Heterologous expression of antimicrobial peptides S-thanatin and bovine lactoferricin in the marine diatom Phaeodactylum tricornutum enhances native antimicrobial activity against Gram-negative bacteria

The increase in resistance of bacterial strains to existing antibiotics represents a major public health concern. Antibiotics are an invaluable tool still frequently used in agriculture as they can both improve feed/energy conversion ratios in livestock and treat pathogenic infections. However, the misuse of these compounds has contributed to escalating antimicrobial resistance (AMR). The rapid increase in drug-resistant infections has presented a serious challenge to antimicrobial therapies. Research is currently being carried out to identify alternative antimicrobial compounds to use in animal production although to date, no effective alternatives to production-purpose antibiotics (PPAs) have been developed. Recent reports indicate that lysate of marine diatom Phaeodactylum tricornutum was able to significantly lower the maximal growth rate of Gram-negative bacteria. We have engineered P. tricornutum to accumulate antimicrobial recombinant proteins, S-thanatin and bovine lactoferricin (bLFcin). Transgenic P. tricornutum expressing heterologous S-thanatin and bLFcin demonstrated significantly enhanced antimicrobial activity against Gram-negative bacteria Enterobacter cloacae ATCC 13047 and Vibrio natriegens ATCC 14048 respectively. The lysate of transgenic strain Pt_LFcin1.15, expressing bLFcin, lowered maximal growth rate (μmax) of V. natriegens by 34.5%, which was a 17.3% further reduction than by wild type (WT) lysate. The lysate of the optimized bLFcin-expressing transgenic strain Pt_LFcin2.3 lowered V. natriegens μmax by 43.9%, which was a 18.8% further reduction than by WT lysate. The S-thanatin- expressing transgenic cell lysate of strain Pt_S-Tha1.11 lowered E. cloacae μmax by 61.3%, which was a 38.1% greater reduction than by WT lysate. These statistically significant results (p < 0.01) demonstrate that microalgae can be engineered to express a range of naturally occurring antimicrobial and anti-inflammatory compounds which can substitute for PPAs and treat serious bacterial infections.