Minimum inhibitory concentration should be considered in the care of patients with microbial diseases. Of key importance in selecting an antibiotic to treat a surgical bacterial infection is the susceptibility of the isolate to the administered drug. One way to measure this is calculation of the minimum inhibitory concentration (MIC) of several possible antibiotic choices against the bacterial isolate.
Minimum inhibitory concentration is defined as the lowest (i.e., most dilute) concentration of an antibiotic that inhibits the growth of the microbe in specific test media known to be hospital to the organism.
The minimum inhibitory concentration (MIC) value is reported in microg/L, not in multiples of dilution. So, the more dilutions a drug can sustain while still having growth retardant ability means a lower mg/L value. The key point is that the lower MIC in microg/L (i.e., the more times the antibiotic has been diluted) the more sensitive the bacterial isolate is to the drug.
There are three general methods of determining minimum inhibitory concentration. These are the serial dilution, disc diffusion, and gradient diffusion methods. In practice, there are commercially available methods that are widely used to make determining the MIC efficient and affordable.
Serial Dilution Method of Minimum Inhibitory Concentration
In this method, the antibiotic is diluted in several two-fold decreasing concentrations. The broth or agar containing the dilution multiples is then inoculated with bacterial isolate of interest and incubated for 18-24 hours at 35 degrees C.
A media sample without antibiotic is also inoculated with the isolate for control purposes.
The broth dilution tubes or agar plates are then inspected to determine the lowest concentration of antibiotic that prevents of turbidity (dilution tubes) or colony growth (agar plates.)
Disc Diffusion Method
In this method, the bacterium to be tested is placed on agar plates. Paper discs that have been coated with a standardized amount of the antibiotic and then placed on the plates. The growth of the bacterium is retarded in a varying size circumferential area around the disk, depending on the sensitivity of the bacterium to the antibiotic.
After a set time period, the diameter of the growth free zone is measured and the MIC is then indirectly determined by a previously known correlation between the diameter of the growth free zone and the MIC.
Gradient Diffusion Method
Similar to the Disc Diffusion Method, the bacterium to be tested is placed on agar plates. Test strips coated with antibiotic arranged in a concentration dependent manner along the length of the strip. The strip is also marked with lines that correlate with varying minimum inhibitory concentration. The strips are then placed on the agar plates and incubated. The area where the growth free zone on the agar plate intersects the strip corresponds to the MIC of that particular antibiotic.
Interpreting the MIC value
Typically, MIC values are reported along with the qualifiers “susceptible”, “intermediate”, and “resistant.” It must be noted however that MIC values for the “susceptible” denomination differ among bacteria.
For instance, a minimum inhibitory concentration of 8 micrograms/mL or less is usually interpreted as meaning a strain of enterococcus is susceptible to penicillin. However, the cutoff for susceptibility of a streptococcus viridans strain to penicillin is a MIC less than .12 micrograms/mL.
There are other susceptibility tests that microbiology labs run, depending on the suspected organism. If the bacterium isolated is oxacillin sensitive Staphylococcus aureus, the presence of penicillinase is tested for to evaluate the sensitivity to penicillin G.
For clindamycin-susceptible, erythromycin-resistent Staph aureusa D-zone test is often performed. A positive D-zone test occurs when the inhibition zone around clindamycin/erythromycin disk pair is smaller than the zone around a clindamycin disk only.
This blunted response correlates with the presence of erm genes which code for ribosomal methylases. These genes can act to increase resistance to clindamycin, but somewhat paradoxically, their presence is best elicited through stimulation of erythromycin.
The presence of extended spectrum beta-lactamases (ESBLs) must also be investigated when Klebsiella or E. Coli microbes show little inhibition to beta-lactam containing antibiotics. If ESBLs are found to be present, the reports for all cephalosporins are rated “resistant,” regardless of the initially reported MIC.
Although testing for resistent bacterial genes is possible for many microbes, the standard practice remains phenotypic resistance typing using the growth inibition parameters described above for most microbial sensitivity testing.
The one exception occurs with testing for methicillin-resistant Staph aureus (MRSA) which uses the presence of the mecA gene or its product penicllin-binding protein 2a to score the strain as resistant.