Microbiology Final

How do you determine MIC (minimal inhibitory concentration)

Identify the last tube without bacterial growth

How do you determine MBC (minimal bactericidal concentration)

plate each tube with no growth and determine which plate also has no growth (also called MLC)

• MLC will be greater than or equal to MIC

Which antibiotics target the cell wall (peptidoglycan synthesis)

Beta-lactam antibiotics and vancomycin

• penicillin, methicillin, amoxicillin, ampicillin, dicloxacillin are all beta lactams

• all are bactericidal

How do beta-lactams work

they resemble the d-ala-d-ala ring of the peptidoglycan, allowing it to bind to transpeptidase (side chain) and transglycolase (polymerizes proteins), blocking synthesis

natural penicillin

• narrow spectrum

• for gram positive bacteria (and some G-)

penicillinase-resistant penicillin

• side chains prevent inactivation by penicillinase enzymes

• includes methicillin (MRSA) and dicloxacillin (acid resistant)

• these semi-synthetic drugs are made by modifying the R group on the beta-lactam ring

broad spectrum penicillin

• acid resistant

• can effect G + and -

• ampicillin and amoxicillin (the more active option)

vancomycin

• directly binds to the d-ala-d-ala terminal, blocking transpeptidase and transglycolase

• only works on G + (too large for G -, also given intranvenously due to poor intestinal absorption)

• a large glycopeptide made by streptomycete

which antibiotics affect the cell membrane

• gramicidin and polymyxin

• toxic to human cells and bactericidal

• only work on Gram - bacteria

gramicidin

• cyclic peptide produced by bacillus brevis

• forms a cation channel for ions to leak out of

polymyxin (colistin)

• only for topical use; destroys cell membrane like detergent

• produced by bacillus polymyxa

which antibiotics effect dna synthesis

quinolones and sulfa drugs

quinolones

• flouroquinolones, nalidixic acid, ciprofloxacin

• block dna gyrase, stopping replication

• bactericidal

sulfonamides

• first commercialized antimicrobial

• analogs of PABA (folic acid precursor), preventing folic acid (and therefore dna) synthesis

gerhard domagk put it in diethylene glycol raising the need for the FDA

*mimicry like penicillin

which anitbiotics effect rna synthesis

• rifamycin B and actinomycin D

• bactericidal b/c they inhibit transcription

• best for stopping bacteria that is growing

Rifamycin B

• bind to beta subunit of rna pol. preventing elongation in transcription

Actinomycin D

• nonselectively binds to dna, preventing initiation

• toxic to host as well

which antibiotics effect protein synthesis

30s subunit: aminoglycosides and tetracyclines

50s subunit: macrolides, chloramphenicol, and clindamycin

aminoglycosides

• change the 30s shape, causing translational misreading

• bactericidal*

• streptomycin and gentamicin

tetracyclines

• blocks binding of charged tRNA’s to the A-site

• bacteriostatic*

• doxycycline

macrolides (erythromycin)

inhibit translocation

chlorophamenicol

inhibits peptidyl transferase activity

clindamycin and metronidazole

• kill cdiff in microbiome

• bind at the chloramphenicol ribosomal binding site preventing peptide bond formation

• active in anaerobic envrionments

• metronidazole exclusively kills obigate anaerobes and protozoa

which virus causes the common cold

rhinovirus

which antiviral is the flu vulnerable to

• amantadine

• prevents entry into host cell

*has developed some resistance and must be taken early in infection

what are zanamivir and tamiflu

neuraminidase inhibitors (prevent release of a mature virus)

how do some antivirals inhibit viral dna synthesis

• resemble normal dna nucleotides but lack a 3` OH end, causing chain termination

• replace the OH with NH3

• zidovudine is an analog of thymine

why are fungi hard to treat

because they are eukaryotic and resemble human cells, it is difficult to develop selectively toxic drugs

what are the synthetic antifungals

azoles and terbinafines

azoles

inhibit membrane sterol synthesis

• clotrimazole, intraconazole, and fluconazole

terbinafines

inhibit ergosterol synthesis

• humans don’t have this in our cell membranes

what are the non-synthetic antifungals

polyenes (made by streptomyces and amphotericin B. nystatin) and Griseofulvin (made by penicillium)

Polycenes

form pores in fungal membranes

griseofulvin

disrupts mitotic spindle

which antibiotic will kill bacteria but not archaea

• must affect peptidoglycan or dna replication, not cell membrane

what causes antibiotic resistance

• overprescription and overuse

• animal feed

• unfinished treatments

• poor infection control / sanitation

• lack of new antibiotics

integrons

highly mobile gene expression elements causing resistance

resistance by expellation or prevention of entry

• beta-lactamase enzyme

• G - inherent resistance to peptidoglycan effectors

• MDR efflux pumps expell beta lactams, tetracyclines, and fluoroquinoles (similar to cancer cells and chemo)

resistance by binding prevention

• mutations in penicillin-binding proteins (against methicillin) and ribosomal proteins (streptomycin)

• add modifying groups to cyclohexane ring of aminoglycosides, preventing it from binding to 16s RNA

reversing the binding of antibiotics

• G + microbes can release proteins that knock off erythromycin antibiotics bound on peptidyltransferase site

what are the ESKAPE pathogens

• E. coli

• Staphylococcus

• Klebsiella pneumoniae

• Actinobacter baunaminnii

• Pseudomonas aeuroginosa

• Enterococcus faecalis

what are the 4 ways to fight rising drug resistance

1) clavulanic acid (dummy compound inactivating beta-lactamase) and amoxicillin form augmenten

2) alter antibiotic structure (amikacin is a modified version of gentamicin)

3) find new antibiotics through genome sequencing, quorum sensing mechanisms, crispr-based resistance reversal

4)antibiofilm approaches: induce biofilm dispersal

Which LD50 is more virulent: 5×10^4 or 5×10^7

5x10^4

what are the molecular postulates

1. phenotype associated with pathogenic strain

2. virulence gene isolated

3. causes specific inactivation and loss of pathogenicity

4. replacement should restore pathogenicity

characteristics of pathogenicity islands

• high GC % compared to rest of genome

• flanked by phage/plasmid genes

• linked to tRNA genes

E.coli v.f.

• type 1 pili

• type 3 ss

• shiga

neisseria meningitis v.f.

• type 4 pili

• lps

streptococcus pyogenes v.f.

• M protein (non-pilus adhesion)

• strep pneumonia has a thick capsule for extracellular avoidance

Bordetella pertusis

• pertacin (non-pilus adhesion)

• type 4 secretion system

Staph aureus

• alpha toxin endotoxin

• antibody neutralizing proteins for extracellular avoidance

anthrax

• 2 subunit a/b toxins

cholera

• 2 subunit a/b toxins

shiga

• 2 subunit a/b toxins

• type 3 ss

• hemolysis of phagosome

G - bacteria

• lps endotoxins

pseudomonas aeruginosa and vibrio cholerae

• type 2 ss

salmonella

• type 3 ss

• fusion of lysosome prevention

pili attachment factor types

• type 1: adhere to mannose residues on membrane

• type 4: re/disassembling from inner membrane

non pilus attachment types

• M protein: bidns to fibronectin

• Pertactin: binds to host cell integrin

exotoxins

• alpha toxin: cause hemolysis

• ab toxins: cause cyclic AMP to raise, → dehydration

endotoxins

G - lps causes cytokinasr release triggering fever shock and death

secretion system factors

• T2ss: proteins extend and retract like pili, going through inner and outer membrane

• T3ss: molecular syringe triggers by cell contact

• T4ss: conjugation of proteins instead of dna

survival factors

• thick capsules

• antibody neutralization

• apoptosis/phagocytosis

• hemolysis of phagosomes

• prevention of lysosome fusion

• maturation in acidic environments

mycobacterium tubercolosis

• shiga

• fusion of lysosome prevention