It was a bit of exaggeration. The bacteria doesn’t want to literally “eat” the brain – it just have the ability to invade your body, and some sneaky bacteria can sometimes go so deep it could end up in your brain. Now your body’s immune system will normally detect that and try hysterically to fight it off to prevent further threat to your life. This process often causes collateral damage to the immediate surroundings, which in the case are brain tissues. This damage is often irreversible and have long-term consequences on people who survived such an ordeal. One may argue that it’s like having part of your brain put out of action or “eaten” by the bacterial culprit!