I n the race to find a drug to defeat Alzheimer’s — the memory-robbing disease affecting millions of older Americans — leaders of a small South San Francisco company believe they’ve nailed an overlooked culprit: a brain-infecting bacteria. Now Cortexyme Inc., which has received funding from Big Pharma and deep-pocketed foundations alike, is taking its lead drug into its first human studies in attempt to prove that an infection is the source of Alzheimer’s. The undisclosed bacteria that Cortexyme’s drug is targeting has been found in the brain tissue and cerebral spinal fluid of Alzheimer’s patients.