Antibiotics and Antibiotic Resistance Flashcards
Disinfectants
antimicrobial agents that are applied to inanimate objects (e.g. floors, tables, walls)
Antiseptics
antimicrobial agents that are sufficiently nontoxic to be applied to living tissues (e.g hand sanitizers)
Antibiotics
antimicrobial agents produced by bacteria and fungi that are exploited by humans (delivered topically and internally)
Who is Alexander Fleming?
- discovered penicillin in 1928
- produced from Penicillium
- colonies of staphylococci couldn’t grow around a contaminating mold
- Nobel Prize in Physiology and Medicine (1945)
What are two major problems related to antibiotics?
- Diminished interest from pharmaceutical companies to develop new antibiotics
- Bacterial resistance to antibiotics always happens
How is antibiotic overused/misused?
- Empiric use (blinded use)
- Increased use of broad spectrum agents
- Pediatric use for viral infections
- Patients who do not complete course (chronic disease, e.g. TB)
- Antibiotics in animal feeds
Minimum Inhibitory Concentration (MIC) purpose
- used to measure antibiotic activity
- lowest amount of antibiotic that will inhibit growth
What was the old way to test MIC
Have aSeries of culture tubes with varying concentration of agent and see which one is effective at the lowest concentration
Modern way to test MIC
- Use antibiotic strips
- Plate bacteria
- Put antibiotic strips (multiple antibiotics)
- Allow bacteria to grow
- Zone of inhibition - where bacteria cannot grow
What do antibiotics target?
- Cell wall synthesis
- Protein synthesis
- DNA/RNA synthesis
- Folate Synthesis
- Cell membrane alteration
- targets are not present (or are different) in eukaryotic cells
B - Lactam Antibiotics
- e.g. Penicillin
- contain a “β lactam ring”
- function to inhibit cell wall synthesis in bacteria
β lactams bind the bacterial “penicillin-binding proteins (PBPs)” - PBPs are transpeptidases
- no peptide cross-links = weak cell wall = cell death
- but some bacteria can produce a β lactamase, an enzyme that destroys the ring and thus the antibiotic (bacteria select against this antibiotic)
Modified B Lactam Antibiotics
- e.g. Methicillin
- contains a “ β lactam ring”
- chemically modified penicillin
- can’t be cleaved by β lactamases
- but some bacteria can produce a different “penicillin-binding protein” (e.g. PBP2a) – encoded by ‘mec’
- PBP2a doesn’t bind methicillin (or other β lactams), so antibiotic no longer works
T/F: antibiotic resistance is always present
TRUE: it is always present, but the use of the antibiotic makes it favoured it in the environment and therefore selects for it
Vancomycin
- a glycopeptide antibiotic
- inhibits cell wall synthesis in Gram positives
- often a drug of “last resort” (e.g. HA-MRSA)
- Vancomycin binds the peptide linkage at terminal D-Ala-D-Ala residues and inhibits transpeptidation
- resistance genes change these to D-Ala-D-Lac and vancomycin can no longer bind
- resistance is encoded by the van genes
Protein Synthesis Inhibitors
- Bacteria contain 70S (30S+50S) ribosomes
- Eukaryotes contain 80S (40S+60S) ribosomes
Many antibiotics target bacterial ribosomes and block translation (inhibit the synthesis of proteins)
Name two 50s inhibitors
Erythromycin, Chloramphenicol
Name two 30s inhibitors
Tetracycline, Kanamycin
Folic Acid Synthesis Inhibitors
- e.g. Trimethoprim and Sulfonamides
- folic acid is a vitamin (B9) for humans that we consume
- bacteria need folic acid for thymidine synthesis
- but bacteria cannot absorb folic acid so they must synthesize their own
- inhibition of folic acid synthesis blocks DNA replication
DNA/RNA Synthesis Inhibitors - Fluoroquinolones
- interfere with DNA gyrase needed for supercoiling of DNA
DNA/RNA Synthesis Inhibitors - Rifampicin
inhibits bacterial RNA polymerase
Cell Membrane Alteration
- e.g. Polymyxin B (polysporin has this)
- used for Gram negatives
bind to LPS - hydrophobic tail inserts and disrupts outer and inner membranes
Selection for Antibiotics
Paradoxically, the use of antibiotics actively selects for antibiotic resistant bacteria
- NO antibiotic: the number of resistant bacteria remain the same (very low population)
- WITH antibiotic: eventually the whole population is only resistant bacteria
- Same thing can happen in an infectious environment
Bacterial Strategies for Antibiotic resistance
Prevention of antibiotic entry
- Gram negative outer membrane and mycobacteria cell wall
Antibiotic modification
- β lactamase (enzyme destroys antibiotic)
Efflux of antibiotic
- actively pump out the antibiotic
Alteration of antibiotic target
- PBPs, Ribosome modifications
Bypassing the antibiotic action
- use environmental folic acid
Which class of bacteria is more resistant to antibiotics (gram neg or pos) and why?
Gram negative, because it is harder to get through the membrane
Antibiotic resistance genes
- many mechanisms of antibiotic resistance are genetically encoded (e.g. mec, β lactamase, efflux pumps)
- can produce very high levels of antibiotic resistance
- often encoded on mobile genetic elements (e.g. plasmids) allowing for horizontal gene transfer “superbugs”
Horizontal Gene Transfer
rather than alter gene function through mutations, new genes are acquired from another source
Name the three methods of HGT
Transformation, transduction, conjugation
Transformation
– recipient cell takes up DNA from donor cell (dead) in the the environment – not passive diffusion, there are mechanisms that do this
Transduction
- transfer of DNA via virus (bacteriophage)
- donor cell has resistant gene
- gets infected with virus
- package genome into virus particle during replication
- sometimes, they make mistakes and package pieces of chromosome in there
- virus get out, and inject resistant gene into another cell
Conjugation
- typically on plasmid, machinery that actively moves DNA across and to a recipient cell
- very high efficiency
Klebsiella pneumoniae
- Gram negative
- an important cause of nosocomial pneumonia
- produces a capsule and is commonly resistant to multiple antibiotics
- first documented source of “NDM-1” (New Delhi Metallo-beta-lactamase-1)
- also known as a carbapenemase (enzyme)
What are carbapenem antibiotics
- carbapenem antibiotics are β-lactamase resistant β-lactams with broad spectrum activity
- NDM-1 is now widespread in other Gram negatives
= CRE (carbapenem resistant Enterobacteriaceae)
Clostridia
- Gram-positive, rod shaped, endospore-formers
- Strict anaerobes, vegetative cells killed by O2
- Generally found in soil and intestinal tracts of animals
- can cause life threatening diseases mediated by exotoxins
what does clostridium difficile cause?
pseudomembranous colitis
what does Clostridium tetani cause?
tetnus
what does clostridium botulinum cause?
botulism
what does perfringens cause?
food-borne illness and gas gangrene
Clostridium difficile (“C. diff”) exists as:
- asymptomatic carrier state in the large intestine (most common)
- cause of mild to moderate diarrhea
- cause of life-threatening pseudomembranous colitis
Where is C. diff often found?
- often found in nursing homes and hospital environments
- a nosocomial pathogen
What is the mode of transmission of C. diff?
through the spore: fecal-oral route
T/F: Endospores are very difficult to eradicate from the environment.
TRUE: can be cultured from floor, bed pans, toilets, hands and clothing of medical personnel
Pseudomembranous colitis - most important risk factor
- most symptomatic patients have recently received an antimicrobial agent
What is pseudomembranous colitis?
- an inflammatory condition of the large intestine
- offensive smelling diarrhea, abdominal pain, fever, nausea, dehydration
- symptoms may occur 1-2 days after antibiotics or several weeks after the antibiotic is discontinued
- endoscopy can show characteristic yellow lesions
- lesions can enlarge to cover substantial portions of inflamed mucosa and can be stripped off (the pseudomembrane)
How does the disease, pseuduomembranous colitis manifest?
- antibiotics are used to cure infections, but they also kill the normal microbiota
- suppression of normal microbiota + persistence of C. difficile endospores
- after the antibiotic is stopped, spores germinate, overgrowth of C. difficile occurs with production of toxins
- C. difficile does not itself invade, but the toxins damage the intestinal lining of the large intestine
How does C.diff cause damage?
- C. difficile produces A-B toxins called the large clostridial cytotoxins
- “A-B” serves to designate two domains
- The A (active) domain denotes the active portion of the toxin that carries the enzymatic activity
- The B (binding) domain denotes the portion of the toxin molecule responsible for binding and uptake by the host cell
- “A” domain functions to inactivate key regulatory proteins of host cells which causes disregulation of multiple cell processes including cytoskeletal rearrangements, cell death and inflammation
Diagnosis and Treatment of Pseudomembranous Colitis
- history (antibiotic use), symptoms and laboratory tests to confirm C. difficile
- endoscopy and toxin detection assays
- discontinue inciting antibiotic if still being used
fluids - antibiotics more specific for “C. diff” – oral vancomycin or I.V. metronidazole
- avoid antidiarrheal agents (would cause decreased toxin clearance)
Fecal Microbiota Transplantation
- Treatment: transferring someone else’s fecal microbiota into your intestine
- Get microbiota from healthy patient, and see if you can cure C diff. (it works!) –> New England Journal study
