Chapter 42- Antimicrobial Sensitivity Testing Flashcards
-One of the worlds most pressing public and animal health problems
-Occurs when a microbe changes or mutates in some way that reduces or eliminates the effectiveness of drugs, chemicals, or other agents designed to cure or prevent infections
Antibiotic resistance
Why does antibiotic resistance happen?
when antibiotics are used to kill or inhibit the growth of susceptible bacteria, sometimes one of the bacteria survives
Antibiotic resistance can also occur through
mutation of their genetic material or by acquiring pieces of DNA that code for the resistance properties
Antibiotic resistant bacteria can quickly spread to
other mammals and threaten communities w/ a new strain of infectious disease that is hard to cure
The most common mutation and form of antimicrobial resistance involves
extended-spectrum beta-lactamase (ESBL)
Beta-lactamases aka
penicillinase
ESBLs can cause
serious infection and are a common source of nosocomial infection
Once a bacteria has been isolated from a patient, _ _ testing can be performed to determine the susceptibility or resistance to specific antimicrobial drugs
antimicrobial testing
For an antimicrobial sensitivity test, the sample must be collected BEFORE
any tx begins
-Most preferred method
-Uses paper discs impregnated with antimicrobial
-Quantitive and requires the measurement of inhibitory zone sizes
agar diffusion method
Agar diffusion method techniques
Modified Kirby-Bauer technique or standardized disk susceptibility method (most used), U.S. Food & Drug administration method, International collaborative disc technique
Antimicrobial discs should be kept in the _ and _ should not be used
refrigerator; outdated
Indirect sensitivity testing requires that colony samples be taken from
a culture plate, subcultured in broth media, and incubated to achieve turbidity to match a standardized 0.5 McFarland suspension
Direct testing method
application of undiluted samples (ex: urine) directly to a Mueller-Hinton plate
Indirect testing is more _ than direct testing but results are not available as quickly
precise
Discs should be placed
10-15mm part and avoid overlapping of Zones of Inhibition (ZOI)
ZOI measured using a
caliper transparent ruler or template and includes the antibiotic disc
Measurement is compared to a chart of ZOI for each antibiotic to determine the
relative resistance of the bacterium to the antimicrobial
The zone sizes are divided into two major categories
resistant or susceptible
Intermediate susceptibility
drug may be used for systemic infection if a high dosage is safe
Strains of staphylococci are resistant to
methicillin and other similar drugs
Minimum inhibitory concentration (MIC) of an antimicrobial is
the lowest concentration of a specific antimicrobial that can inhibit the growth of a given bacteria
Choice of a specific concentration of antimicrobial is based on the
MIC, site of infection, and breakpoint of the antimicrobial (the dilution of the antimicrobial at which the bacteria begins to show resistance)
MIC methods
agar diffusion or Microwell
The presence of pathogenic bacteria does not necessarily indicate
infection
Colony counts on cultured urine can help support a diagnoses of
UTI
_ samples are not recommended for a colony count
Voided
