Slight variations are present due to differences in bacterial strains. Several bacterial strains isolated from sewage treatment plants efficiently degrade EDTA. Intrinsic antibacterial resistance may be part of the genetic makeup of bacterial strains. There are several antimicrobial resistant strains of this bacterium. Phage therapy is another method for treating antibiotic-resistant strains of bacteria. MHC genes were first identified in inbred mice strains. The genomes of all known human rhinovirus strains have been sequenced. This led to transmission of more dangerous strains of bacteria among groups of patients. Genome sequencing projects for several other "Wolbachia" strains are in progress. Some people are resistant to certain strains of HIV. Certain types of malaria, resistant strains, and complicated cases require different or additional medication. These strains are therefore called "attenuated strains". This may lead to better control of outbreak strains. The number of known quinolone-resistant strains is growing. The mid-1970s were important for the evolution of flu strains.