Exercise 15-16 Flashcards
Kirby-Bauer Method
Determines susceptibility of pathogenic bacteria to chemotherapeutic agents
Disk Diffusion Method
Disadvantage of disk diffusion method
Cannot distinguish bactericidal and bacteriostatic effects
The growth phase of organism where it is most sensitive to antibiotics
Exponential Phase
Considered best medium for the disk diffusion method
MH agar
Why is the MH agar the best medium for the disk diffusion methods?
Reproducibility
Low in sulfonamide, trimethoprim, and tetracycline inhibitors
Support satisfactory growth
Factors affecting zone of inhibition
Susceptibility test medium Concentration of test organism Rate of growth or test organism Concentration of antibiotic disk Diffusion of antibiotic in the agar Susceptibility of the organism
Disadvantage of disk diffusion method
Not a perfect indication on how well the chemotherapeutic agent will perform in vivo
Suspensions of either barium sulfate or latex principles that allow visual comparison of bacterial density
McFarland Standards
0.5 McFarland Standard
1.5 x 10^8 CFU/mL
Purpose of McFarland Standard
To standardize the test
To be able to compare between microorganisms and antibiotics
Aerobic and facultative anaerobic
Gram-negative
Nonspore-forming
Lactose fermenter
Coliforms
Not a coliform because it cannot ferment lactose
Salmonella
Organisms under coliforms
Enterobacteriaceae E. coli Enterobacter Klebsiella Citrobacter
Indicators of fecal contamination
Indicators of water potability
Not a pathogen
May give false positive or negative result
Coliforms
Characteristics of an ideal indicator organism
Always present when pathogens are present
Never present when pathogens are absent
Easy to culture
Basic techniques accepted for coliform analysis
Membrane Filtration
Multiple tube fermentation or Most Probable Number (MPN method)
MPN test is performed in 3 steps
Presumptive Test
Confirmatory Test
Completed Test
Purpose of presumptive test
To detect presence of coliforms
Medium used in presumptive test
Lactose broth or lauryl tryptose broth
Positive result for presumptive test
Acid and gas formation
Purpose of confirmatory test
To detect presence of mcgs other than coliforms that also produce acid and gas from lactose fermentation
Medium used in confirmatory test
Lactose broth or brilliant green lactose bile broth
Positive result for confirmatory test
Acid and gas formation
Medium used for completed test
EMBA
One of the most widely used medium for the detection of coliform bacteria in water, wastewater, foods, and milk and dairy products
Brilliant Green Bile Broth 2%
Source of essential nutrients
Peptone
Fermentable carbohydrate
Lactose
Inhibits gram-positive bacteria
Bile
Inhibits gram-negative bacteria
Brilliant green
Do not produce gas
Non-faecal coliforms
Advantages of MPN
Ease of interpretation
Sample toxins are diluted
Effective for highly turbid samples
Disadvantages of MPN
Long time to get results
Not very accurate results
Requires more glassware and media
Probability of false positives
