Q fever is a zoonotic disease caused by Coxiella burnetii, an obligate intracellular bacterium that resides inside
a phagolysosome-like niche. Chronic Q fever is typified by endocarditis, and is treated with multiple antibiotics for at
least 18 months. The discovery of clinical C. burnetii isolates resistant to the first-line antibiotic doxycycline, and the
problematic nature of chronic Q fever treatment have demonstrated the need for improved treatment regimes. To search
for alternative antimicrobial agents, we assessed the effect of 26 antimicrobial peptides (AMPs) on the intracellular
growth of C. burnetii in L929 cells at a concentration of 25 µM or their maximal non-cytotoxic concentration. Among the
peptides tested, A3-APO, Cath-BF, δ-Hemolysin, Octa-1, P5 and Pleurocidin were able to significantly reduce both the total
bacterial cell number and the host cell bacterial burden (average bacterial number per host cell). Combining selected
AMPs with Chariot, a non-covalent carrier peptide, did not increase treatment potency when non-cytotoxic concentrations
were used, with the exception of P5, which remained active at a concentration of 1.6 µM (1.8 µg/mL). Combining AMPs
with each other did not further improve AMP potency, with some treatment combinations increasing the growth rate of C.
burnetii by >3-fold. This is the first description of AMP cellular penetration to exhibit inhibitory affect on intracellular C.
burnetii growth. These results are the first step in the development of a non-traditional antibiotic treatment for Q fever.