The present invention provides a method of treating bacterial respiratory infections by pulmonary administration of protonated/acidified nucleic acids. These modified nucleic acids are effective as bactericidal and/or bacteriostatic agents without regard to the class of bacteria, so are especially useful when diagnosis is difficult or when multiple infectious organisms are present. The antibiotic activity of nucleic acids of the invention is not dependent on either the specific sequence of the nucleic acid or the length of the nucleic acid molecule.