From Cow

Tetracyclines are a 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 Escherichia coli isolates.

Even though there is general cross-resistance among tetracyclines, doxycycline and minocycline usually are more effective against staphylococci.