A type of gram-positive bacterium (see GRAM'S STAIN; BACTERIA) which under the microscope appears in small masses like bunches of grapes. It is one of the most common infectious micro-organisms and is found, for example, in the PUS discharged from BOILS (FURUNCULOSIS). (See also MICROBIOLOGY.) Most respond to certain penicillins but there is great concern over the increase in resistant staphylococci (MRSA).