Abstract: A predetermined amount of parent bacteriophage capable of infecting a target microorganism is added to a sample to create a bacteriophage-exposed sample; the sample is incubated for a defined incubation time and assayed to determine the level of a bacteriophage or bacterial marker in the sample; and if the measured marker level has increased, then the initial concentration of the microorganism exceeds a specific threshold value. An antibiotic in different concentrations is added to different and separate portions of the sample and tested to determine if the bacteriophage marker is present and thereby determine the Minimum Inhibitory Concentration (MIC) of a given antibiotic. The antibiotic preferably is an antibiotic that inhibits DNA replication or protein synthesis.