All tetracyclines are about equally active and typically have about the same broad spectrum, which comprises both aerobic and anaerobic gram-positive and gram-negative bacteria, mycoplasmas, rickettsiae, chlamydiae, and even some protozoa (amebae). Strains of Pseudomonas aeruginosa, Proteus, Serratia, Klebsiella, and Arcanobacterium spp frequently are resistant, as are many pathogenic E coli isolates. Even though there is general crossresistance among tetracyclines, doxycycline and minocycline usually are more effective against staphylococci.