(Section C: Bacteriology) Lecture 20 Flashcards
What are the 3 classifications of antimicrobial agents?
- Disinfectants
- Antiseptics
- Antibiotics
Disinfectants
Antimicrobial agents that are applied to inanimate objects
* Floors, tables, walls etc.
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
How are antibiotics delivered/administered?
- Topically
- Internally
What are 2 major problems with antibiotics?
- Diminished interest from pharmaceutical companies to develop new antibiotics
- Bacterial resistance to antibiotics always happens
What are 5 examples of misuse of antibiotics?
- Empiric/blinded use
- Increased use of broad-spectrum agents
- Pediatric use for viral infections
- Patients who do not complete course of treatment
- Antibiotics in animal feeds
Blinded use
Using random antibiotics for diseases
Increased use of broad-spectrum agents
Antibiotic that is effective against wide variety of bacteria
* Causes large-scale development of resistance
Pediatric use for viral infections
Misuse, not useful for viral infections and may cause resistance to develop by bacteria
Patients who do not complete course of treatment
E.g. Tuberculosis
* Treatment usually lasts 6 months or more
* Bacteria may develop resistance if the treatment is not carried through all the way
Antibiotics in animal feeds
Spreads to environment through defecation
* Causes bacteria in the environment to gain resistance
How is antibiotic activity measured?
Minimum inhibitory concentration (MIC)
What is the MIC defined as?
The lowest concentration of agent that inhibits growth completely
How is MIC conducted (traditionally)?
Series of culture tubes with varying concentration of antibiotic
* MIC is the lowest concentration where there is no growth of bacteria
How is MIC conducted (modern)?
Antibiotic strips
* Faster to do
* Can test multiple antibiotics at the same time
How do antibiotics work?
Target essential bacterial components
What components do antibiotics target?
- Cell wall synthesis
- Protein synthesis
- DNA/RNA synthesis
- Folate Synthesis
- Cell membrane alteration
How do antibiotics ensure they target bacterial cells?
Targets are either
* Not present in eukaryotic cells
* Different in eukaryotic cells
β Lactam Antibodies
* Example
* Structure
* Function
* Penicillin, Methicillin
* Contain a "β Lactam ring"
* Function to inhibit cell wall synthesis in bacteria
How do β Lactam antibiotics work to inhibit cell wall synthesis?
- Bind to bacterial “penicillin-binding proteins” (PBPs)
- PBPs are transpeptidases
- No peptide cross links = Weak cell wall = Cell death
How do bacteria counter β Lactam Antibiotics?
(Unmodified, use penicillin as example)
Produce β Lactamase
* Enzyme that destroys the β Lactam ring
How is methicillin different from penicillin?
- Chemically modified penicillin
- Cannot be cleaved by β lactamases
How do bacteria counter β Lactam Antibiotics?
(Modified, use methicillin as example)
Produce a different “penicillin-binding protein” (PBP)
* Called PBP2a or ‘mec’
* Doesn’t bind to methicillin or other β lactams
Vancomycin
* Type of antibiotic
* Function
- Glycopeptide antibiotic
- Inhibits cell wall synthesis in Gram positives
True or False:
Vancomycin is used last in a treatment
True, it is a drug of “last resort”
How does Vancomycin work?
Binds the peptide linkage at terminal D-Ala-D-Ala residues
* Inhibits transpeptidation, weakens cell walls
How do bacteria counter Vancomycin?
Changes terminal to D-Ala-D-Lac
* Vancomycin can no longer bind
* Encoded by ‘van’ genes
What are methods that bacteria use for antibiotic resistance?
- Prevention of antibiotic entry
- Antibiotic modification
- Efflux of antibiotic
- Alteration of antibiotic target
- Bypassing the antibiotic action
Examples:
Prevention of antibiotic entry
- Gram negative outer membrane
- Mycobacteria cell envelope
Examples:
Antibiotic modification
β lactamase
Definition:
Efflux of antibiotic
Actively pumping out the antibiotic
Examples:
Alteration of antibiotic targett
- PBPs
- Ribosome modifications
Examples:
Bypassing the antibiotic action
Using environmental folic acid
True or False:
Many mechanisms of antibiotic resistance are genetically encoded
True
Where are antibiotic resistance mechanisms encoded on?
Mobile genetic elements (plasmids)
* Allows for horizontal gene transfer
What can horizontal gene transfer create?
Superbugs
What is horizontal gene transfer?
New genes are acquired from another source
* Not acquired through gene mutations
What are forms of horizontal gene transfer? Explain each
- Bacterial transformation: Cell lysis releases genes to other cells
- Bacterial transduction: Virus picks up gene and implants in another cell
- Bacterial conjugation: Gene directly transferred to another cell when they join
Klebsiella pneumoniae
* Type
* Disease it causes
* Antibiotic
- Gram negative
- Causes nosocomial pneumonia
- Carbapenem antibiotics
What are carbapenem antibiotics?
β lactamse resistant β-lactams with broad spectrum activity
What does Klebsiella pneumoniae do in its life cycle?
Produces a capsule
How is Klebsiella penumoniae resistant to carbapenem?
“NDM-1” enzyme
* Known as carbapenemase
True or False:
NDM-1 is only present in Klebsiella pneumoniae
False, it is now widespread in other Gram negatives too (e.x. CRE - carbapenem resistant Entrobacteriaceae)
Clostridia
* Type
* Shape
* Characteristics
- Gram positive
- Rod shaped
- Endospore formers, strict anaerobes
Where is Clostridia generally found?
- Soil
- Intestinal tracts of animals
List:
Important human pathogens in Clostridia
- Clostridiodes difficile (pseudomembraneous colitis)
- Clostridium tetani (tetanus)
- Clostridium botulinum (botulism)
- Clostridium perfringens (food-borne illness, gas gangrene)
How do Clostridia cause life threatening disease?
Exotoxins
How can Clostridioides difficile exist as?
- Asymptomatic carrier state in the large intestine
- Cause of mild/moderate diarrhea
- Cause of life-threatening pseudomembranous colitis
What type of pathogen is C. difficile? Where is it often found?
A nosocomial pathogen
* Nursing homes, hospital environments
Why is C. difficile so difficult to eradicate?
Endospores are difficult to eradicate
* Cultured from floor, bed pans, toilets, hands, and clothing of medical personnel
Mode of transmission of C. difficile
Fecal-oral route
What condition can C. difficile cause?
Pseudomembraneous colitis (aka Antibiotic-associated diarrhea)
What is pseudomembraneous colitis?
Inflammatory condition of the large intestine
* Risks: Recently received an antimicrobial agent
Signs/symptoms of pseudomembranous colitis
- Diarrhea
- Abdominal pain
- Fever
- Nausea
- Dehydration
How long does it take for signs/symptoms of pseudomembranous colitis to show?
May occur
* 1-2 days after antibiotics
* Several weeks after the antibiotic is discontinued
What do endoscopy in pseudomembranous colitis patients show?
Characteristic lesions
* Enlarge to cover substantial portions of inflamed mucosa
* Can be stripped off
What is the etiology behind pseudomembranous colitis?
Antibiotics cure infections, but also kill normal microbiota
* Suppression of normal microbiota + Persistence of C. difficile endospores = Pseudomembraneous colitis
Is C. difficile considered an invasive bacterium?
No
* Exotoxins cause damage and inflammation to the intestinal lining of the large intestine
What toxins do C. difficile produce?
A-B toxins known as “Large clostidial cytotoxins”
What does the “A-B” stand for in A-B toxins?
- A domain: Active portion of the toxin that carries the enzymatic activity
- B domain: Binding portion, responsible for binding and uptake by the host cells
How does the A domain work in large clostridial cytotoxins?
Inactivates key regulatory proteins of host cells
* Causes dysregulation of multiple cell processes including cytoskeletal rearrangements, cell death and inflammation
How is pseudomembraneous colitis diagnosed?
- History: Antibiotic use
- Symptoms
- Laboratory tests: C. difficile
- Endoscopy
- Toxin detection assays
What is the treatment for pseudomembranous colitis?
- Discontinue antibiotic
- Fluids
- Use antibiotics specific for C. difficile (oral vancomycin or I.V. metronidazole)
Why should antidiarrheal agents be avoided in pseudomembranous colitis?
Would cause decreased toxin clearance
What is a new treatment method for pseudomembranous colitis?
Fecal microbiota transplantation
What is fecal microbiota transplantation? Why is it used?
Literally taking fecal matter from healthy patient and implanting into patients with pseudomembranous colitis
* Builds the microbiota lost to antibiotic use
