In 1884, the Danish scientist Hans Christian Gram developed a diagnostic dye that, after washing, is retained by bacteria with thick peptidoglycan walls, but not by those with thin walls. Molecular sequence comparisons confirm that most gram-positive bacteria (those that retain the dye) form a well-defined branch of the bacterial tree (Fig. 26.15). One species in this group, Bacillus subtilis, is the focus of many studies about basic molecular function in bacteria. All of us harbor Streptococcus species in our mouths, and many of us have suffered from strep throat, produced by streptococcal pathogens. More serious infections, including some forms of meningitis, are also associated with streptococci. Staphylococcus bacteria are commonly found on human skin, but only occasionally do we suffer from maladies such as staph infection, toxic shock syndrome, or food poisoning induced by staphylococcal bacteria.
In contrast to these pathogens, gram-positive bacteria called streptomycetes have proved invaluable to medicine because of a remarkable property. More than half of the species in this group secrete compounds that kill other bacteria and fungi. Streptomycetes have provided us with tetracycline, streptomycin, erythromycin, and numerous other antibiotics that combat infectious disease.