Antimicrobial susceptibility
Antibiogram is an in-vitro testing for the sensitivity of an isolated bacteria strain
to different antibiotics. Frequently it follows the strain identification and
represents the final goal of a bacteriolgy analysis.
There are 2 antibiogram methods: diffusimetric (most used and easy to do)
and quantitative.
- diffusimetric method - based on the diffusion property of the antibiotic from
the impregnated paper disks. Disks are dispensed on the surface of an
bacterial strain inoculated media (in one or more Petri dishes). EUCAST
recommends the utilization of Mueller-Hinton agar without supplements for non-
fastidious organisms, including enterococci, and Mueller-Hinton with 5 % horse
blood and 20 mg β-NAD/L for Streptococcus spp. including Streptococcus
pneumoniae, Haemophilus spp. and other fastidious organisms.
Minimum distance between disks must be 30 mm. Recommended density of
inoculum is 0.5 on McFarland scale.
The plates are incubated at 35 ± 1 ºC for 18 ± 2 h within 15 minutes from
application of the disks. Plates are incubated in air for non-fastidious and in air
with 5% CO2 for fastidious organisms. During the incubation the antibiotic will
diffuse in the area surrounding each disk, and a rounded clear area (where
the strain's growth is inhibited) will appear. The measured diameter (in
millimeters) of the clear area is compared with standard tables. See video and
images sections for more.
- quantitative method - based on dilution: serial dilutions of an antimicrobial
agent of known concentration are prepared in growth medium. All tubes are
then inoculated with the isolated strain and incubated for a defined period of
time. The MIC is defined as the lowest concentration of antimicrobial agent
needed to inhibit the growth of a test organism. Once the MIC is calculated, it
can be compared to known values for a given bacterium and antibiotic.
Note: before you perform an antibiogram, you need to identify the strain and
be sure you have a pure culture. The antibiotics used for antibiogram are not
the same for different bacteria genera.
The same principle may be applied for Antifungigram by changing antibiotic
with antimycotic impregnated disks.
to different antibiotics. Frequently it follows the strain identification and
represents the final goal of a bacteriolgy analysis.
There are 2 antibiogram methods: diffusimetric (most used and easy to do)
and quantitative.
- diffusimetric method - based on the diffusion property of the antibiotic from
the impregnated paper disks. Disks are dispensed on the surface of an
bacterial strain inoculated media (in one or more Petri dishes). EUCAST
recommends the utilization of Mueller-Hinton agar without supplements for non-
fastidious organisms, including enterococci, and Mueller-Hinton with 5 % horse
blood and 20 mg β-NAD/L for Streptococcus spp. including Streptococcus
pneumoniae, Haemophilus spp. and other fastidious organisms.
Minimum distance between disks must be 30 mm. Recommended density of
inoculum is 0.5 on McFarland scale.
The plates are incubated at 35 ± 1 ºC for 18 ± 2 h within 15 minutes from
application of the disks. Plates are incubated in air for non-fastidious and in air
with 5% CO2 for fastidious organisms. During the incubation the antibiotic will
diffuse in the area surrounding each disk, and a rounded clear area (where
the strain's growth is inhibited) will appear. The measured diameter (in
millimeters) of the clear area is compared with standard tables. See video and
images sections for more.
- quantitative method - based on dilution: serial dilutions of an antimicrobial
agent of known concentration are prepared in growth medium. All tubes are
then inoculated with the isolated strain and incubated for a defined period of
time. The MIC is defined as the lowest concentration of antimicrobial agent
needed to inhibit the growth of a test organism. Once the MIC is calculated, it
can be compared to known values for a given bacterium and antibiotic.
Note: before you perform an antibiogram, you need to identify the strain and
be sure you have a pure culture. The antibiotics used for antibiogram are not
the same for different bacteria genera.
The same principle may be applied for Antifungigram by changing antibiotic
with antimycotic impregnated disks.
| Antibiogram, diffusion method |
| Antifungigram, diffusion method (Candida spp.) |
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