Are natural products still the best source for antibacterial discovery? The bacterial entry factor

Background: One of the reasons for the low output of new antibacterial agents from recent discovery efforts has been the reliance on synthetic chemicals in screening for inhibitors of new bacterial targets. As the bulk of antibacterials are natural product-derived, is a return to natural products for screening warranted? Objective: As bacterial entry is required for inhibition of many targets, this review concentrates on the potential for natural products and compounds from synthetic libraries to enter and be retained in the bacterial cytoplasm. Methods: Papers investigating the physicochemical nature of synthetic libraries, natural products and antibacterials were reviewed; the requirements for entry into the bacterial cytoplasm were delineated. Results/conclusion: Until rules for cytoplasmic entry are developed and routinely used for design of synthetic libraries, natural products still provide a rich resource for antibacterial discovery.