Antibiotic resistance has emerged as a serious threat to society and is responsible for at least 2 million illnesses each year. A common contribution to the spread of antibiotic resistance is the incorrect use of antibiotics. Prescription of the correct drug increases the chance of completely eliminating the infection early on and lowers the risk of selecting for resistant strains. Antimicrobial susceptibility testing (AST) results guide physicians in their decision of which antibiotic to prescribe. Current AST is carried out in a standardized manner in vitro which unfortunately does not consistently represent the outcome in the host environment. Improvements in AST methods must be implemented in order to provide a more accurate prediction of treatment outcome. We have introduced a novel approach to performing AST by substituting the standard media with host-mimicking media. We found in several cases that when host-mimicking media is used for testing, the susceptibility or resistance of a strain to a drug can be significantly altered and even change the prediction of whether drug will succeed or fail in the host. We also observed treatment outcomes in vivo that were consistent with the susceptibility profiles obtained with host-mimicking media but inconsistent with the susceptibility profiles obtained with standard media. Further, the addition of sodium bicarbonate (NaHCO3) to standard testing medium often resulted in the same susceptibility profiles that were obtained with host-mimicking media and linked to in vivo treatment outcomes. This indicates that the presence of certain signaling molecules in testing media could be a simple change that improves the accuracy of AST. Lastly, we investigated the role of fetal bovine serum (FBS) as an extracellular signal that contributed to changes in susceptibility profiles. Overall, these findings emphasize the importance of recapitulating the host environment to obtain more accurate AST results, correct prescription of drugs, and in turn reduced cases of antibiotic resistance.
