Merry -
It's a way to quantify the amount of bacteria that grows on a culture, instead of saying "positive" and "few, some, many, significant or numerous", they say +1, +2, etc. The higher the number, the greater the bacterial load. There is a scale the tech uses of how many bacteria in the field of view of the microscope, at a specific magnification, to determine the number assigned. With TB, which requires treatment nearly as long as MAC, but with a single drug, +1 is not usually treated unless you are symptomatic or work with a vulnerable population (eg. nurse, elder care, child care.) My pulmonologist does not treat +1 MAC unless you are symptomatic or have visible nodules (indicating pockets of infection) on a CT. Instead he follows closely (visit & sputum test every 3 months, CT twice a year.) to watch for changes.