Minimum inhibitory concentrations

The MIC is the lowest concentration of an antimicrobial agent that will inhibit the visible growth of a microorganism after overnight incubation. 

The broth dilution method is a simple procedure for testing a small number of isolates, even a single isolate. It has the added advantage that the same tubes can also be used for minimum bactericidal concentration(s) (MBC) tests. 

Sterile graduated pipettes of 10, 5, 2 and 1 ml, sterile capped 7.5 x 1.3 cm tubes/ small screw-capped bottles, Pasteur pipettes, an overnight broth culture of the test and control organisms (same as for disc diffusion tests), the required antibiotic in powder form, the required solvent for the antibiotic, sterile distilled water - 500 ml and suitable nutrient broth medium. A suitable rack to hold 22 tubes in two rows, i.e., 11 tubes in each row. 

Prepare stock dilutions of antibiotic concentrations of 1,000 and 100 pg/1, as required. 

The MIC is expressed as the lowest dilution which inhibited microbial growth and is judged by a lack of turbidity in the tube. Because very faint turbidity may result from the inoculum itself, the inoculated tube kept in the refrigerator overnight may be used as the standard to determine complete inhibition. A standard strain of known MIC value, run with the test, is used as the control to check the reagents and conditions allowed microbial growth. 

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Table 3. Representative Minimum Inhibitory Concentrations for Urinary Tract Organisms by Sulfamethoxazole (SMX), Trimethoprim (TMP), and Their Combination...

Table 1. Minimum Inhibitory Concentration (MIC) of Chloramphenicol and Thiamphenicol Against Sensitive Organisms...

Table 5. Minimum Inhibitory Concentrations (MIC q ) of Florfenicol (2) and Other Antibiotics Against Bacterial Pathogens Isolated from Fish in Japan, Jg/mL...

Minimum Inhibitory Concentration measured by dilution of test compound in agar inoculated with microorganism. 

Minimum inhibitory concentration for 90% of the strains used. jJ-g/ruL All are nonfermentors except the Enterohacteriaceae. 

Although most /3- lactam antibiotics bind covalently to some or all of the same six proteins, there are decided differences among them in terms of their relative affinities. For example, cefoxitin (see Table 1 for structures) fails to bind to protein 2 while cephacetrile binds very slowly to proteins 5 and 6. Cephaloridine binds most avidly to protein 1, the transpeptidase, and inhibits cell elongation and causes lysis at its minimum inhibitory concentration. On the other hand, cephalexin binds preferentially to protein 3 and causes inhibition of cell division and filament formation (75PNA2999, 77MI51002). 

Activities are expressed as minimum inhibitory concentration (MIC, pg mL ) defined as the lowest concentration required for complete inhibition of growth of bacterial strain. 

Vancomycin is bactericidal to most susceptible bacteria at concentrations near its minimum inhibitory concentration (MIC) and is an inhibitor of bacterial cell wall peptidoglycan synthesis, although at a site different from that of j3-lactam antibiotics (Chapter 9). 

Table 11.2 Effect of inoculum size on the minimum inhibitory concentration (MIC) of three antiseptics against Staphylococcus aureus... 

Graded doses ofthe test substance are incorporated into broth dispensed in McCartney boules and the bottles inoculated with the test organism and incubated. The point at which no growth occurs is taken as the bacteriostatic concentration (minimum inhibitory concentration, MIC). It is essential when performing these tests to determine the size of the inoculum as the position of the end-point varies considerably with inoculum size, which should always be defined in any description of result. 

LFT, liver function tests MIC, minimum inhibitory concentration. Adapted, with permission, from ref. 14. 

LFT, liver function tests MIC, minimum inhibitory concentration. 

Infective endocarditis caused by these streptococci typically has a subacute clinical course. The current cure rate is often over 90% unless complications occur, which is the case in more than 30% of patients.17 The majority of viridans streptococci remain very susceptible to penicillin, with most strains having a minimum inhibitory concentration (MIC) of less than 0.125 mcg/mL.15,18 Organisms with decreased susceptibilities are increasing. Therefore, antibiotic susceptibilities need to be assessed in order to determine the most appropriate treatment regimen. 

Minimum inhibitory concentration less than or equal to 0.1 2 mcg/mL. 

Penicillin C 24 million units/24 hours IV in 4 to 6 equally divided doses may be used in place of nafcillin or oxacillin if strain is penicillin-susceptible (minimum inhibitory concentration less than or equal to 0.1 mcg/mL) and does not produce P-lactamase. cGentamicin should be administered in close temporal proximity to vancomycin, nafcillin, or oxacillin dosing. 

IVDUs intravenous drug users MIC minimum inhibitory concentration... 

Minimum inhibitory concentration The lowest concentration of a drug that will visually inhibit the growth of a microorganism. 

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