Oxytetracycline was the second of the broad-spectrum tetracycline group of antibiotics to be discovered. Oxytetracycline works by interfering with the ability of bacteria to produce essential proteins. Without these proteins, the bacteria cannot grow, multiply and increase in numbers. Oxytetracycline therefore stops the spread of the infection and the remaining bacteria are killed by the immune system or eventually die. Oxytetracycline is a broad-spectrum antibiotic, active against a wide variety of bacteria. However, some strains of bacteria have developed resistance to this antibiotic, which has reduced its effectiveness for treating some types of infections. Oxytetracycline is still used to treat infections caused by Chlamydia (e.g. the chest infection psittacosis, the eye infection trachoma, and the genital infection urethritis) and infections caused by Mycoplasma organisms (e.g. pneumonia). Oxytetracycline is also used to treat acne, due to its activity against the bacteria on the skin that cause acne (Propionibacterium acnes). It is used to treat flare-ups of chronic bronchitis, due to its activity against the bacteria usually responsible, Haemophilus influenzae. Oxytetracycline may also be used to treat other rarer infections, such as those caused by a group of micro-organisms called rickettsiae (e.g. Q fever). To make sure the bacteria causing an infection are susceptible to it, a tissue sample is usually taken, for example a swab from the infected area, or a urine or blood sample.