Generally, arthropods combat pathogenic infections with antibacterial peptides that are inducible in response to injury or bacterial infection. Ornithodoros moubata shows higher up-regulated gene expression of defensin in response to blood feeding than bacterial infection. Four defensin genes have been previously identified in O. moubata . In the present study synthetic tick defensin was used to investigate the spectrum and mode of action of natural tick defensin. Although originally purified as a S. aureus -killing antibacterial peptide, synthetic tick defensin shows a broad spectrum that kills many other Gram-positive bacteria including medically important E. faecalis , S. aureus and MRSA at low concentrations. Synthetic tick defensin shows extremely low hemolytic activity against human erythrocytes. The mode of action of defensin was confirmed to be due to the peptides interacting with and disrupting the cytoplasmic membrane. Tick defensin seems to also interfere with cell separation as shown by the presence of multiple aberrant cross-walls. This is the first report of abnormal cell division as a result of defensin treatment.