Methods are provided for performing antibiotic susceptibility testing based on the detection of RNA, such as tmRNA, from microbial cells after exposure to antibiotics. In some embodiments, aliquots are obtained from a sample, one of which contains a selected antibiotic. The aliquots, which include growth media, are incubated under conditions suitable for microbial growth, and the microbial cells in each aliquot are removed and lysed, and the lysate is subjected to reverse transcription and amplification in infer the effect of the selected antibiotic on the microbial cells. In one embodiment, a sample containing microbial cells is incubated in the presence of a selected antibiotic and a stimulus is provided to induce the production on m RNA within the microbial cells. The microbial cells are subsequently lysed without substantial degradation of the m RNA within the lysate, and the m RNA is detected to determine the effect of the antibiotic on the microbial cells.