There are a number of pathogens resistant to traditional treatment, including drug-resistant influenza, treatment resistant tuberculosis, and antibiotic-resistant bacteria. These pathogens require novel treatments and are the subjects of a number of investigations into new therapies to overcome infection and save lives. The resistance occurs because of mutations in the pathogen genomes, which includes the transfer and insertion of new genes into plasmids, the circular bacterial genome.
Horizontal gene transfer is the transfer of genetic information between bacteria. Referred to as strains, different types of bacteria acquire DNA sequences, including whole genes, from other bacteria they come into contact with. Drug resistance is conferred by genes, often by encoding enzymes that allow the bacteria to deactivate or neutralize the drug. These same genes are often taken advantage of in the lab to screen for plasmids and constructs of interest. This process has also contributed to the variety of the bacterial phylogenic tree, driving bacterial evolution. Essentially, bacteria are evolving to survive in today's medical environment.
Exposing bacteria to antibiotics allows those that are less susceptible to the treatment to survive, and actually thrive, when the other bacteria are removed from competition. When two resistant strains co-infect an individual, they have an opportunity to share resistance genes, potentially creating a double antibiotic resistant strain. There is evidence in scientific journals that the cocolonization of patients with resistant strains, like Staphylococcus aureus and Enterococcus faecalis, is currently occurring. This is the potential explanation for double resistant Staph infections.
A quarter of all hospital-acquired infections are estimated to be drug resistant, and hospital-acquired infections result in 100,000 deaths annually. A return to basic hygiene consciousness, more diligent hand-washing and sterilization, may be able to cut the numbers. Limiting antibiotic treatment to proven bacterial infections, and taking the full course of treatment, is also necessary to reducing the further mutation of bacterial strains.
Example: MRSA and V-MRSA
S. aureus usually colonizes the skin, but open wounds and surgical incisions allow the bacterium to enter and gain access to the internal organs or bloodstream. Methicillin-resistant S. aureus (MRSA) is resistant to methicillin treatment, requiring vancomycin treatment as it is the only effective antiobiotic against the strain. MRSA was first identified in the general population in 1981, but remaining mainly as a hospital-acquired infection until 1997 when four school children died from a community strain. In 2002 and 2003, vancomycin resistant MRSA strains were identified in three states, making treatment almost impossible.
Example: Treatment resistant Tuberculosis
Tuberculosis is caused by a mycobacterium called Mycobacterium tuberculosis. The TB strains resistant to treatment with two first choice drugs, like INH and rifampin, are called multi-drug resistant TB (MDR-TB). There is also extremely drug-resistant TB (XDR-TB), which is due to mismanaged TB cases in undeveloped countries and prison populations. The large population of HIV positive individuals is providing a reservoir for TB resurgence because it takes advantage of the immune deficiency to infect the individuals, reactivate later, and mutate against treatment regimens. There is also a possibility that the presence of INH in cough medicines in Asia, where XDR-TB is common, has contributed to the development of resistance. Regulation of drug exposure prior to TB infection and better control of HIV infection are being promoted as limiting factors in TB resistance.