control: Resistant strains are highly contagious and can be spread through coughing, sneezing, or poor hygiene (e.g., failure to change gloves between patients).

Questions & Discussion

• Question: So we want the bacteria to be susceptible?
• Response: Yes, susceptibility means the antibiotic kills them. We do not want them to be resistant.
• Question: Is the picture showing the antibiotic wiping out the susceptible ones and leaving the resistant ones?
• Response: Yes. It leaves space and nutrients for the resistant ones to create a new population that the original drug cannot treat. This is common in elderly populations who have used antibiotics for many years or are immunocompromised.
• Question: Why isn't the DNA destroyed in the process [of cell lysis]?
• Response: It depends on the target. If the antibiotic targets the cell wall or membrane, the DNA is released into the environment rather than being specifically dismantled.
• Question: What is in a Z-Pak?
• Response: A Z-Pak contains azithromycin (an antibiotic) and may include additives to reduce inflammation. While it is an antibiotic, its ability to reduce inflammation can help in respiratory cases to prevent fluid accumulation (pneumonia), though it shouldn't be the primary treatment for a simple virus.
• Question: Should I stop taking eye drops when I feel better?
• Response: No. Even with medications like eye drops every two hours, the entire course must be finished. Stopping early can allow resistant bacteria to return with a vengeance.