The latter smells faintly of ethanol; a 1% aqueous solution has a pH of 2–3; and the specific rotation is 
  
    
      
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   −110° cm³/dm·g in 0.01 N methanolic hydrochloric acid. History
After penicillin revolutionized the treatment of bacterial infections in WWII, many chemical companies moved into the field of discovering antibiotics by bioprospecting. American Cyanamid was one of these, and in the late 1940s chemists there discovered chlortetracycline, the first member of the tetracycline class of antibiotics. Shortly thereafter, scientists at Pfizer discovered terramycin and it was brought to market. Both compounds, like penicillin, were natural products and it was commonly believed that nature had perfected them, and further chemical changes could only degrade their effectiveness.
It is also effective against Yersinia pestis (the infectious agent of bubonic plague), and is prescribed for the treatment of Lyme disease, ehrlichiosis, and Rocky Mountain spotted fever.Doxycycline is indicated for treatment of:
Rocky Mountain spotted fever, typhus fever and the typhus group, Q fever, rickettsialpox, and tick fevers caused by Rickettsia
Respiratory tract infections caused by Mycoplasma pneumoniae
Lymphogranuloma venereum, trachoma, inclusion conjunctivitis, and uncomplicated urethral, endocervical, or rectal infections in adults caused by Chlamydia trachomatis
Psittacosis
Nongonococcal urethritis caused by Ureaplasma urealyticum
Relapsing fever due to Borrelia recurrentis
Chancroid caused by Haemophilus ducreyi
Plague due to Yersinia pestis
Tularemia
Cholera
Campylobacter fetus infections
Brucellosis caused by Brucella species (in conjunction with streptomycin)
Bartonellosis
Granuloma inguinale (Klebsiella species)
Lyme diseaseWhen bacteriologic testing indicates appropriate susceptibility to the drug, doxycycline may be used to treat these infections caused by Gram-negative bacteria:
Escherichia coli infections
Enterobacter aerogenes (formerly Aerobacter aerogenes) infections
Shigella species infections
Acinetobacter species (formerly Mima species and Herellea species) infections
Respiratory tract infections caused by Haemophilus influenzae
Respiratory tract and urinary tract infections caused by Klebsiella speciesSome Gram-positive bacteria have developed resistance to doxycycline. Up to 44% of Streptococcus pyogenes and up to 74% of S. faecalis specimens have developed resistance to the tetracycline group of antibiotics. Up to 57% of P. acnes strains developed resistance to doxycycline.