What Is Minimum Bactericidal Concentration?

Minimum bactericidal concentration (MBC) usually relates to antibiotics. It is a measure of how much of a particular antibiotic is required to kill bacteria. Laboratory analysts can assess the MBC of a drug on a bacterial infection to aid a doctor in treating a patient.

Antibiotics are drugs that adversely affect bacteria. Certain bacterial species are susceptible to certain antibiotics, but a doctor may not know which dose of the antibiotic is appropriate, or he or she may not know if the infection is treatable by the antibiotic. A low dose of the drug may inhibit the growth of the bacteria and therefore stop the infection from getting worse. This dose concentration is called the minimum inhibitory concentration (MIC). A dose of antibiotic at the MIC concentration does not kill the bacteria already present.

On the other hand, the minimum bactericidal concentration is the concentration at which the antibiotic kills the bacteria. This information is useful to a doctor, who may not want to expose his or her patient to unnecessary levels of drugs in case of side effects. A known minimum bactericidal concentration can also lead to a shorter course of antibiotics.

Bacteriostatic antibiotics are those that typically inhibit further growth of bacteria, and drugs that usually kill bacteria at the normal dose are called bactericidal. Drugs that are bacteriostatic usually have an MIC that is much less than the MBC, and bactericidal drugs generally have a similar MIC and MBC. Both MIC and MBC can be determined in the laboratory, although many antibiotics have known MBCs expressed in the manufacturers' recommended dosages, which doctors can use to treat patients.

A common lab method for figuring out MIC for a particular bacterial strain uses test tubes and a bacterial culture. A series of tubes receive a nutrient broth base. Then, a microbiologist makes dilutions of an antibiotic, each one more dilute than the one before. He or she places individual dilutions into the tubes and adds samples of the bacteria causing the infection.

After the solutions incubate in a controlled and heated environment for a specified amount of time, the analyst inspects each tube. The MIC of the antibiotic is the tube holding the lowest dilution of the drug that shows no bacterial growth. To figure out the minimum bactericidal concentration, the analyst can then place a sample from the inhibited tubes onto growth medium. The MBC of the antibiotic is represented by the lowest dilution that does not show any growth on the agar plate.