Researchers have discovered a compound that could outmaneuver resistance to an antibiotic long used as a crucial last option. In animal experiments, this prospective antibiotic was highly potent against dangerous opportunistic pathogens like Acinetobacter baumannii, the most common cause of infections in health care settings. For years, public-health experts have sounded the alarm about the next phase in humanity’s coexistence with bacteria—a dark future where emerging strains have rendered once-powerful antibiotics useless. The United Nations recently projected that, unless new drugs are developed, multidrug-resistant infections will force up to 24 million people into extreme poverty within the next decade and cause 10 million annual deaths by 2050. Scientists are especially apprehensive about a broad group of bacteria that circulate in hospitals and can dodge not only blockbuster drugs like penicillin and tetracycline, but even colistin, an antibiotic used as a last resort. When colistin fails, there is often no effective antibiotics for patients …