Image source: http://www.pnas.org/content/106/14.cover-expansion
The Gram Positive cocci that have gained recent notoriety in the medical and subsequently bench literature are the USA300 and 400 CA-MRSA isolates; so named due to their prevalence in the United States. Ever since the emergence of the beta lactam antibiotics such as methicillin in the 60’s, horizontal gene transfer of mobile genetic elements such as phage, plasmids, and transposons have resulted in an increased resistance to said antibiotics, finally yielding the clinically significant strains listed above. A recent revelation, and what makes these strains so significant, is that not only are these methicillin resistant strains a severe problem in a nosocomial sense, in that they tend to infect hospital patients with risk factors such as recent surgery and open wounds, but they can also be passed to healthy individuals. It is surprising then to note that the bacterium’s natural reservoir is the human population, with 20% of the anterior nares (nostrils) of individuals colonized at any given time. Necrotizing pneumonia, a severe form of pneumonia that often follows infection with influenza, is caused by CA-MRSA and is generally lethal. Necrotizing skin lesions and other soft tissue and superficial lesions are also caused by the bacterium.