Control of Microbial Populations (Bacterial and Fungal) Flashcards
Biocides
- physical or chemical agents used to control microbes
- disinfectant, antiseptic or temperature
Disinfection
- destruction of VEGETATIVE pathogens on inanimate surfaces
- spores and other relatively resistant organisms (i.e. mycobacteria, viruses and fungi) may remain viable
- too harsh to be used on tissues
- may use physical or chemical methods
Antisepsis
- destruction of vegetative pathogens on living tissues
- almost always be chemical methods
- sporicidal action not implied
- commonly used as components in soaps, hand gels
- effectiveness determined by:
a) microorganisms present
b) the level of toxicity of the chemical to the tissues
Degerming
- removal of microbes from a limited area (i.e. skin around injection site)
- mostly mechanical removal by alcohol-soaked swab
Sterilisation
- absolute term - either something is sterile or it isn’t
- destruction and removal of all forms of microbial life (including endospores)
- prions may not be removed
- i.e steam under pressure, sterilising gas (ethylene oxide)
Sanitisation
- treatment intended to lower microbial counts to safe public health levels
- usually eating/drinking utensils
- i.e. high-temp washing, dipping into chemical disinfectant
-Static
- inhibition of further growth
- bacteria enters stationary phase
-Cidal
- decreases cell numbers (killing effect)
- bacteria enters death phase
Moist heat
- physical sterilizing agent
- steam at 121 degrees celsius and greater than atmospheric pressure (i.e. 2 atm)
- sterilizes in 15 minutes
- not for heat-sensitive objects (solutions, plastics)
Ethylene Oxide Gas
- chemical sterilizing agent (used in gaseous form, it’s considered a physical agent)
- objects placed in chemiclave and gas pumped in
- alkylating agent; alkylation of terminal hydroxyl, carboxyl, amino, and sulfhydryl groups. This process blocks the reactive groups required for many essential metabolic processes (i.e. alkylates guanine and funtional groups of proteins -> prevents DNA replication)
- used to sterilize heat-sensitive objects like sutures, bandages, and grafts
- flammable, explosive and carcinogenic to lab animals
Generations
- lab has altered the chemical composition of a naturally-occuring antibiotic
- by altering the side chains, can change the properties of the antibiotic
Benzyl penicillin
- natural penicillin
Ampicillin
- aminopenicillin
- can be taken orally
Methicillin
- penicillinase-resistant penicillins
- overcome degrading enzymes that some bacteria possess to break down natural penicillin
Piperacillin
- extended spectrum penicillin
- active against some gram - bacteria in addition to gram + bacteria
Beta-lactams
- Penicillins, Cephalosporins
- cell wall synthesis inhibitor
- binds penicillin-binding proteins (PBPs) - serine proteases (transpeptidases) that perform cross-linking of the peptidoglycan layer
- binding inhibits assembly of peptidoglycan layer -> activates AUTOLYSIS (degrades cell wall -> cell death)
- ineffective against mycobacteria (b/c cell wall is impenetrable) and mycoplasmas (b/c they lack a cell wall)
Isoniazid/ Ethionamide/ Ethambutol/ Cycloserine
- cell wall synthesis inhibitors
- treat mycobacterial infections
Cycloserine:
- inhibits D-alanine-D-alanine synthetase and alanine racemase (both enzymes catalyze cell wall synthesis)
Bacitracin
- cell wall synthesis inhibitors
- topical treatment
- treats gram + bacteria (Staphylococcus, Streptococcus)
- gram - bacteria are resistant
- interferes with dephosphorylation and recycling of lipid carrier responsible for moving peptidoglycan precursors through cytoplasmic membrane to cell wall
Aminoglycosides
- streptomycin, gentamicin
- bind irreversibly to the 30S ribosomal subunit
- induces codon misreading (changes the shape of the 30S portion and causes the code on the mRNA to be misread incorrectly)
- not taken up by mammalian tissues
- treats gram - bacterial infections
- not effective against anaerobes (b/c oxidative phosphorylation is absent in anaerobes)
Tetracyclines
- binds reversibly to the 30S ribosomal subunit
- blocks binding of amioacylated tRNA to A site (on the mRNA-ribosome complex)
Macrolides
- erythromycin
- binds 50S ribosomal subunit
- inhibits transpeptidation and translocation
Lincosamides
- clindamycin
- binds 50S ribosomal subunit
- targets binding at A and P sites
Streptogramins
- chloramphenicol
- binds 50S ribosomal subunit
- inhibit peptide bond formation
Quinolones
- ciprofloxacin, levofloxacin (broad spectrum)
- nucleic acid synthesis inhibitors
- binds to both the alpha-subunit of DNA gyrase and topoisomerase IV -> inhibits DNA synthesis
Rifampin
- nucleic acid synthesis inhibitors
- binds to the beta-subunit of DNA dependant RNA polymerase complex -> inhibits transcription of mRNA
Glycopeptides
- vancomycin
- cell wall synthesis inhibitor
- interacts with the D-alanine-D-alanine termini of the pentapeptide side chains -> interferes sterically with the formation of the bridges between the peptidoglycan chain -> disrupts cell wall peptidoglycan synthesis in growing gram-positive bacteria.
- ineffective against gram - bacteria (the molecule is large and cannot penetrate into the cell)
Nitroimidazoles
- metronidazole
- Interact with DNA
- Inhibit metabolism of glucose
- Interfere with mitochondrial function
- ineffective against aerobes (molecule requires activation by flavodoxin, which is absent in aerobes)
Sulfamethoxazole/Trimethoprim combination treatment
Sulfamethoxazole (nucleic acid synthesis inhibitor)
- inhibits dihydropteroate synthetase (converts PABA -> dihydrofolic acid)
Trimethoprim (metabolism inhibitor)
- inhibits dihydrofolate reductase (converts Dihydrofolic acid -> tetrahydrofolic acid)
- most bacteria synthesize their own folic acid, so this treatment works best against those bacteria
- **exception: Enterococci use exogenous folic acid
Antibiotic target modification (mechanism of resistance)
- bacteria possess MecA gene - allows bacteria to produce alternative transpeptidase enzymes in the presence of antibiotics (i.e. beta-lactams like meticillin) so that the antibiotic cannot recognize the transpeptidase and the peptidoglycan layer can continue to grow in the presence of the antibiotic
Inactivating enzymes (mechanism of resistance)
i. e. Beta-lactamases
- either constitutively produced, or produced in the presence of an antibiotic
- inactivate beta-lactam antibiotics
***can give the patient beta-lactamase inhibitors (i.e. clavulanic acid)
Conjugation
- only in gram - bacteria (only gram - bacteria have the F pilus)
prior to exchange:
donor: F+ cell (i.e. penicillin sensitive)
recipient: F- cell (i.e. penicillin resistant)
after exchange:
recipient: F+ cell (i.e. penicillin resistant)
- single strand of plasmid transferred to recipient cell
- once strand present in the recipient, complimentary strands formed in both cells
High Frequency Recombination
- happens in conjugation when there is a complete transfer of the F plasmid and it’s integration into the chromosome of the recipient cell
Transformation
- happens during the late lag phase
- “free” DNA expelled into the environment and taken up by DNA binding protein on recipient cell
1st strand - degraded
2nd strand - recombination and incorportation into chromosome
Transduction
- DNA from bacteriophage incorporated into bacteria’s chromosome
- Virulent (lytic) bacteriophages - death of cell by lysis
- Temperate bacteriophages - switch between virulent and lytic phase
Lysogeny
- when bacteria are carrying a prophage
- The bacteria’s gene expression is repressed
- Phage gene expression and replication are triggered by certain conditions
Transposition
Transposons (Tn)/ insertion sequences = jumping genes
chromosome -> plasmid
chromosome -> chromosome
- transposons move around via non-homologous recombination, via the action of site-specific recombinases (Transposase)
- simple transposon - no selectable genes
- complex/ composite transposon - antibiotic resistance or other trait
- plasmids may be numerous transposons together
Polyenes
- Amphotericin B
- direct fungal membrane damage
- lipid-loving compounds; bind to ergosterol in the membrane and form pores that result in leakage of internal cell contents out of cell
- “cidal” to the fungi
- doesn’t discriminate sterol attachment; also used as an antiparasitic (for Leishmania spp.) and -can be toxic to host cells as well
Flucytosine
- nucleic acid synthesis inhibitor/ antimetabolite
- flucytosine (5-FC, 5-fluorocytosine) is an analog of cytosine
- converted by fungal cytosine deaminase into 5-fluorouracil -> undergoes phosphorylation steps -> 5-fluorouridine triphosphate and 5-fluorodeoxyuridine monophosphate are incorporated into RNA/DNA synthesis process -> inhibition of synthesis
Azoles
- fluconazole
- ergosterol biosynthesis inhibitor
- inhibits 14alpha-demethylase (converts lanosterol -> ergosterol)
Allylamines
- terbinafine
- ergosterol biosynthesis inhibitor
- inhibits squalene expoxidase -> accumulation of squaline (which is incorporated into the cell membrane)
- in addition, no lanosterol or ergosterol formed
- total result is an increased membrane permeability
Echinocandins
- caspofungin
- beta-glucan synthesis inhibitors
- block (1,3)-beta-D-glucan synthetase
*active against Candida and Aspergillus, but not C. neoformas
Nikkomycin Z
- chitin synthesis inhibitors
- competes with UDP-N-acetylglucosamine for chitin synthase
- still under investigation for therapeutic potential
