2) Iodine - mordant; enhances color of crystal violet and forms complex which is retained by thick Gram + cell wall but not Gram - wall
3) Decolorization - extracts dye from gram - but not gram +
4) Counterstain - safranin; bind to gram - cells showing up as red/pink
what color would a gram + cell be?
what color would a gram - cell be?
What is the difference in composition between a gram - and gram + wall?
A gram - wall has thin layers of peptidoglycan that the alcohol creates pores in therefore releasing the crystal violet. The gram + wall has a thick layer of peptidoglycan that dehydrates and compresses in response to alcohol which traps the crystal violet thus retaining the dye and showing up as purple/blue.
What is a capsule composed of?
Glycolax or slime layer composed of polysaccharides
How is a capsule stain performed?
1) Bacterial cells are chemically adhered to slide
2) Add acidic/negative stain (congo red or nigrosin) to stain background
3) Add basic stain to (crystal violet or safranin) to stain cell
Is a capsule stain heat fixed? why?
No, because it causes the cells to shrink and the appearance of an artificial white halo that can be misinterpreted as a capsule
B) Bacillus spp; gram +
What is shown here?
E. coli (bacillus-coccus); gram -
What is shown here?
S. aureus, gram +
What is mycolic acid?
A hydrophobic molecule present in some cell walls that requires cabolfuschin to acid-fast stain it
What is malachite green stain used for?
An endospore is....
A bacterial cell that has become dormant and most dyes will not stain because it is resistant; malachite green forces the stain into the cell by steaming.
An antiobiotic is bactericidal if:
They kill the bacteria
An antimicrobial is bacteriostatic if:
They inhibit the growth of the bacteria
An antibiotic is:
A substance produced naturally by microbes (fungi)
What is MIC?
Minimum Inhibitory Concentration; the basis quantitative measure of an antimicrobial
What is the therapeutic index?
the ratio of the dose toxic to the host to the effective therapeutic dose. Higher index = better antibiotic.
Why would you want an antimicrobial with a selective toxicity towards the bacterium?
Because you do not want side effects to the host
What is the Kirby-Bauer test?
Test for antibiotic susceptibility. It is a disk diffusion test where zones of inhibition are measured with selective antimicrobial discs which are placed on plates of agar. Zones of inhibition are measured.
What are the three things that influence the zone of inhibition?
1) Diffusion rate of the antimicrobial
2) Degree of sensitivity of the microorganism
3) Growth rate of the bacterium
What are the 4 main targets that antimicrobial agents interefere with?
1) Protein synthesis
2) Cell wall synthesis
3) Folic acid synthesis
4) Nucleic acid synthesis
What is an antibiotic that intereferes with protein synthesis?
What is an antibiotic that intereferes with Nucleic acid synthesis?
What is an antibiotic that intereferes with Folic Acid
What is an antibiotic that intereferes with the cell wall?
How can a bacterium acquire resistance to an antimicrobial?
By mutation or acquiring DNA carrying a resistance gene
What are the mechanisms of bacterial resistance?
1) Altered permeability of the antimicrobial agent
2) Inactivation of the antimicrobial agent
3) Altered target site
4) Replacement of an antimicrobial sensitive pathway
What are the 3 most common nosocomial infections?
2) surgical site infections
3) pneumonia or bloodstream infections
What is MRSA and why is it so deadly?
methicillin-resistant staphylococcus aureus; spreads quickly and is resistant to a lot of antimicrobials
Glycerol yeast extract agar - selects for growth of hyphae like soil bacteria - Actinomycetes
Nutrient Agar - grows numerous bacteria but not fungal growth
Sabouraud Dextrose Agar - selects for growth of fungi