Introduction ...

Question 1

To diagnose infection, what 4 topics are beneficial to know?

Answer 1

1. site of infection2. infecting pathogen3. spectrum of activity of antibiotic4. host characteristics

Question 2

If you isolate an organism, does that mean that there’s an infection?

Answer 2

No

Question 3

What are normal flora?

Answer 3

Harmless bacteria that are naturally found and suppress growth of pathogenic bacteria

Question 4

Can normal flora become pathogenic?

Answer 4

Yes, if host defenses are impaired or disrupted

Question 5

Define Contamination

Answer 5

When a organism is accidentally introduced during collection or processing of a specimen

Question 6

Give an example of contamination

Answer 6

Coag-neg staph in blood of patient

Question 7

Define colonization

Answer 7

when an organism is present but not invading the host or inducing signs/symptoms of infection

Question 8

Give an example of colonization

Answer 8

Psudomonas aeruginosa in patient’s sputum but they aren’t displaying signs of pneumonia

Question 9

Define infection

Answer 9

pathogen is damaging host tissue and eliciting signs/symptoms of an infection

Question 10

Give an example of infection

Answer 10

Strep pneumoniae in sputum of patient who has fever, cough, and shortness of breath

Question 11

What bacteria are associated with throat infections?

Answer 11

Group A Strep

Question 12

What bacteria are associated with infections in urine?

Answer 12

E. Coli

Question 13

What bacteria are associated with skin abscesses?

Answer 13

S. aureus (or group a strep)

Question 14

What bacteria are associated with lung infections?

Answer 14

pneumococcus (aka strep pneumoniae)

Question 15

Define empirical antibiotic therapy

Answer 15

when antibiotics are chosen to be given based on preliminary info (before culture results are available)*antibiotics chosen have activity against likely pathogens**this is done to try to avoid morbidity/mortality due to a delay in treatment

Question 16

What preliminary info is used to determine empirical antibiotic therapy?

Answer 16

Site of InfectionLikely causative organismGram-stain result

Question 17

Is normal flora found everywhere in the host?

Answer 17

No, some body sites are sterile (see lecture for a list)

Question 18

What is the purpose of susceptibility testing?

Answer 18

To see if the proper antibiotic was chosen for empiric therapy

Question 19

What is directed or targeted therapy?

Answer 19

When the antibiotic regiment is modified once culture and susceptibility results are available

Question 20

Define antimicrobial spectrum of activity. What are the 2 possible spectra?

Answer 20

General list of bacteria killed or inhibited by an antibiotic*established during early clinical/pre-clinical trials and could change with the emergence of resistanceNarrow or Broad spectrum

Question 21

What is Narrow spectrum?

Answer 21

antibiotic has activity against a LIMITED group of bacteria

Question 22

Give an example of a narrow spectrum antibiotic used to treat Methacillin resistant bacteria

Answer 22

Nafcillin

Question 23

What is broad spectrum?

Answer 23

antibiotic with activity against a WIDE variety of bacteria

Question 24

Give an example of a broad spectrum antibiotic class that is used for anaerobes, all gram positive bacteria, etc.

Answer 24

Carbapenem

Question 25

What is MIC?

Answer 25

Minimum inhibitory concentrationlowest concentration of antibiotic that inhibits visible bacterial growth (ex: in broth culture system where we can see growth in tubes)

Question 26

What is MBC?

Answer 26

minimum bactericidal concentrationlowest concentration that kills bacteria*decreases bacterial inoculum by 99.9% (3 logs)

Question 27

What is the relationship between MIC and MBC?

Answer 27

MIC is less than or equal to MBC*so it takes more antibiotic to kill an organism than to inhibit it’s growth

Question 28

Define susceptibility breakpoint

Answer 28

interpretive guidelines by CLSI that categorize antibiotic MIC values as susceptible, intermediate, or resistant (SIR)

Question 29

Why are the interpretive guidelines for SIR different for each antibiotic and bacteria?

Answer 29

they are based on PK (achievable serum/tissue conc)depends on site of infectiondepends on antibiotic activity*thus…MIC values should NOT be compared for diff antibiotics

Question 30

Define Susceptible

Answer 30

organism is most likely eradicated with normal doses of antibiotic –> is “sensitive”

Question 31

Define Intermediate

Answer 31

treatment MAY b successful using max doses of antibioticex: urine infection where antibiotic gets concentrated in that one region

Question 32

Define Resistant

Answer 32

MIC exceeds usual serum concentration of antibiotic, so less than optimal results are expected

Question 33

What are methods that can be used to determine the in-vitro activity of an antibiotic?

Answer 33

Quantitative determination of MIC: Broth dilution (macro and micro) and E testQualitative: Kirby Bauer Disk Diffusion(*note: zone diameters are correlated to S and R but you can’t determine the exact MIC)

Question 34

Describe macrodilution (process)

Answer 34

2-fold serial dilutions of antibiotic are incubated with standard inoculum of bacteria in test tubesMIC = 1st tube without visible growth)

Question 35

Is macrodilution used commonly?

Answer 35

No because it’s labor and resource intensive

Question 36

What is not typically determined by macrodilution?

Answer 36

MBC (if you were to determine it, it would be via growing colonies on agar plates and finding the one where there’s no growth)

Question 37

Describe the microdilution process

Answer 37

microtiter plates/cassettes with serial dilutions of several antibiotics are tested simultaneously (1st clear well = mic)is an automated process and the most common!note: size restraints limit # of concentrations that can be tested…MIC may be given as a range

Question 38

How does the Kirby Bauer Disk Diffusion test work?

Answer 38

filter paper disks with a known concentration of antibiotic are put on agar plates with a standard conc/log (or lag??) phase growth of bacteriaclear zone around disk is seen where antibiotic was effective in inhibiting bacterial growth

Question 39

How does the E test work?

Answer 39

plastic strip containing a gradient of antibiotic concentrations is put on an agar plate that has a standard concentration of bacteriaMIC = where the clear ellipse shaped zone crosses the strip

Question 40

What is PCR used for in relation to bacteria?

Answer 40

45 minute result from blood culture gives bacteria identification as well as tests probes for specific antibiotic resistance genes (ex: mecA, KPC, vanA/B)

Question 41

What are susceptibility reports?

Answer 41

Gives antibiotic tested, MIC, and SIR for individual bacteria*are used for antibiotic selection (directed therapy) but are only an INDICATION of antibiotic activity

Question 42

What is a hospital antibiogram?

Answer 42

Report with susceptibility data for most common bacteria that are seen in hospitals*helps guide choices for empiric antibiotic therapy (prior to identification of bacteria)