What is an MIC? The MIC, or minimum
inhibitory concentration, is the lowest concentration (in μg/mL) of an
antibiotic that inhibits the growth of a given strain of bacteria. How tested??
Nowadays commercial automated system is used to
determine MICs..This implies ,therefore that minimal inhibitory concentration.( MIC )=
is the lowest concentration of drug that inhibits the growth of the organism. ...
After a standardized incubation, the lowest concentration of drug that prevents visible growth of the organism. Susceptibility
is individual to the organism tested.
Why tested?? Ans:-Antimicrobial susceptibility tests can
guide the physician in drug choice and dosage for difficult-to-treat infections
To selct the antibiotic?? Antimicrobial susceptibility tests are used to
determine which specific antibiotics a particular bacteria or fungus is
sensitive to. Most often, this testing complements a Gram stain and culture,
the results of which are obtained much sooner. Antimicrobial susceptibility
tests can guide the physician in drug choice and dosage for difficult-to-treat
infections. Results are commonly reported as the minimal inhibitory
concentration (MIC), which is the lowest concentration of drug that inhibits
the growth of the organism. Reports typically contain a quantitative result in
µg/mL and a qualitative interpretation
.
What was done earlier?? Historically,
in vitro susceptibility testing was routinely performed by disk diffusion
(Kirby-Bauer) method. The size of the growth-free zone determined whether the
bacterium was considered to be susceptible, resistant, or intermediate to a
particular antibiotic. Although a useful guide for selecting an effective
antibiotic, Kirby-Bauer testing could not tell the clinician the exact
concentration of antibiotic needed to achieve a therapeutic result. The VITEK®
2 (BioMerieux) automated platform, used in all our microbiology laboratories,
supports rapid and accurate quantitative antibiotic susceptibility test (AST)
reporting, including minimum inhibitory concentration (MIC). The MIC provides
the ability to precisely determine the concentration of antibiotic required to
inhibit growth of a pathogen. Your IDEXX microbiology results will show the
identity of the organism and the appropriate antibiotic sensitivity pattern
against each organism. Most antibiograms will include MICs in order to
determine the most effective antibiotic that will result in effective
treatment. This guide provides a detailed explanation of the following concepts
important in implementing the MIC: • The MIC number is the lowest concentration
(in μg/mL) of an antibiotic that inhibits the growth of a given strain of
bacteria. (See the “What is an MIC?” section.) • An MIC number for one
antibiotic CANNOT be compared to the MIC number for another antibiotic. (See
the “How are MICs used?” section.) • The choice of antibiotic should be based
on the MIC number, the site of infection, and an antibiotic’s breakpoint.
Consider safety, ease of use, and cost when determining the optimum antibiotic.
• The attached tables will aid in MIC interpretation and antibiotic selection.
What is an MIC? The MIC, or minimum inhibitory concentration, is the lowest
concentration (in μg/mL) of an antibiotic that inhibits the growth of a given
strain of bacteria.
At IDEXX, a commercial automated system is
used to determine MICs. A quantitative method of susceptibility testing, an MIC
helps determine which class of antibiotic is most effective. This information
can lead to an appropriate choice of an antibiotic that will increase chances
of treatment success and help in the fight to slow antibiotic resistance. How
is the MIC reported? Next to each antibiotic is the susceptibility
interpretation: S (sensitive), I (intermediate), or R (resistant), followed by
the MIC in μg/mL. Sensitive implies that the organism is inhibited by the serum
concentration of the drug that is achieved using the usual dosage; intermediate
implies that the organisms are inhibited only by the maximum recommended dosage;
and resistant implies that the organisms are resistant to the usually
achievable serum drug levels. These interpretive standards have been
established by the Clinical and Laboratory Standards Institute (CLSI).
Microbiology guide to interpreting minimum inhibitory concentration (MIC)
Diagnostic update Antibiotics MIC (μg/mL) Interpretation Amoxicillin ≤2
Sensitive Amoxicillin/clavulanic acid ≤2 Sensitive Cephalexin 8 Sensitive
Cefpodoxime ≤0.25 Sensitive Ceftiofur ≤1 Sensitive Cefovecin ≤2 Sensitive Ceftazidime
≤4 Sensitive Cefotaxime Sensitive
Imipenem 2 Sensitive Amikacin ≤2 Sensitive Gentamicin ≤0.5 Sensitive Tobramycin
≤1 Sensitive Enrofloxacin 0.5 Sensitive Marbofloxacin 0.5 Sensitive
Ciprofloxacin ≤1 Sensitive Doxycycline ≥16 Resistant Nitrofurantoin 32
Sensitive Chloramphenicol 8 Sensitive Trimethoprim/sulfa ≤10 Sensitive *MIC not
available Urine culture susceptibility result Organism: E.coli Class-reference
antibiotics Some antibiotics are used to determine the susceptibility of other
antibiotics in the same class. For example, the presence of
methicillin-resistant staphylococci (MRS) is tested in the laboratory with
oxacillin and not methicillin. The name MRS is used because of convention over
years of use in scientific articles and textbooks. For example: Amoxicillin is
four dilutions away from its breakpoint while cefovecin is only two dilutions
away from its breakpoint. Therefore, in this case, the E. coli strain is more
susceptible to amoxicillin. Our consultants are available to help you interpret
test results. When are MICs not performed? MICs are not performed when: • The
growth requirements of some organisms require the sensitivity testing to be
performed by another method. • Interpretive criteria is not available from
CLSI. In these cases, recommended antibiotics will usually be reported based on
clinical efficacy studies. • Certain antibiotics are not available on our
commercial system. • The drug is known to be clinically ineffective against the
organism, regardless of the in vitro results. How are MICs used? The breakpoint
and range of dilutions differ by drug and bacterial species (see chart on next
page). Therefore, comparing MICs of different antibiotics is not based solely
on the numerical value but on how far the MIC is from the breakpoint, the site
of the infection, and other considerations, such as the age, species, and
health of the animal. Possible side effects of the drug, price, frequency, and
route of administration are also important factors. For example: A strain of
Escherichia coli has an MIC of 2 μg/mL for amoxicillin and for cefovecin.
Looking at the dilutions for amoxicillin, at 2 μg/mL, this strain of E. coli is
four dilutions away from the breakpoint. For cefovecin, the same strain of E.
coli at an MIC of 2 μg/mL is two dilutions away from the breakpoint. So, based
on MICs, this strain of E. coli is more susceptible to amoxicillin than
cefovecin. Other factors to take into consideration are the site of the
infection, the animal’s health, frequency and route of administration, and cost
of the antibiotic. An antibiotic breakpoint is the dilution where bacteria
begin to show resistance. In vitro efficacy of amoxicillin (predicts
ampicillin) In vitro efficacy of cefovecin Sensitive (MIC) Intermediate
Resistant 2 4 8 16 32 Tested concentrations of amoxicillin (µg/mL) Sensitive
(MIC) Intermediate Resistant 2 4 8 Tested concentrations of cefovecin (µg/mL)
Antibiotic Further application Amoxicillin/clavulanic acid Predicts
susceptibility of Clavamox®. Amoxicillin Predicts susceptibility of ampicillin.
Cephalexin Predicts susceptibility of all first-generation cephalosporins,
except cefazolin. Clindamycin Predicts susceptibility of lincomycin. Should not
be used in horses, rabbits, and other herbivores. Not effective against aerobic
gram-negative bacteria. Erythromycin Predicts susceptibility of azithromycin
and clarithromycin. Not effective against aerobic gram-negative bacteria.
Oxacillin Predicts susceptibility to methicillin. Trimethoprim/sulfa Predicts
susceptibility of other potentiated sulfonamides. Antibiotics When selecting an
antibiotic, keep in mind that other factors in addition to the MIC are
important. The location of the infection is important because lipid-soluble
drugs reach higher levels in the tissue than they do in serum. Drugs excreted
by the kidney reach much higher bladder levels than serum levels. Also, some
drugs are more effective against gram-negative bacteria than gram-positive
bacteria and vice versa. Species considerations are also important because
certain antibiotics are toxic in some species. Breakpoint Breakpoint Published
March 2019 © 2019 IDEXX Laboratories, Inc. All rights reserved. • 09-67064-03
VITEK is a registered trademark of bioMérieux, Inc. Clavamox is a registered
trademark of the GSK group of companies. All other ®/TM marks are owned by
IDEXX Laboratories, Inc. or its affiliates in the United States and/or other
countries. The IDEXX Privacy Policy is available at idexx.com. Current
antibiotic MIC ranges for canine and feline patients are listed below: Customer
support services IDEXX supports your practice with our customer support,
technical support, and medical consulting services teams, including our
diagnostic support veterinarians and board-certified veterinary specialists.
Call 1-888-433-9987 if you have questions. The information contained herein is
intended to provide general guidance only. As with any diagnosis or treatment,
you should use clinical discretion with each patient based on a complete
evaluation of the patient, including history, physical presentation and
complete laboratory data. With respect to any drug therapy or monitoring
program, you should refer to product inserts for a complete description of
dosages, indications, interactions and cautions. Antibiotic Susceptible Intermediate
Resistant breakpoint Amikacin ≤4 8 ≥16 Amoxicillin (skin and soft tissue)*
≤0.25 0.5 ≥1 Amoxicillin (urine)* ≤8 16 ≥32 Amoxicillin/clavulanic acid (skin
and soft tissue)* ≤0.25 0.5 ≥1 Amoxicillin/clavulanic acid (urine)* ≤8 16 ≥32
Benzylpenicillin Enterococcus ≤8 ≥16 Benzylpenicillin Staphylococcus ≤0.12
≥0.25 Cefovecin (skin and soft tissue) ≤0.5 1 ≥2 Cefovecin (urine) ≤2 4 ≥8
Cefpodoxime ≤2 4 ≥8 Ceftazidime Enterobacteriaceae ≤4 8 ≥16 Ceftazidime
Pseudomonas ≤8 16 ≥32 Cephalexin (skin and soft tissue) ≤2 4 ≥8 Cephalexin
(urine) ≤16 ≥32 Chloramphenicol ≤8 16 ≥32 Clindamycin gram-positive ≤0.5 1–2 ≥4
Ciprofloxacin gram-negative ≤1 2 ≥4 Doxycycline (respiratory, skin, and soft
tissue)† ≤0.12 0.25 ≥0.5 Doxycycline (urine)† ≤4 8 ≥16 Enrofloxacin ≤0.5 1–2 ≥4
Erythromycin ≤0.5 1–4 ≥8 Florfenicol ≤2 4‡ ≥8 Gentamicin gram-negative ≤2 4 ≥8
Gentamicin Staphylococcus ≤4 8 ≥16 Imipenem gram-negative ≤2 4 ≥8 Marbofloxacin
≤1 2 ≥4 Minocycline (skin and soft tissue) ≤0.5 1 ≥2 Nitrofurantoin (only
reported on urine cultures) ≤32 64 ≥128 Oxacillin Staphylococcus aureus ≤2 ≥4
Oxacillin Staphylococcus (non-S. aureus) ≤0.25 ≥0.5 Polymyxin B (not reported
on urine cultures) ≤2 4 ≥8 Pradofloxacin ≤0.25 0.5–1 ≥2 Trimethoprim/sulfa ≤40
≥80 *Amoxicillin and amoxicillin/clavulanic acid may appear sensitive when
tested in vitro, but they may not achieve effective levels at the site of skin
and soft-tissue infections with standard dosing. † CLSI breakpoints for
doxycycline against staphylococci isolated from respiratory, skin, and soft
tissue are ≤0.12 (susceptible), 0.25 (intermediate), and ≥0.5 (resistant).
Currently, the lowest concentration reportable for doxycycline in the IDEXX
automated platform is 0.5. An internal study of 120 Staphylococcus isolates
submitted to IDEXX Reference Laboratories with an MIC result of
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