Download Lecture 1 Bacterial meningitis

Lecture 3
Antimicrobials and Susceptibility
tests
Dr. Abdelraouf A. Elmanama
Islamic University-Gaza
Medical Technology Department
Lecture outlines
• Kirby-Bauer susceptibility test
• Antimicrobial profiles selection
• Reporting susceptibility test
What Does the Laboratory Need to Know
about Antimicrobial Susceptibility Testing (AST) ?
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Which organisms to test?
What methods to use?
What antibiotics to test?
How to report results?
What Does a Laboratory Need to Know
about AST? (con’t)
• How to determine the clinical significance
of results?
• How to ensure accuracy of results?
–Quality control / quality assurance
• When to call the MD, infection control,
public health?
What Does a Laboratory Need to Know
about AST? (con’t)
• When to ask for help?
• Where to go for help?
Brief Review of Routine AST
Methods
Routine Susceptibility Tests
• Disk diffusion (Kirby Bauer)
• Broth micro-dilution MIC
– NCCLS reference method
• Etest
Disk Diffusion
Test
Prepare inoculum
suspension
Select colonies
Mix well
Standardize inoculum
suspension
Swab plate
Remove sample
Incubate overnight
Add disks
Measure Zones
Transmitted Light
Reflected Light
Zone Interpretive Criteria (mm)
Disk
content
(ug)
Res
Int
Susc
Cefazolin
30
 14
15-17
 18
Gentamicin
10
 12
13-14
 15
Drug
Flash presentation for summary
Disk Diffusion Test
• Qualitative results
– Susceptible
– Intermediate – may respond if infection
is at body site where drug concentrates
(e.g. urine) or if higher than normal dose
can be safely given
– Resistant
Modify methods for fastidious
bacteria
Clinical Conditions when MICs are
Useful
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Endocarditis
Meningitis
Septicemia
Osteomyelitis
Immunosuppressed patients (HIV, cancer, etc.)
Prosthetic devices
Patients not responding despite “Sensitive results”
MIC
• Minimal inhibitory concentration
• The lowest concentration of
antimicrobial agent that inhibits the
growth of a bacterium
• Interpret:
– Susceptible
– Intermediate
– Resistant
Inoculum Preparation
MIC Testing
(NCCLS Reference Method)
• Standardize inoculum suspension
• Final inoculum concentration
– 3 – 5 x 105 CFU/ml
– (3 – 5 x 104 CFU/well)
Prepare inoculum
suspension
Microdilution MIC tray
Dilute & mix inoculum
suspension
Pour inoculum
into reservoir and
inoculate MIC tray
Incubate overni
Inoculate
purity plate
Reflected light
Transmitted light
Examining
purity plate
Read MICs
MICs
0.5
1
- +
2
4
8
16
32
64
>64
>64
MIC on a
strip
S. pneumoniae
Penicillin MIC = 3 g/ml
MIC Interpretive Criteria (g/ml)
Drug
Susc
Int
Res
cefazolin
8
16
 32
gentamicin
4
8
 16
Empirical Treatment
Infants 1-3 mos
Immunocompetent children > 3 mos
and adults <55
Ampicillin + cefotaxime or
ceftriaxone
Cefotaxime or ceftriaxone +
vancomycin
Adults > 55 and adults of any age
with alcoholism or other debilitating
illnesses
Ampicillin + cefotaxime or
ceftriaxone + vancomycin
Hospital-acquired meningitis,
posttraumatic or postneurosurgery
meningitis, neutropenic patients, or
patients with impaired cell-mediated
immunity
Ampicillin + ceftazidime +
vancomycin
• Ceftazidime should be substituted for
ceftriaxone or cefotaxime in neurosurgical
patients and in neutropenic patients
Specific treatment
• N. meningitidis
– Penicillin sensitive Penicillin G or Ampicillin
– Penicillin-resistant Ceftriaxone or
cefotaxime
Chemoprophylaxis for N. meningitidis
• Rifampin 600 mg every 12 h for 2 days in
adults and 10 mg/kg every 12 h for 2 days in
children >1 year
•
Or
• One dose of ciprofloxacin (750 mg)
• One dose of azithromycin (500 mg)
• One intramuscular dose of ceftriaxone (250
mg)
•
Rifampicin is not recommended in pregnant
women.
• Pneumococci
– Penicillin-sensitive Penicillin G
– Penicillin-intermediate  Ceftriaxone or
cefotaxime
– Penicillin-resistant  (Ceftriaxone or
cefotaxime) + vancomycin
• Gram-negative bacilli (except Pseudomonas
spp.)  Ceftriaxone or cefotaxime
• Pseudomonas aeruginosa  Ceftazidime
• Staphylococci spp.
– Methicillin-sensitive  Nafcillin
– Methicillin-resistant  Vancomycin
• Listeria monocytogenes  Ampicillin + gentamicin
• Haemophilus influenzae  Ceftriaxone or
cefotaxime
• Streptococcus agalactiae Penicillin G or
ampicillin
• Bacteroides fragilis  Metronidazole
• Fusobacterium spp.  Metronidazole
• Local Data and protocols should be
observed and reviewed periodically
Thank you