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Liz's Microbiology Module 2 > Antimicrobial Susceptibility Testing > Flashcards

Antimicrobial Susceptibility Testing Flashcards

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1
Q

What three classes of antimicrobials fall under the category of beta-lactams?

A
  • Pencillins
  • Cephalosporins
  • Carbapenems
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2
Q

What antimicrobial class is penicillin in?

A

Narrow spectrum penicillins

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3
Q

What antimicrobial class is oxacillin in?

A

Penicillins (beta-lactamase resistant)

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4
Q

What antimicrobial class is piperacillin in?

A

Broad-spectrum penicillin (ureidopenicillins)

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5
Q

What antimicrobial class is cefazolin in?

A

Cephalosporins (1st generation)

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6
Q

What antimicrobial class is cefuroxime in?

A

Cephalosporins (2nd generation)

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7
Q

What antimicrobial class is ceftazidime in?

A

Cephalosporins (3rd generation)

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8
Q

What antimicrobial class is cefepime in?

A

Cephalosporins (4th generation)

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9
Q

What antimicrobial class is imipenem in?

A

Carbapenems (beta-lactams)

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10
Q

What antimicrobial class is meropenem in?

A

Carbapenems (beta-lactams)

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11
Q

What antimicrobial class is doripenem in?

A

Carbapenems (beta-lactams)

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12
Q

What antimicrobial class is vancomycin in?

A

Glycopeptides

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13
Q

What antimicrobial class is ciprofloxacin in?

A

Floroquinolones

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14
Q

What antimicrobial class is levofloxacin in?

A

Fluoroquinolones

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15
Q

What antimicrobial class is gentamicin in?

A

Aminoglycosides

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16
Q

What antimicrobial class is tobramycin in?

A

Aminoglycosides

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17
Q

What antimicrobial class is is amikacin in?

A

Aminoglycosides

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18
Q

What antimicrobial class is tetracycline in?

A

Tetracyclines

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19
Q

What antimicrobial class is doxycycline in?

A

Tetracyclines

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20
Q

What antimicrobial class is clindamycin in?

A

Under its own category

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21
Q

What antimicrobial class is is erythromycin in?

A

Macrolides

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22
Q

What antimicrobial class is trimethoprim-sulfamethoxazole (Bacitrim) in?

A

Antimetabolites

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23
Q

Penicillin

  • Spectrum of activity
  • Mechanism of action
A
  • Against GP and GN (changes depending upon class)

- Inhibits cell wall synthesis

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24
Q

Cephalosporins (4th generation)

  • Spectrum of activity
  • Mechanism of action
A
  • GNRs

- Inhibits peptidoglycan

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25
Q

Tetracyclines

  • Spectrum of activity
  • Mechanism of action
A
  • Broad spectrum: GP, GN, mycoplasma, chlamydiae, rickettsiae
  • Inhibits protein synthesis
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26
Q

Clindamycin

  • Spectrum of activity
  • Mechanism of action
A
  • Broad spectrum (aerobic GP + anaerobes)

- Inhibits protein synthesis

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27
Q

Fluoroquinolones

  • Spectrum of activity
  • Mechanism of action
A
  • Broad spectrum: GP, GN

- Inhibits DNA synthesis

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28
Q

Cephalosporins (2nd generation)

  • Spectrum of activity
  • Mechanism of action
A
  • GPC, some GPR

- Inhibits peptidoglycan synthesis

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29
Q

Antimetabolites

  • Spectrum of activity
  • Mechanism of action
A
  • Specific clinical uses…UTI, S. maltophilia

- Inhibits folic acid synthesis

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30
Q

Macrolides

  • Spectrum of activity
  • Mechanism of action
A
  • Broad spectrum

- Inhibits protein synthesis

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31
Q

Cephalosporins (3rd generation)

  • Spectrum of activity
  • Mechanism of action
A
  • GNR, some GPC

- Inhibits peptidoglycan synthesis

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32
Q

Aminoglycosides

  • Spectrum of activity
  • Mechanism of action
A
  • Active against GNRs and S. aureus

- Inhibits protein synthesis

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33
Q

Cephalosporins (1st generation)

  • Spectrum of activity
  • Mechanism of action
A
  • GPC

- Inhibits peptidoglycan synthesis

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34
Q

Glycopeptides

  • Spectrum of activity
  • Mechanism of action
A
  • GP only

- Inhibits peptidoglycan synthesis

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35
Q

How many interactions occur b/w a patient, an organism, and an antimicrobial?

A

Six

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36
Q

Substance naturally produced by living organisms such as bacteria and fungi and able, in a dilute solution, to inhibit or kill another microorganism

A

Antibiotic

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37
Q

Chemical substance produced by a microorganism that has the capability of killing of inhibiting the growth of another organism

A

Antimicrobial agent

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38
Q

Range of activity of an antimicrobial agent against certain groups of bacteria

A

Spectrum of activity

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39
Q

Difference b/w intrinsic and acquired resistance

A
  • Intrinsic: all members of the species are resistant

- Acquired: not all members of the species are resistant

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40
Q

Examples of intrinsic resistance

A

Staphylococcus saprophyticus and novobiocin

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41
Q

Examples of acquired resistance

A

Staphylococcus aureus and methicillin/oxacillin

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42
Q

Interpret susceptibility/resistance to oxacillin and cefoxitin as MSSA

A

MSSA is susceptible to oxacillin and cefoxitin

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43
Q

Interpret susceptibility/resistance to oxacillin and cefoxitin as MRSA

A

MRSA is resistant to oxacillin and cefoxitin

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44
Q

Three mechanisms used by bacteria in order to exchange genetic material resulting in antimicrobial resistance

A
  • Genes
  • Transfer
  • Expression
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45
Q

Three mechanisms of bacterial resistance

A
  • Impermeability
  • Alterations in target molecules
  • Enzymatic inactivation
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46
Q

Two mechanisms of impermeability

A
  • Altered outer membrane porins

- Altered transport systems

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47
Q

Three mechanisms in altering the target molecules

A
  • Methylation of rRNA
  • Alterations of ribosomes
  • Altered penicillin binding proteins
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48
Q

Three mechanisms in enzymatic inactivation

A
  • Beta-lactamases
  • Chloramphenicol acetyltransferases
  • Aminoglycosides modifying enzymes
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49
Q

Three effects of combining antimicrobials

A
  • Autonomous/indifferent
  • Antagonistic (a substance that interferes w/ or inhibits the physiological action of another)
  • Synergistic (one drug increases the other’s effectiveness)
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50
Q

What organism is universally susceptible to penicillin?

A

Streptococcus pyogenes

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51
Q

Standardizing susceptibility testing

- Growth medium

A

Mueller-Hinton Agar or broth

- Depth 3-5mm

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52
Q

Standardizing susceptibility testing

- pH

A

7.2-7.4

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53
Q

Standardizing susceptibility testing

- Cation concentration

A
  • Mg2+
  • Ca2+
  • NaCl
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54
Q

Standardizing susceptibility testing

- Incubation conditions

A

35°C, ambient air, duration varies

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55
Q

Standardizing susceptibility testing

- Inoculum density

A

If not enough organism then not enough enzyme which could lead to a false susceptibility

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56
Q

Standardizing susceptibility testing

- Inoculum prep

A

3-5 colonies made into a McFarland standard

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57
Q

In a disk diffusion test (or an E-test), the agar depth is 1mm, what will the result be?

A

The antibiotic diffuses farther due to less agar → false susceptibility

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58
Q

In a disk diffusion test (or an E-test), the agar depth is 6 mm, what will the result be?

A

The antibiotic cannot diffuse as far → false resistance

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59
Q

In a disk diffusion test (or an E-test), a 0.25 McFarland is used, what will the result be?

A

There is less bacteria in the inoculum that is spread over the plate → Larger zone → false susceptibility

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60
Q

In a disk diffusion test (or an E-test), a 2.0 McFarland is used, what will the result be?

A

There is more bacteria in the inoculum that is spread over the plate → smaller zone → false resistance

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61
Q

In a broth dilution, a 0.25 McFarland is used, what will the result be?

A

There is less bacteria in the inoculum → ↓ MIC

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62
Q

In a broth dilution, a 2.0 McFarland is used, what will the result be?

A

There is more bacteria in the inoculum → ↑ MIC

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63
Q

Disk diffusion test

- What is the McFarland standard?

A

Uses barium sulfate to make a 1.5x10^8 CFU/mL standard which is a 0.5 McFarland

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64
Q

Disk diffusion test

- Inoculation of agar

A

W/in 15 minutes of prep, streak plates in 3 planes so that there is a lawn of confluent growth

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65
Q

Disk diffusion test

- Application of disks

A

W/in 15 minutes of inoculation, NEVER relocate a disk

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66
Q

Disk diffusion test

- Incubation

A
  • 16-18 hours (Staph and oxacillin/methicillin for 24 hours)

- 35°C ambient air

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67
Q

Disk diffusion test

- Reading results

A

Read circular zone side in mm;

- If it’s resistant, zone size is still 6mm! NEVER 0!

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68
Q

Disk diffusion test

  • Qualitative or quantitative?
  • Interpretation of results
A

Qualitative → inform MD if its S, I, or R using CLSI charts

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69
Q

E-test

- What is the McFarland standard?

A

Uses barium sulfate to make a 1.5x10^8 CFU/mL standard which is a 0.5 McFarland

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70
Q

E-test

- Inoculation of agar

A

W/in 15 minutes of prep, streak plates in 3 planes so that there is a lawn of confluent growth

71
Q

E-test

- Application of gradient strips

A

?

72
Q

E-test

- Incubation

A
  • 16-18 hours

- 35°C ambient air

73
Q

E-test

- Reading results

A

Read elliptical zone side in mcg/mL

74
Q

E-test

- MIC

A

Where growth intersects strip

  • No zone: MIC > highest concentration
  • Zone below strip: MIC < lowest concentration
75
Q

E-test

  • Qualitative or quantitative?
  • Interpretation of results
A

Quantitative → informing MD of concentration (mcg/mL) and if its S, I, or R using CLSI tables

76
Q

Broth dilution

- Inoculum prep

A

3.5 colonies and make a McFarland standard

77
Q

Broth dilution

- What is the McFarland standard?

A

Use barium sulfate to make a 1.5x10^8 CFU/mL standard which is a 0.5 McFarland
- Final concentration: 5x10^5 CFU/mL

78
Q

Broth dilution Macrotube

- Inoculation procedure

A

??

79
Q

Broth dilution microtiter tray

- Inoculation procedure

A
  • Deliver inoculum, via delivery prongs, into sterility tray with sterility well (neg) and growth control well (pos)
  • Make a PURITY PLATE!
80
Q 
Broth dilution (Macrotube and microtiter tray)
- Incubation
A

35°C ambient air, 16-20 hours

81
Q

Broth dilution

- Reading results

A

MIC is the lowest concentration that inhibits growth

- Needs to be reproducible w/ one well margin of error

82
Q

Broth dilution

- Qualitative or quantitative

A

Quantitative → informing MD of concentration (mcg/mL) and if it’s S, I, or R using CLSI tables

83
Q

What is the significance of a regressive curve?

A

Linear and inversely proportional

- As zone size ↑, MIC ↓

84
Q

Regressive curve

  • X axis
  • Y axis
A
  • Y axis: MIC (mcg/mL)
  • X axis: Zone size (mm)
  • Top left = resistant
  • Bottom right = susceptible
85
Q

MIC/MBC procedure

A

Take clear tubes from MIC and incubate again on a plate. If < 50 colonies can grow on a plate thats considered killed!

86
Q

Minimal bactericidal concentration

A

Killing capacity of 99.9% of original inoculum

87
Q

MID/MBD procedure

A

?

88
Q

MID procedure

A

Patient’s serum is serially diluted and a standard inoculum (5x10^5 CFU/ml) of patient organism is added. Incubate and read highest dilution which inhibits growth.

89
Q

MBD procedure

A
  • Highest dilution of patient serum that kills 99.9% of an original inoculum of patients organism
  • Must be performed w/ MID
  • Results are dilutions 1:8 to 1:32 is adequate
90
Q

SXT is read at what % inhibition?

A

80%

91
Q

Automated methods: Dade Microscan

- Three methods

A
  • Traditional MIC methods
  • Photometric reading (turbidity)
  • Fluorometric reading (best method) (degradation of substrates by viable bacteria; detects bacterial inhibition by antibiotics)
92
Q

Automated methods: BD Phoenix

- Three methods

A
  • Gravity based inoculation process
  • Redox indicator system
  • Data management system
93
Q

Automated methods: Vitek

- Three methods

A
  • Computer-assisted analysis of growth
  • Algorithm derived MIC
  • Calculated MIC
94
Q

Three methods of beta-lactamase detection

A
  • Chromogenic cephalosporin: cefinase, nitrocefin
  • Acidometric
  • Iodometric
95
Q

Results of beta-lactamase test

A
  • Positive = organism produces enzyme to degrade penicilin, ampicillin, and amoxicillin
  • Negative = does not produce enzyme but it may have some other mechanism of resistance to the same drug
96
Q

What organism requires beta-lactamase testing?

A

Haemophilus influenzae

97
Q

Which antibiotics which will not be effective against beta-lactamase producers

A
  • Penicillin, ampicillin, amoxicillin
  • Cephalosporins
  • Carbapenems
98
Q

What is the value of measuring serum levels?

A

Want to know how much antibiotic is at site of infection so that therapeutic range can be determined

99
Q

Agar dilution

- Prep

A

Antibiotic is diluted into agar in 2-fold concentrations

100
Q

Agar dilution is used for what type of organisms?

A

Fastidious organisms (N. gonorrhoeae, N. meningitidis)

101
Q

Agar dilution, steers replicator

- What is the standard inoculum per spot?

A

10^4 CFU/mL

102
Q

Agar dilution, steers replicator

- Incubation requirements

A

35°C, ambient air, 18-20 hours

103
Q

What is the reference method for susceptibility testing?

A

Agar dilution

104
Q

Five mechanisms of action a drug can have

A
  • Inhibit cell wall synthesis
  • Inhibit protein synthesis
  • Inhibit nucleic acid synthesis
  • Antimetabolites
  • Alteration of cell membranes
105
Q

What drugs work by inhibiting cell wall synthesis?

A
  • Penicillins
  • Cephalosporins
  • Glycopeptides (inhibitis peptidoglycan synthesis; precursors of cell wall synthesis)
106
Q

What drugs work by inhibiting protein synthesis?

A
  • Aminoglycosides
  • Tetracyclines
  • Macrolides
  • Clindamycin
107
Q

What drug work by inhibiting the folic acid pathway?

A

Antimetabolites

108
Q

What drug works by inhibiting DNA synthesis?

A

Fluoroquinolones

109
Q

What drug alters cell membranes?

A

Bactracin

110
Q

Mechanism of resistance

- DRSP

A
  • Altered target sight

- PBP-2 pencillin binding protein

111
Q

Mechanism of resistance

- ESBL

A
  • Arise from point mutations of common beta-lactamases

- TEM-1 and SHV-1 genes coding for enzyme

112
Q

Mechanism of resistance

- VRSA/GRSA

A
  • May involve alterations in the cell wall and hyperexpression of PBP
  • vanA gene present
113
Q

Mechanism of resistance

- MRSA

A
  • Alteration of target site (classic and modified)

- Hyperproduction of beta-lactamases

114
Q

Mechanism of resistance

- VRE

A
  • Altered peptidoglycan synthesis

- Carried by transposons or plasmids

115
Q

Mechanism of resistance

- ARHI

A

Plasmid-mediated beta-lactamases (most common)

116
Q

Mechanism of resistance

- CRE

A
  • Carbapenemase (breaks down antibiotic)

- Cephalosporinase combined with porin loss

117
Q

Lab detection of MRSA

- Cefoxitin disk diffusion

A

Induces mecA gene (best drug to detect MRSA)

  • Media: CAMHB with 2% NaCl
  • Incubation: 24 Hours, 35°C, ambient air
118
Q

Lab detection of DRSP

- Disk diffusion w/ oxacillin

A

Disk Diffusion using Oxacillin;

  • Media: Mueller-Hinton Agar, 5% SBA;
  • Incubation: 20-24 Hrs, 35°C in CO2;
119
Q

Lab detection of ARHI

- Disk diffusion

A
  • Media: HTM agar

- Incubation: 16-18 Hrs, 35°C in CO2

120
Q

Lab detection of VRE

- Disk diffusion

A
  • Media: Mueller-Hinton Agar;

- Incubation: 24 Hrs, 35’C, Ambient Air;

121
Q

Lab detection of VRSA/GRSA

A

Disk Diffusion and Broth Dilution tests are held for 24 Hrs;
Agar Screen:
- Media: BHI with 6ug/mL Vancomycin,
- Incubation: 24 Hrs, 35’C, ambient air;
D-Test Disk Induction: Erythromycin and Clindamycin disks are placed 15mm apart and flattening is noted; Pos=Resistant to clindamycin, Neg=susceptible

122
Q

Lab detection of ESBL

A

Confirmatory Test: Use cefotaxime and ceftazidime disks and each disk combined with clavulanic acid; >5mm increase with clav=pos;
Can also use E-Test Strips, Vitek-2, and Microscan

123
Q

Lab detection of CRE

A

Modified Hodge Test: 1:10 Dilution of 0.5 Mcfarland (E.coli lawn)(because larger zone size with decreased inoculum), streaked with known + and - and Patient.
- Shouldering occurs if enzyme present to break down meropenam = pos result/resistance

124
Q

MRSA

- Infection types

A
  • HA-MRSA

- CA-MRSA

125
Q

What is HA-MRSA?

A
  • Found in ICUs

- S. aureus has acquired several genes that are responsible for its resistance

126
Q

What is CA-MRSA?

A
  • S. aureus has only gene responsible for its resistance
127
Q

VRE

- Infection types

A
  • Nosocomial bacteremia
  • Surgical wound infection
  • UTI
128
Q

DRSP

- Infection types

A
  • Otitis media
  • Sinusitis
  • Community acquired pneumonia
  • Bacteremia
  • Meningitis
129
Q

ARHI

- Infection types

A

?

130
Q

VISA/VRSA

- Infection types

A

?

131
Q

ESBL

- Infection types

A
  • E. coli
  • Klebseilla spp
  • Proteus mirabilis
132
Q

CRE

- Infection types

A
  • Class A: K. pneumoniae
  • Class B: P. aeruginosa, A. baumannii
  • Class D: A. baumannii
133
Q

Effective treatment or prevention

- MRSA

A
  • Linezolid
  • Synercid
  • Daptomycin
  • Vancomycin to treat
134
Q

Effective treatment or prevention

- ARHI

A
  • Type b vaccine

- Empiric treatment w/ cefotaxime or ceftriaxone

135
Q

Effective treatment or prevention

- DRSP??

A

??

136
Q

Effective treatment or prevention

- VRE??

A

??

137
Q

Effective treatment or prevention

- VRSA/VISA??

A

??

138
Q

Effective treatment or prevention

- ESBL??

A

??

139
Q

Effective treatment or prevention

- CREs??

A

??

140
Q

Synercid treats which problematic bacteria?

A
  • MRSA

- VRE

141
Q

Daptomycin treats which problematic bacteria?

A
  • MRSA

- VRE

142
Q

Telithromycin treats which problematic organisms?

A

Macrolide-resistant pneumococci

143
Q

What test detects the inducible clindamycin resistance mechanism in GP organisms?

A

D-test

144
Q

Desribe the prinicple behind the D-test

A

Erythromycin is used to induce the resistance mechanism (ribosome alteration) to clindamycin w/in the GP organism
- If using the D-test, the GP organism should already be resistant to erythromycin

145
Q

Describe the procedure of the D-test

A

Erythromycin and clindamycin disks are placed 15mm apart and flattening is noted around the clindamycin disk

146
Q

Interpret the results of the D-test

A
  • Pos: GP organism is RESISTANT to clindamycin (CANNOT USE FOR TREATMENT!)
  • Neg: GP organism is SUSCEPTIBLE to clindamycin
147
Q

Rationale for identifying VRE to the species level

A

To confirm if we truly have VRE (E. casseliflavus or E. gallinarium)

148
Q

Resistance mechanism responsible for vancomycin resistance in S. aureus (VRSA)

A

Alteration of cell wall

149
Q

Resistance mechanism responsible for penicillin resistance in S. pneumoniae

A

Alteration of target site

150
Q

What antibiotic disk is used to predict penicillin resistance in S. pneumoniae?

A

Oxacillin

151
Q

What antibiotic disk is used to predict oxacillin resistance in S. aureus?

A

Cefoxitin

152
Q

What is the purpose of the confirmatory ESBL test?

A

To detect the presence of extended spectrum beta-lactamase enzymes produced by an isolate

153
Q

Four disks used in the confirmatory ESBL test

A
  • Cephalosporin (CTX)
  • Clavulanic acid (CLA)
  • Cephalosporin/clavulanic acid (CTX/CLA)
  • Ceftazidime/clavulanic acid (CAZ/CLA)
154
Q

What is the purpose of the cefoxitin screen?

A

To determine if an isolate produces the mecA gene (differentiates MSSA vs. MRSA)

155
Q

What size zone indicates a “positive” (resistant) screen?

A

≤ 21mm (MRSA)

156
Q

What size zone indicates a “negative” (susceptible) screen?

A

≥ 22mm (MSSA)

157
Q

CRE/modified Hodge test

- Purpose

A

To detect the presence of carbapenemases

158
Q

CRE/modified Hodge test

- What organism is streaked all over the plate?

A

E. coli

159
Q

CRE/modified Hodge test

What disk is placed in the center of the plate?

A

Meropenem

160
Q

What does “MRSA” stand for?

A

Methicillin resistant Staphylococcus aureus

161
Q

What does “DRSP” stand for?

A

Drug resistant Streptococcus pneumoniae

162
Q

What does “ARHI” stand for?

A

Ampicillin resistant Haemophilus influenzae

163
Q

What does “VRE” stand for?

A

Vancomycin resistant Enterococcus spp

164
Q

What does VRSA/VISA” stand for?

A

Vancomycin resistant Staphylococcus aureus

165
Q

What does “ESBL” stand for?

A

Extended spectrumm beta-lactamases

166
Q

What does “CRE” stand for?

A

Carbapenem resistant enterics

167
Q

Lab detection of MRSA

- MRSA agar screen

A

Meuller-Hinton Agar + 4% NaCl + 6 mcg/mL Oxacillin

168
Q

Lab detection of DRSP

- Broth dilution

A
  • Media CAMHB with 2-5% LHB;

- Incubation: 20-24 Hrs at 35°C, ambient air

169
Q

Lab detection of DRSP

- E-test

A
  • Media: Mueller-Hinton agar, 5% SBA;

- Incubation: 20-24 Hours 35°C in CO2

170
Q

Lab detection of ARHI

- Broth dilution

A
  • Media: HTM broth

- Incubation: 20-24 Hours, 35°C, ambient air

171
Q

Lab detection ARHI

- Nitrocefin assay

A

Chromogenic cephalosporin

  • Pos: isolate is resistant to penicillin, ampicillin, and amoxicillin
  • Neg: isolate can be resistant by other mechanisms
172
Q

Lab detection of VRE

- Broth dilution

A

Broth Dilution:

  • Media: CAMHB;
  • Incubation: 24 Hrs, 35’C, ambient air; Agar
173
Q

Lab detection

- Screening test

A
  • Media: BHI with 6ug/mL vacomycin;
  • Incubation: 24 Hrs, 35’C, Ambient air, any growth = resistance;
    Confirmation of VRE with gram stain

Liz's Microbiology Module 2 (11 decks)

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