refers to a procedure for visualizing bacteria under the microscope. It was devised by the Danish scientist Hans Christian Gram in 1884 and is still used widely today. The technique involves soaking bacteria on a microscope slide in a solution of the purple dye crystal violet. This is then complexed with a dilute iodine solution and the cells are then washed with a solvent such as acetone or ethanol. Finally the bacteria are treated with a pink-colored stain such as safranin. Bacteria referred to as Gram-positive retain the original crystal violet/iodine complex and appear purple when viewed microscopically, while Gram-negative types lose the crystal/violet complex when washed with solvent and are stained pink by the safranin.

We now appreciate that Gram-positive and Gram- negative bacteria differ markedly in cell structure. Gram-positive bacteria have a cell envelope composed largely of a unique polymer of sugars and amino acids called peptidoglycan. This large, net-like polymer provides strength to the cell wall in much the same way as cellulose supports the plant cell wall. Gram-negative bacteria have far less peptidoglycan than their Gram-positive counterparts and additionally have a lipid covering termed the “outer membrane.” This has important implications for brewing microbiology because the outer membrane impedes entry of hop iso- alpha acids and therefore renders Gram-negative bacteria tolerant to these inhibitory compounds. See iso-alpha acids. Most Gram-positive bacteria are killed by iso-alpha acids but some strains of Pediococcus and Lactobacillus have acquired resistance, either through additional genes (e.g., horA) that expel the acid or modifications to their physiology that enable tolerance. See lactobacillus, pediococcus.